PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER PLATE ...

PREDICTION OF WORKING-LOAD DISPLACEMENTSUNDER PLATE-LOADING TESTS FROM SEISMIC STIFFNESS MEASUREMENTS

RESEARCH REPORT ICAR - 501-3

Sponsored by the Aggregates Foundation

for Technology, Research and Education

Technical Report Documentation Page

1

1. Report No. ICAR 501-3

2. Government Accession No.

3. Recipient's Catalog No.

5. Report Date October 1998

4. Title and Subtitle PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER

PLATE LOADING TESTS FROM SEISMIC STIFFNESS MEASUREMENTS

6. Performing Organization Code

7. Author(s) Michael L. Myers, Kenneth H. Stokoe, II, and John J. Allen

8. Performing Organization Report No. Research Report ICAR 501-3

10. Work Unit No. (TRAIS)

9. Performing Organization Name and Address International Center for Aggregates Research The University of Texas at Austin ECJ 5.200 Austin, Texas 78712-1076

11. Contract or Grant No. Project No. ICAR-501

13. Type of Report and Period Covered Research Report March 1997-October 1998

12. Sponsoring Agency Name and Address Aggregates Foundation for Technology, Research and Education 1415 Elliott Place NW Washington, D.C. 20007

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract

A study was conducted to evaluate the feasibility of compacting unbound aggregate base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test section using a variety of material types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone. Three crushed granite test sections were built as part of a road widening project in Georgia, and two test pads were constructed of uncrushed and partially crushed gravel with loess fines at a gravel production facility near Memphis, Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of Surface-Waves (SASW) technique, was used to evaluate stiffness profiles within the compacted lifts. Plate load tests were conducted on the surface of the crushed limestone test pads by means of the Rolling Dynamic Deflectometer specially modified for this fixed site application. Low frequency cyclic loads were applied to determine axial stiffness under transient working loads of varying magnitude. The base courses were tested at to moisture contents. The results were evaluated and compared with small strain seismic tests result. Strain amplitudes in the plate load tests led to a 5% to 25% reduction in measured stiffness as compared to the seismic results.

17. Key Words Seismic-Analysis-of-Surface-Waves, Nuclear Density Gauge, Compaction, Granular Base Course, Lift Thickness, Thick Lifts, Plate Load Testing

18. Distribution Statement No restrictions.

19. Security Classif.(of this report) Unclassified

20. Security Classif.(of this page) Unclassified

21. No. of Pages

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

2

PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER PLATE LOADING TESTS FROM SEISMIC STIFFNESS MEASUREMENTS

by

Michael L. Myers Captain, United States Air Force

820th Red Horse Civil Engineering Squardron Nellis Air force Base, Nevada

Kenneth H. Stokoe, II

Cockrell Family Regents Chair No. 9 Civil Engineering Department

The University of Texas at Austin

and

John J. Allen Director, Roadway Research Initiative

Center for Transportation Research The University of Texas at Austin

and Texas Transportation Institute

Texas A&M University

Research Report ICAR 501-3 Research Project Number ICAR 501

Sponsored by the Aggregates Foundation for Technology, Research, and Education

October 1998

International Center for Aggregates Research The University of Texas at Austin

Austin, Texas

and Texas A&M University College Station, Texas

iii

ABSTRACT

A study was conducted to evaluate the feasibility of compacting unbound

aggregate base courses in thicker lifts than currently permitted by state departments of

transportation (DOTs). At present, the majority of states allow a maximum lift thickness

of 8 inches or less. This project constructed and tested full-scale test section using a

variety of material types. Two test pads were constructed in an aggregate quarry in Texas

utilizing crushed limestone. Three crushed granite test sections were built as part of a

road widening project in Georgia, and two test pads were constructed of uncrushed and

partially crushed gravel with loess fines at a gravel production facility near Memphis,

Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents

and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive

seismic testing, using the Spectral-Analysis-of Surface-Waves (SASW) technique, was

used to evaluate stiffness profiles within the compacted lifts. Plate load tests were

conducted on the surface of the crushed limestone test pads by means of the Rolling

Dynamic Deflectometer specially modified for this fixed site application. Low frequency

cyclic loads were applied to determine axial stiffness under transient working loads of

varying magnitude. The base courses were tested at to moisture contents. The results were

evaluated and compared with small strain seismic tests result. Strain amplitudes in the

plate load tests led to a 5% to 25% reduction in measured stiffness as compared to the

seismic results.

TABLE OF CONTENTS

ACKNOWLEDGMENTS ........................................................................................................... vi LIST OF TABLES ..................................................................................................................... xii LIST OF FIGURES .................................................................................................................... xv CHAPTER 1. INTRODUCTION 1.1 Increasing the Single-Lift Thickness of Compacted Aggregate Base Courses ............ 1 1.2 Characterizing Thick Aggregate Lifts .......................................................................... 2 1.3 Scope of Plate-Load and Seismic Testing .................................................................... 3 1.4 Objectives of This Study............................................................................................... 4 1.5 Correlations Between Plate-Load Settlement and Other Field Tests............................ 5 1.6 Organization of This Report ......................................................................................... 5 CHAPTER 2. MATERIAL and TEST SITE 2.1 Construction of Test Pad............................................................................................... 7 2.2 Location of Plate-Load Tests ...................................................................................... 11 2.3 Flexible Base Material ................................................................................................ 12 2.4 In-Situ Density Testing ............................................................................................... 14 CHAPTER 3. PLATE-LOAD TESTING with the ROLLING DYNAMIC

DEFLECTOMETER 3.1 Introduction................................................................................................................. 17 3.2 Plate-Load Testing Equipment ................................................................................... 18 3.2.1 Loading Mechanism........................................................................................... 18 3.2.2 Load-Plate Details.............................................................................................. 21 3.2.3 Load Cell............................................................................................................ 22 3.2.4 Displacement Transducers ................................................................................. 23 3.2.5 Support Frame for DC-LVDTS ......................................................................... 24 3.2.6 Data Acquisition System.................................................................................... 28 3.3 Data Reduction............................................................................................................ 30 3.4 Overview of Plate-Load Testing................................................................................. 30 3.4.1 Modulus of Soil Reaction, Ku’ .......................................................................... 31 3.4.2 Material Equivalent Spring Constant (Stiffness) ............................................... 32 3.4.3 Summary of Plate-Load Tests Performed.......................................................... 32

vi

CHAPTER 4. PRELIMINARY PLATE-LOAD TESTS at LOCATION A on the TEST PAD 4.1 Introduction................................................................................................................. 34 4.2 Test A1 - Incremental Plate-Load Test from 0 to 5 Kips (22.2 kN).......................... 34 4.3 Test A2 - Loading from 0 to 11 Kips (48.9 kN) in 36 Seconds Followed by

Complete Unloading ................................................................................................... 35 4.4 Test A3 - 32 Cycles of 0 to 9 Kips (40.0 kN) in 20 Minutes...................................... 41 4.4.1 Determining Stiffness During Loading.............................................................. 44 4.4.2 Determining Stiffness During Unloading .......................................................... 49 4.4.3 Measurement of Permanent Displacement ........................................................ 52 4.5 Test A4 - Loading from 0 to 40 Kips (178 kN) in 6 Minutes Followed by 56

Complete Unloading ................................................................................................... 56 4.6 Summary of Observations from Preliminary Plate-Load Tests at Location A........... 63 CHAPTER 5. PLATE-LOAD TESTS on DRIER MATERIAL at LOCATION B on the

TEST PAD 5.1 Introduction................................................................................................................. 65 5.2 Test B1 - Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on

Drier Material (Moisture Content ~ 4.7 %) ................................................................ 65 5.2.1 Adjusted Load-Settlement Curve from Test B1 ................................................ 66 5.2.2 Plotting the Unload-Reload Loops on the Adjusted Load-Settlement

Curve from Test B1 ........................................................................................... 71 5.2.3 Stiffness of the Test Pad Material as Determined from the Overall

Plate-Load Test .................................................................................................. 75 5.2.4 Presentation of Cyclic Stiffness and Overall Stiffness Measurements .............. 77 5.2.5 Calculation of Modulus of Soil Reaction, Ku’, for Drier Material

(Moisture Content ~ 4.7 %) ............................................................................... 79 5.3 Summary of Observations from Incremental Plate-Load Test on

Drier Material (Moisture Content ~ 4.7 %) at Location B.......................................... 79 CHAPTER 6. PLATE-LOAD TESTS on WETTER MATERIAL at LOCATION C on the

TEST PAD 6.1 Introduction................................................................................................................. 81 6.2 Test C1 - Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN)

on Wetter Material (Moisture Content ~ 6.1%).......................................................... 82 6.2.1 Adjusted Load-Settlement Curve from Test C1 ................................................ 85 6.2.2 Plotting the Unload-Reload Loops on the Adjusted Load-Settlement

Curve from Test C1 ........................................................................................... 88 6.2.3 Stiffness of the Test Pad Material as Determined from the Overall

Plate-Load Test .................................................................................................. 91 6.2.4 Presentation of Cyclic Stiffness and Overall Stiffness Measurements .............. 93 6.2.5 Calculation of Modulus of Soil Reaction, Ku’, for Wetter Material

(Moisture Content ~ 6.1 %) ............................................................................... 94

vii

6.3. Test C2 - 5 Cycles of 0 to 8 Kips (35.6 kN) in 7 Minutes on Wetter Material (Moisture Content ~ 6.1%) ......................................................................................... 96

6.4 Summary of Observations from Incremental Plate-Load Tests on Wetter Material (Moisture Content ~ 6.1 %) at Location C................................................. 102

CHAPTER7. SEISMIC TESTING and STIFFNESS EVALUATIONS 7.1 Overview................................................................................................................... 104 7.2 SASW Testing .......................................................................................................... 104 7.3 Non-Linear Moduli ................................................................................................... 105 7.4 Determination of Small-Strain Shear Modulus......................................................... 108 7.5 Determination of Small-Strain Equivalent Spring Constant..................................... 108 7.6 Shear Wave Velocity Profiles................................................................................... 110 7.7 Measured Small-Strain Stiffness............................................................................... 113 7.8 Adjustment to the Small-Strain Stiffness for Variations in the

State of Stress............................................................................................................ 114 CHAPTER 8. COMPARISON of STIFFNESS VALUES from PLATE-LOAD and

SEISMIC TEST 8.1 Introduction............................................................................................................... 115 8.1.1 Estimation of Strain Amplitude ....................................................................... 115 8.1.2 Adjustments to Small-Strain Shear Moduli for increase in Strain

Amplitude and increase in State of Stress........................................................ 117 8.2 Small-Strain Stiffness Comparisons ......................................................................... 120 8.2.1 Adjustment to Small-Strain Stiffnesses for increased Strain Amplitude......... 122 8.2.2 Adjustment to Small-Strain Stiffnesses for increased State of Stress

Due to Wetting and Due to increasing the Load on the Plate.......................... 125 8.2.3 Conclusions from Small-Strain Moduli Comparisons..................................... 128 8.3 Large-Strain Stiffness Comparisons ......................................................................... 129 8.3.1 Short Duration Load to Failure ........................................................................ 130 8.3.2 Stiffness Under Large-Amplitude, Short-Duration Cyclic Loading................ 133 8.3.3 Stiffnesses from Incremental Plate-Load Tests ............................................... 141 8.3.4 Sensitivity of the Material to Duration of Loading.......................................... 143 8.4 Summary of Stiffness Comparison ........................................................................... 147 CHAPTER9. SUMMARY, CONCLUSIONS, and RECOMMENDATIONS 9.1 Summary of Testing Program................................................................................... 148 9.2 Summary of Observations......................................................................................... 150 9.3 Conclusions from Plate-Load and Seismic Tests...................................................... 152 9.3 Recommendations for Future Work.......................................................................... 153

viii

APPENDIX A. NUCLEAR DENSITY GAUGE DATA ....................................................... 155 APPENDIX B. INDIVIDUAL LOADING CYCLE PLOTS FOR TEST A3...................... 159 APPENDIX C. INDIVIDUAL UNLOAD-RELOAD LOOPS FOR TEST B1.................... 192 APPENDIX D. INDIVIDUAL UNLOAD-RELOAD LOOPS FOR TEST C1 ................... 198 APPENDIX E. INDIVIDUAL LOADING CYCLE PLOTS FOR TEST C2...................... 205 REFERENCES .......................................................................................................................... 211

ix

LIST OF TABLES

Table 2.1. Index Properties of Grade 2 Crushed Limestone Base Material from the

Georgetown, Texas Capitol Aggregates Quarry...................................................... 12 Table 2.2. Gradation of Grade 2 Crushed Limestone Base Material from the

Georgetown, Texas Capitol Aggregates Quarry...................................................... 14 Table 2.3. Density and Moisture Content Data at Plate-Load Test Locations A and B

(Drier Material) ........................................................................................................ 16 Table 2.4. Density and Moisture Content Data at Plate-Load Test Location C (Wetter

Material)................................................................................................................... 16 Table 3.1. Direct Current Linear Variable Differential Transformer (DC-LVDTs)

Calibration Factors................................................................................................... 23 Table 5.1. Average Stiffnesses Measured at Various Loading Conditions for Cyclic

Plate-Load Tests on Drier Material (Moisture Content ~ 4.7 %) ............................ 72 Table 5.2. Overall Stiffnesses Measured Over Various Ranges of Loading for Static

Plate-Load Tests on Drier Material (Moisture Content ~ 4.7 %) ............................ 75 Table 6.1. Average Stiffnesses Measured During Small Amplitude Unload-Reload

Loops at Various Loading Conditions for Cyclic Plate-Load Tests on Wetter Material (Moisture Content ~ 6.1 %)....................................................................... 90

Table 6.2. Overall Stiffnesses Measured Over Various Ranges of Loading for Static

Plate-Load Tests on Wetter Material (Moisture Content ~ 6.1 %).......................... 93 Table 7.1. Average Small-Strain Stiffness for an Effective Depth of 12 in. (30.5 Cm)

(One Plate Diameter) ............................................................................................. 113 Table 8.1. Estimated Strain Amplitudes for Small-Strain, Unload-Reload Loops in

Tests B1 and C1 ..................................................................................................... 123 Table 8.2. Young’s Modulus Values Calculated Directly from Unload-Reload Loops in

Plate-Load Tests and from Small-Strain Seismic Tests......................................... 126 Table 8.3. Comparison of Stiffness Values Determined from Large Amplitude Plate-

Load Tests and from Seismic Tests ....................................................................... 136 Table 8.4 AASHTO T-222 Moduli of Soil Reaction for Georgetown Crushed

Limestone Test Pad................................................................................................ 143

x

Table A.1. Nuclear Density Gauge Data for Drier Material (W~ 4.7 %); Collected by

Gene Schlieker, Texas Transportation Institute..................................................... 156 Table A.2. Nuclear Density Gauge Data for Wetter Material (W~ 6.1 %); Collected

by Gene Schlieker, Texas Transportation Institute................................................ 158 Table C.1. Stiffnesses of Individual Small-Amplitude [1 Kip (4.4 kN)] Unload-Reload

Loops; Test B1, Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ...................................................... 197

Table D.1. Stiffnesses of Individual Small-Amplitude [1 Kip (4.4 kN)] Unload-Reload

Loops; Test C1, Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %) ................................... 204

xi

LIST of FIGURES Fig. 2.1. Unbound Base Material Being Delivered to the Test Pad Site.................................. 8 Fig. 2.2. Spreading Unbound Base Material Before Compaction of the Test Pad................... 9 Fig. 2.3. Vibratory Pad-Foot Roller Used to Compact Unbound Base Course Material

Over One-Half of the Test Pad .................................................................................. 9 Fig. 2.4. Vibratory Smooth-Drum Roller Used to Compact Unbound Base Course

Material Over One-Half of the Test Pad.................................................................. 10 Fig. 2.5. Cross Section of Unbound Aggregate Base Course Test Pad Constructed in

Georgetown, Texas .................................................................................................. 10 Fig. 2.6. Plan View of Test Pad and Plate-Load Test Locations............................................ 11 Fig. 2.7. Gradation Curve for Capitol Aggregates Crushed Limestone Base Course............ 13 Fig. 3.1. Rolling Dynamic Deflectometer Configured for Static Plate-Load Testing ........... 18 Fig. 3.2. Plan View of the RDD Showing the Locations of the Truck Tires Relative to

the Location of the Plate-Load Tests ....................................................................... 20 Fig. 3.3. Plate Setup Showing 6 in. (15.2 Cm) Diameter Plate Stacked on Top of the

12 in. (30.5 Cm) Diameter Plate .............................................................................. 22 Fig. 3.4. Measurement Frame Assembled for Plate-Load Testing......................................... 25

Fig. 3.5. Aluminum Adapter Block Attached to the Top of a Tripod.................................... 26 Fig. 3.6. Connection Between Rectangular Tubing and Round Bars [6-In. (15-Cm)

Ruler in Foreground]................................................................................................ 26 Fig. 3.7. Connection of Round Bars [6-In. (15-Cm) Ruler in Foreground] ........................... 27 Fig. 3.8. DC-LVDTs Mounting Block with the Capability of Connecting In-Line with

the Round Bar or at a 90-Degree Angle to the Round Bar ...................................... 27 Fig. 3.9. Schematic of Data Acquisition Hardware................................................................ 29 Fig. 3.10. Data Acquisition System installed in the Cab of the RDD...................................... 29

xii

Fig. 4.1. Estimates of Vertical Stress Delivered to the Top of a Pavement Base Course Beneath the Center of a 12 in. (30.5 cm) Diameter Circular Plate Under a 9000 lb (40.0 kN) Load.............................................................................. 37

Fig. 4.2. Variation of Force with Time; Test A1 - Incremental Pate-Load Test from 0

to 5 Kips (22.2 kN) ................................................................................................. 38 Fig. 4.3. Variation of Displacement with Time; Test A1 - Incremental Plate-Load

Test from 0 to 5 Kips (22.2 kN).............................................................................. 39 Fig. 4.4. Continuous Load-Settlement Curve from Test A1 - Incremental Plate-Load

Test from 0 to 5 Kips (22.2 kN).............................................................................. 40 Fig. 4.5. Variation of Force with Time; Test A2 - Loading from 0 to 11 Kips

(48.9 kN) in 36 Seconds Followed by Complete Unloading ................................... 42 Fig. 4.6. Variation of Displacement with Time; Test A2 - Loading from 0 to 11 Kips

(48.9 kN) in 36 Seconds Followed by Complete Unloading ................................... 43 Fig. 4.7. Continuous Load-Settlement Curve from Test A2 - Loading from 0 to 11

Kips (48.9 kN) in 36 Seconds and Unloading to Zero............................................. 45 Fig. 4.8. Variation of Force with Time; Test A3 - 32 Cycles of 0 to 9 Kips

(40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %) ................ 46 Fig. 4.9. Variation of Displacement with Time; Test A3 - 32 Cycles of 0 to 9 Kips

(40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %) ................ 47 Fig. 4.10. Continuous Cyclic Load-Settlement Curve; Test A3 - 32 Cycles of 0 to 9

Kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %) ........ 48 Fig. 4.11. Illustration of Bi-Linear Stiffness Behavior Upon Loading; Test A3 - 32

Cycles of 0 to 9 Kips (40.0 kN) in 20 Minutes....................................................... 50 Fig. 4.12. Bi-Linear Stiffnesses of Each Cyclic Loop in the Loading Section;

Test A3 - 32 Cycles of 0 to 9 Kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)....................................................................... 51

Fig. 4.13. Illustration of Stiffness Behavior Upon Unloading; Test A3 - 32 Cycles of 0

to 9 Kips (40.0 kN) in 20 Minutes .......................................................................... 53 Fig. 4.14. Stiffnesses of Individual Loops During Rebound; Test A3 - 32 Loading

Cycles from 0 to 9 Kips on Drier Material (W~ 4.7 %) ......................................... 54 Fig. 4.15. Variation of Force with Time; Test A3 - 32 Cycles of 0 to 9 Kips (40.0 kN)

in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %) ................................. 55

xiii

Fig. 4.16. Variation of Force with Time; Test A4 – Loading to 40 Kips (178 kN) in 6

Minutes Followed by Complete Unloading on Drier Material (Moisture Content ~ 4.7 %) ...................................................................................................... 57

Fig. 4.17. Variation of Displacement with Time; Test A4 – Loading to 40 Kip

(178 kN) in 6 Minutes Followed by Complete Unloading on Dried Material (Moisture Content ~ 4.7 %) ..................................................................................... 58

Fig. 4.18. Continuous Load-Settlement Curve from Test A4 – Loading to 40 Kips (178

kN) in 6 Minutes Followed by Complete Unloading on Drier Material (Moisture Content ~ 4.7 %) ..................................................................................... 59

Fig. 4.19. Load-Settlement Curve Annotated with Values of Relative Stiffness During

Loading; Test A4 – Loading to 40 Kips (178 kN) in 6 Minutes and Unloading to Zero on Drier Material (Moisture Content ~ 4.7 %).......................... 61

Fig. 4.20. Load-Settlement Curve Annotated with Values of Relative Stiffness During

Unloading; Test A4 – Loading to 40 Kips (178 kN) in 6 Minutes and Unloading to Zero on Drier Material (Moisture Content ~ 4.7 %).......................... 62

Fig. 5.1. Variation of Force with Time; Test B1 - Incremental Plate-Load Test from 0

to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) .................... 67 Fig. 5.2. Variation of Displacement with Time; Test B1 - Incremental Plate-Load

Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ..................................................................................... 68

Fig. 5.3. Continuous Load-Settlement Curve; Test B1 - Incremental Plate-Load

Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ..................................................................................... 69

Fig. 5.4. Adjusted Load-Settlement Curve; Test B1 - Incremental Plate-Load Test

from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ..................................................................................... 70

Fig. 5.5. Adjusted Load-Settlement Curve with Unload-Reload Loops Plotted; Test

B1 - Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)....................................................................... 72

Fig. 5.6. Adjusted Load-Settlement Curve with Relative Stiffnesses During Loading

and Unloading Annotated; Test B1 - Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ......................... 76

xiv

Fig. 5.7. Representation of Stiffness with Varying Degrees of Load on the Loading Plate; Test B1 - Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ........................................................ 78

Fig. 6.1. Variation of Force with Time; Test C1 - Incremental Plate-Load Test from 0

to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %).................. 83 Fig. 6.2. Variation of Displacement with Time; Test C1 - Incremental Plate-Load

Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %) ..................................................................................... 84

Fig. 6.3. Continuous Load-Settlement Curve; Test C1 - Incremental Plate-Load Test

from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %) ..................................................................................... 86

Fig. 6.4. Adjusted Load-Settlement Curve; Test C1 - Incremental Plate-Load Test

from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %) ..................................................................................... 87

Fig. 6.5. Adjusted Load-Settlement Curve with Unload-Reload Loops Plotted; Test

C1 - Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)....................................................................... 89

Fig. 6.6. Adjusted Load-Settlement Curve with Relative Stiffnesses During Loading

and Unloading Annotated; Test C1 - Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)....................... 92

Fig. 6.7. Representation of Stiffness with Varying Degrees of Load on the Loading

Plate; Test C1 - Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)...................................................... 95

Fig. 6.8. Variation of Force with Time; Test C2 - 5 Cycles of 0 to 8 Kips (35.6 kN)

in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%).................................. 97 Fig. 6.9. Variation of Displacement with Time; Test C2 - 5 Cycles of 0 to 8 Kips

(35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%) ................. 98 Fig. 6.10. Continuous Load-Settlement Curve; Test C2 - 5 Cycles of 0 to 8 Kips (35.6

kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%) .......................... 99 Fig. 6.11. Bi-Linear Stiffnesses of Each Cyclic Loop in the Loading Section;

Test C2 - 5 Cycles of 0 to 8 Kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%) .................................................................................... 100

xv

Fig. 6.12. Illustration of Stabilization Or Closing of Cyclic Loops; Test C2 - 5 Cycles of 0 to 8 Kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%) .................................................................................... 101

Fig. 7.1. Basic Configuration for SASW Testing (from Stokoe Et Al. 1994) ..................... 105 Fig. 7.2. Relationship Between the Monotonic Loading Curve and Material Stiffness

in Shear of a Geotechnical Material....................................................................... 106 Fig. 7.3. Variation in Material Stiffness in Shear with Increasing Strain Amplitude as

Determined from Fig. 7.2....................................................................................... 107 Fig. 7.4. Equivalent Spring Constant, Keff, Determined from Shear Wave Velocity for

Strain Amplitudes in the Small-Strain Range........................................................ 110 Fig. 7.5. Shear Wave Velocity Profile of the Test Pad for the Drier Material

(W~ 4.7 %)............................................................................................................. 111 Fig. 7.6. Shear Wave Velocity Profile of the Test Pad for the Wetter Material

(W~ 6.1 %)............................................................................................................. 111 Fig. 8.1. Variation of Vertical Normal Strain with Depth Beneath A Circular Plate on

Sand After Schmertmann (1970) ........................................................................... 117 Fig. 8.2. Illustration of the Reduction in Shear Modulus with increases in

Strain Amplitude.................................................................................................... 119 Fig. 8. 3. Adjusted Load-Settlement Curve with Unload-Reload Loops for Test B1;

Incremental Plate-Load Test from 0 to 8.3 Kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %) ................................................................................... 121

Fig. 8.4. Adjusted Load-Settlement Curve with Unload-Reload Loops for Test C1;

Incremental Plate-Load Test from 0 to 8.6 Kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)..................................................................... 121

Fig. 8.5. Illustration of Strain Ranges Observed from Multiple Series of Cyclic

Unload-Reload Loops During Plate Load Testing................................................. 124 Fig. 8.6 Relationship Between Pressure Applied to the Plate and Observed increase

in Small-Strain Stiffness ........................................................................................ 128 Fig. 8.7. Illustration of Material Response Under Load to “Failure”; Test A4,

Loading from 0 to 40 Kips (0to 178 kN) in 6 Minutes and Unloading To Zero................................................................................................................... 132

xvi

Fig. 8.8. Continuous Force-Displacement Plot from Test A3; 32 Cycles of 0 to 9 Kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %) ................................................................................... 134

Fig. 8.9. Continuous Force-Displacement Plot from Test C2; 5 Cycles of 0 to 8 Kips

(35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%) ............... 134 Fig. 8.10. Comparison of Stiffness Under Large Amplitude Cyclic Loading; Loop 5

from Tests A3 (Drier Material) and C2 (Wetter Material) .................................... 135 Fig. 8.11. Comparison of Seismically Determined Stiffness with Plate-Load Stiffness

for Large Amplitude Cyclic Plate-Load Tests; Loop 5, Test A3 - 32 Cycles of 0 to 9 Kips (40.0 kN) in 20 Minutes on Drier Material

(Moisture Content ~ 4.7 %) ................................................................................... 137 Fig. 8.12. Comparison of Seismically Determined Stiffness with Plate-Load Stiffness

for Large Amplitude Cyclic Plate-Load Tests; Loop 5, Test C2 - 5 Cycles of 0 to 8 Kips (35.6 kN) in 9 Minutes on Wetter Material

(Moisture Content ~ 6.1%) .................................................................................... 138 Fig. 8.13. Closure of Cyclic Loops from Plate-Load Test A3; 32 Cycles of 0 to 9 Kips

on Drier Material (W~ 4.7 %)................................................................................ 140 Fig. 8.14. Closure of Cyclic Loops from Plate-Load Test C2; 5 Cycles of 0 to 8 Kips

on Wetter Material (W~ 6.1 %) ............................................................................. 140 Fig. 8.15. Closure of Cyclic Loops from Plate-Load Test A3 After Accumulated

Displacement from Application of Previous Loops; 32 Cycles of 0 to 9 Kips on Drier Material (W~ 4.7 %)................................................................................ 141

Fig. 8.16. Long-Term, Adjusted Load-Settlement Curve; Test (B1) on Drier Material

(W~ 4.7 %)............................................................................................................. 144 Fig. 8.17. Long-Term, Adjusted Load-Settlement Curve; Test (C1) on Wetter Material

(W~ 6.1 %)............................................................................................................. 144 Fig. 8.18. Illustration of the Effect of Duration of Loading on Unbound Granular Base

Course Material; Plate-Load Tests at Location C on Drier Material (Moisture Content ~ 4.7%) .................................................................................... 145

Fig. 8.19. Illustration of the Effect of Duration of Loading on Unbound Granular Base

Course Material; Plate-Load Tests at Location C on Wetter Material (Moisture Content ~ 6.1%) .................................................................................... 146

Fig. B.1. Test A3; Loop 1 of 32 - 32 Loading Cycles from 0 to 9 Kips on Drier

Material (W~ 4.7 %) .............................................................................................. 160

xvii


Material (W~ 4.7 %) .............................................................................................. 161 Fig. B.3. Test A3; Loop 3 of 32 - 32 Loading Cycles from 0 to 9 Kips on Drier












Material (W~ 4.7 %) .............................................................................................. 173 Fig. B.15. Test A3; Loop 15 of 32 - 32 Loading Cycles from 0to 9 Kips on Drier Material (W~ 4.7 %) .............................................................. 174 Fig. B.16. Test A3; Loop 16 of 32 - 32 Loading Cycles from 0 to 9 Kips on Drier

Material (W~ 4.7 %) .............................................................................................. 175

xviii

Fig. B.17. Test A3; Loop 17 of 32 - 32 Loading Cycles from 0 to 9 Kips on Drier Material (W~ 4.7 %) .............................................................................................. 176















Material (W~ 4.7 %) .............................................................................................. 190

xix

xx

Fig. B.32. Test A3; Loop 32 of 32 - 32 Loading Cycles from 0 to 9 Kips on Drier Material (W~ 4.7 %) .............................................................................................. 191

Fig. C.1. Test B1, Individual Unload-Reload Loops, Group Ab1 ......................................... 193 Fig. C.2. Test B1, Individual Unload-Reload Loops, Group Bb1.......................................... 194 Fig. C.3. Test B1, Individual Unload-Reload Loops, Group Cb1.......................................... 195 Fig. C.4. Test B1, Individual Unload-Reload Loops, Group Db1 ......................................... 196 Fig. D.1. Test C1, Individual Unload-Reload Loops, Group Ac1 ......................................... 199 Fig. D.2. Test C1, Individual Unload-Reload Loops, Group Bc1.......................................... 200 Fig. D.3. Test C1, Individual Unload-Reload Loops, Group Cc1.......................................... 201 Fig. D.4. Test C1, Individual Unload-Reload Loops, Group Dc1 ......................................... 202 Fig. D.5. Test C1, Individual Unload-Reload Loops, Group Ec1 .......................................... 203 Fig. E.1. Test C2; Loop 1 of 5 - 5 Loading Cycles from 0 to 8 Kips on Wetter

Material (W~ 6.1 %) .............................................................................................. 206 Fig. E.2. Test C2; Loop 2 of 5 - 5 Loading Cycles from 0 to 8 Kips on Wetter




Material (W~ 6.1 %) .............................................................................................. 210

CHAPTER ONE

INTRODUCTION

1.1 Increasing the Single-Lift Thickness of Compacted

Aggregate Base Courses

Unbound granular base course material is a common component of

layered pavement structures. According to a survey conducted by the

International Center for Aggregates Research (ICAR), most state departments

of transportation (DoTs) limit the thickness of aggregate base course which

may be compacted in a single lift. Typical limits on the lift thicknesses of

these bases are within the range of 6 to 8 in. (15 to 20 cm), with a few states

allowing somewhat thicker lifts with the design engineer’s approval (Bueno

et.al., 1998). Increasing the allowable lift thickness of granular base course

materials could make the product more cost effective and could improve

construction efficiency. A research project was, therefore, funded by ICAR

(No. ICAR-501) to determine the feasibility of compacting thicker granular

base course layers while achieving the same density as would be obtained by

compacting thinner lifts.

1

1.2 Characterizing Thick Aggregate Lifts

Unbound granular layers with thicknesses greater than 12 in. (30.5 cm)

are difficult to characterize with standard field tests. Field verification

methods typically include sand-cone density, balloon density, or nuclear

density tests. Sand-cone and balloon density tests sample only the near-

surface material, and the nuclear density gauge is restricted to depths of 12 in.

(30.5 cm) or less. Therefore, as part of the ICAR project, seismic testing was

studied to evaluate the feasibility of using seismically determined stiffness

profiles within the lifts to estimate the degree of compaction of the lifts.

Seismic testing was chosen, because it has some advantages over the

density tests mentioned above. Seismic tests are non-invasive and do not alter

the material during testing. Also, there is no depth limitation when using

certain seismic test procedures. Finally, seismic tests show increases or

decreases in stiffness which can not be detected by density testing and which

can affect the performance of the compacted layer under traffic loads.

To study further the characterization of thick lifts by in-situ seismic

and density measurements, plate-load tests were conducted in conjunction

with the seismic tests conducted for the ICAR project. The objective of the

study presented in this report is to determine the relationship between the

seismically determined stiffnesses of a test pad constructed with an unbound

aggregate base and the displacements induced in the material under working

load stresses.

2

1.3 Scope of Plate-Load and Seismic Testing

The ICAR project involved the construction and testing of test pads of

unbound aggregate base. Two test pads were constructed in Texas and three

test strips were constructed in Georgia. The two test pads in Texas were

constructed at the Capitol Aggregates quarry in Georgetown, Texas. Non-

invasive seismic testing using the Spectral Analysis of Surface Waves

(SASW) method (Stokoe et al. 1994) was conducted to determine shear

stiffness profiles of the material with depth. Comparison of the shear stiffness

profiles of thinner and thicker lifts was made to determine whether the same

stiffness properties could be achieved with compaction of thin and thick lifts.

A complete report of the seismic stiffness study is given in Bueno et al.

(1998).

In the study presented herein, static plate-load tests were conducted on

one of the Georgetown test pads to determine how small-strain shear

stiffnesses from seismic testing are related to displacements under working

loads. A unique piece of equipment developed at The University of Texas at

Austin called the Rolling Dynamic Deflectometer (RDD) was used to conduct

the plate-load tests (Bay 1997). The RDD was used in a stationary mode in

this application.

Low-frequency cyclic loading (0.01 Hz) was applied to the plate to

determine axial stiffnesses under transient working loads. Observations were

made of working-load displacements under a variety of load ranges for two

different moisture conditions of the aggregate base. These observations were

3

coupled with small-strain stiffness measurements to evaluate the potential for

predicting working load displacements with seismic testing. This

combination, if successful, would allow seismic measurements to be used to

evaluate the degree of compaction and also to estimate deformations in the

base.

1.4 Objectives of This Study

The primary goal of this study was to evaluate how well working-load

displacements could be predicted using small-strain seismic moduli. Small-

strain moduli were adjusted for the effects of strain amplitude and state of

stress and then used to predict displacements.

A secondary goal of static plate-load testing was to observe material

stiffness under varying states of stress. Cycles of unloading and reloading

were applied to the loading plate, and an equivalent spring constant or

stiffness was observed from the deformational behavior of the material.

These stiffness measurements (equivalent spring constants) were then

compared to those measured with seismic tests in order to quantify the effects

of changing states of stress in granular material.

Lastly, static plate-load tests and seismic tests were used to show how

the material stiffness was affected by increasing the in-situ moisture content

of the material. Nearly identical tests were performed before and after wetting

the test pad. The change in stiffness was observed and quantified in both the

static plate-load tests and seismic tests.

4

1.5 Correlations Between Plate-Load Settlement and Other

Field Tests

Static plate-load tests have been used regularly to evaluate roadbed

material stiffness under pressures in the range of those exerted by working

loads. Previous attempts have been made to predict footing settlement in

granular materials using in-situ test procedures. Terzaghi and Peck (1948)

introduced a procedure for estimating settlement under a footing on granular

material using standard penetration test (SPT) values. Schmertmann (1970)

introduced a method for predicting settlement on granular material using cone

penetration test (CPT) measurements. Recently, seismic testing has started to

receive more attention for use in predicting settlement. For instance, Ahtchi-

Ali and Santamarina (1994) investigated the use of cross-hole and downhole

seismic testing for the prediction of settlement under building foundations on

granular material.

Extension of seismic testing techniques to the loading conditions

applied to pavements has the potential to improve the design, construction,

and performance of pavement systems. Plate-load tests were conducted in an

effort to increase understanding of the relationship between in-situ seismic

stiffness measurements and working-load displacements in pavement layers.

5

1.6 Organization of This Report

This study begins with a description of the construction of the test

pads in Chapter 2 although only one test pad was used for plate-load testing.

An explanation of the equipment used for plate-load testing is presented in

Chapter 3. Seven plate-load tests were performed on one test pad. The

individual test procedures are described in detail in Chapters 4 through 6.

Background information about the seismic tests is summarized in Chapter 7.

The majority of the analysis of the plate-load and seismic test results is

presented in Chapter 8. The relative stiffnesses measured during seismic

testing and plate-load testing are compared and discussed in this chapter.

Detailed discussion of the strain amplitude for both the seismic and plate-load

tests as well as the influence of the state of stress is also included. The

observations and conclusions made from static plate-load tests and seismic

tests are summarized in Chapter 9.

6

CHAPTER TWO

MATERIAL AND TEST SITE

2.1 Construction of Test Pad

A test pad 40 ft (12 m) long by 35 ft (11 m) wide was constructed at the

Capitol Aggregates Quarry in Georgetown, Texas. Crushed limestone base

material with a nominal maximum particle size of 1.5 in. (37.5 mm) was delivered

from the quarry stockpile to the test pad location. The material was dumped from

the belly-dump tractor-trailer shown in Fig. 2.1 and spread with a motor grader as

shown in Fig 2.2. The test pad was divided lengthwise into two equal areas. One

area was compacted with an Ingersoll Rand SD-100F vibratory pad-foot roller

(Fig. 2.3) while the other area was compacted with an Ingersoll Rand SD-100D

vibratory smooth-drum roller (Fig 2.4). Both rollers had a static drum weight of

13,200 lb (58.7 kN) and delivered 52,500 lb (233.5 kN) peak to peak dynamic

force.

The test pad was constructed in two lifts. The first lift was 12 in. (31 cm)

thick. The second lift had an average thickness of 23 in. (58 cm) on the side of

the pad compacted with the pad-foot roller which is the area discussed in this

report. Details of the compaction procedures and material densities throughout

the pad are given by Bueno et al. (1998). A cross section of the test pad is shown

at Fig. 2.5.

7

The pad was constructed prior to the spring of 1998. Unfortunately, part

of the test pad was destroyed before plate-load testing could begin. As a result,

plate-load tests were conducted only on the side of the test pad compacted by the

pad-foot roller.

Fig. 2.1. Unbound Base Material Being Delivered to the Test Pad Site

8

Fig. 2.2. Spreading Unbound Base Material Before Compaction of the Test Pad

Fig. 2.3. Vibratory Pad-Foot Roller Used to Compact Unbound Base Course Material Over One-Half of the Ttest Pad

9

Fig. 2.4. Vibratory Smooth-Drum Roller Used to Compact Unbound Base Course Material Over One-Half of the Test Pad

23 in. (58 cm)

12 in. (30 cm)

~ 40o

Fig. 2.5. Cross Section of Unbound Aggregate Base Course Test Pad Constructed in Georgetown, Texas

10

2.2 Location of Plate-Load Tests

Three locations were selected for plate-load testing. These locations are

labeled A, B, and C in Fig. 2.6. The geometry of the test equipment dictated that

tests be conducted in the center of the pad-foot lane. This was a convenient

location in that it allowed avoidance of the edge of the pad. Details of the test pad

and the plate-load test locations are shown in Fig. 2.6.

.

C B A

40 ft. (12 m)

NCompactedWith Pad-FootRoller

Effective Plate-LoadTesting Area

5 ft. 5 ft.(1.5 m) (1.5 m)

35 ft

. (11

m)

~ 3 ft(0.9 m)

~ 20 ft (6.1 m)

Fig. 2.6. Plan View of Test Pad and Plate-Load Test Locations

11

2.3 Flexible Base Material

The material used to construct the test pad was produced by Capitol

Aggregates for sale as road base material. This product meets TxDOT

specifications for Grade 2 Flexible Base material. The stockpile material had

moisture content near optimum (6.5 %) and required no added moisture before

compaction. Gradation of the stockpile material showed 12 % passing the #40

(0.475 mm) sieve and 7.1 % passing the # 200 sieve. The fines were non-plastic.

A summary of the index properties is presented in Table 2.1. The gradation curve

is shown in Fig. 2.7. The gradation data are also presented in Table 2.2.

Table 2.1. Index properties of Grade 2 crushed limestone base material from the Georgetown, Texas Capitol Aggregates Quarry Liquid Limit Non-Plastic Plastic Limit Non-Plastic Optimum Moisture Content 6.5 % AASHTO T180 (modified Proctor) Max. Density 141 pcf (2263 kg/m3)

12

100

80

60

40

20

0 890.1

234567891

2345678910

2345

Grain Size, mm

#200#60#20#10#4

U.S. Standard Sieve Number

#40 #1400.317in.

0.75 in.2.0in.

1.5in.1.0in.

Fig. 2.7. Gradation Curve for Capitol Aggregates Crushed Limestone Base Course

13

Table 2.2. Gradation of Grade 2 crushed limestone base material from the

Georgetown, Texas Capitol Aggregates Quarry

Sieve Size % Passing mm

2.0 in. 50.000 100 1.5 in. 37.500 98.8 1.0 in. 25.000 85.1 0.75 in. 19.000 73.0 0.317 in. 9.275 51.5

#4 4.750 35.0 #10 2.000 23.2 #20 0.850 15.3 #40 0.425 12.0 #60 0.250 10.1 #140 0.106 7.9 #200 0.075 7.1

2.4 In-Situ Density Testing

A Troxler nuclear density gauge (NDG) was used to estimate total in-

situ density and moisture content before plate-load tests were performed at

locations A and C (Fig. 2.6). No moisture was added to the flexible base test

pad before tests A and B, and no significant weather changes took place

during the 25 days between the two sets of tests. The NDG data for location

A can, therefore, be considered representative of the material at location B.

Moisture was added, however, to the material before plate-load and seismic

tests were performed at location C as described in Chapter 3. Density

14

measurements were made every 2 in. (5 cm) to a depth of 12 in. (31 cm).

Moisture content specimens were collected at the conclusion of the plate-load

tests and used to correct the NDG data. Relatively large moisture content

specimens were collected to provide an accurate representation of the

moisture of the base course. The moisture content specimens weighed

between 6.2 and 7.3 lb (2.8 kg and 3.3 kg). The difference between the

laboratory moisture content and that measured at a depth of 2 in. (5 cm) in the

field was subtracted from NDG readings at every depth. Average density and

moisture content data for tests at locations A and B are presented in Table 2.3.

The same information for location C after wetting is given in Table 2.4.

Complete records of the nuclear density data are given in Appendix A.

15

16

Table 2.3. Density and moisture content data at plate-load test locations A and B (drier material)

Depth Wet Density Corrected Moisture Content 1

Dry Density

(in.) (lb./ft3) (%) (lb./ft3) Back Scatter 126 4.7 120

2 124 4.9 119 4 133 4.5 128 6 138 4.5 132 8 141 4.5 135 10 143 4.3 137 12 142 4.3 136

1 NDG moisture content data were corrected by subtracting a correction factor from every NDG reading. The correction factor was determined by taking the difference between the surface moisture content determined by the NDG and the surface moisture content determined in the laboratory. (Correction for drier material: w NDG - wLAB = 0.8)

Table 2.4. Density and moisture content data at plate-load test location C (wetter material)

Depth Wet Density Corrected Moisture Content

2

Dry Density

(in.) (lb./ft3) (%) (lb./ft3) Back Scatter 134 6.3 126

2 134 6.3 126 4 140 6.1 132 6 144 5.7 136 8 147 5.9 139 10 147 5.7 139 12 147 5.5 139

2 NDG moisture content data were corrected by subtracting a correction factor from every NDG reading. The correction factor was determined by taking the difference between the surface moisture content determined by the NDG and the surface moisture content determined in the laboratory. (Correction for wetter material: w NDG - wLAB = 1.7.

CHAPTER THREE

PLATE-LOAD TESTING WITH THE ROLLING DYNAMIC

DEFLECTOMETER

3.1 Introduction

Plate-load testing was conducted by adapting the Rolling Dynamic

Deflectometer (RDD) to this static application. The RDD is a vibroseis truck

modified to apply continuous rolling dynamic loads to a pavement surface while

deflections are measured via rolling sensors. The equipment was developed by

Bay (1997) at The University of Texas at Austin and is shown in static

configuration in Fig. 3.1. Static plate load tests were facilitated by the addition of

a 100-kip (445 kN) static load cell in the center of the truck’s hydraulic loading

frame. With these modifications, static load and displacement data could be

continually monitored using the computerized data acquisition system of the

RDD. Electronic data collection provided precise measurement of displacement

over short periods of time which would not have been possible with dial gauges.

This unique arrangement allowed incremental static loading as well as short-

duration cyclic loading and unloading tests to be performed during plate loading.

All of this equipment is discussed below.

17

Fig. 3.1. Rolling Dynamic Deflectometer Configured for Static Plate-Load Testing

3.2 Plate-Load Testing Equipment

3.2.1 Loading Mechanism

The load was applied to the plate through the hydraulic loading

mechanism on the RDD. The pressure in the hydraulic cylinders which control

the load can be increased or decreased from controls inside the cab of the truck.

Complete details of the loading system are given in Bay (1997).

To mount the static load cell under the truck, additional structural supports

had to be designed, machined, and installed on the truck’s loading frame.

Structural sections were machined from 6-in. (15.2 cm) wide by 2-in. (5.1 cm)

18

thick solid steel bar stock to minimize compliance of the loading system. The

structural members were welded and bolted to the loading frame.

Due to the geometry of the truck’s loading mechanism and chassis

configuration, the rear wheels of the truck were within 5-ft (1.5 m) of the edge of

the plate. The locations of the truck tires and loading plate are diagrammed in

plan view in Fig 3.2 to show their relative positions. Standard test methods for

repetitive and non-repetitive plate-load tests published by the American

Association of State Highway and Transportation Officials (AASHTO, 1986)

specify a minimum clearance of 8-ft (2.4 m) between the reaction points for the

load mechanism and the edge of the plate. This was not possible without

significant modifications to the truck. To limit the effects of the truck’s rear

wheels on the material being sampled in the plate-load test, the plate size was

limited to 1-ft (30.5 cm.). Because the sampling depth is related to the diameter

of the plate, a larger plate would sample to a greater depth with greater potential

for influence from the truck’s wheels. Future modifications to the truck may

include the addition of hydraulic jacks mounted on the front and rear bumpers

which would lift the truck and move the reaction points well outside the

deflection basin produced during plate-load testing. This would allow the use of

larger plates and facilitate sampling deeper material.

19

76 in. (193 cm)

Tire Contact Area[Approx 12 in. by 14 in.(30.5 cm by 35.6 cm)]

187

in. (

475

cm)

72 in. (183 cm)

Load Plate12 in. (30.5 cm)

54 in

. (13

7 cm

)

53 in

. (13

5 cm

)

56 in

. (14

2 cm

)

Truck Tires

Fig. 3.2. Plan View of the RDD Showing the Locations of the Truck Tires Relative to the Location of the Plate-Load Tests

20

The advantage of the hydraulic loading system on the RDD is that cycles

of loading and unloading can be easily applied at pre-selected rates to the load

plate. Such cyclic loading more closely simulates the loads induced by repeated

traffic. In addition, small cycles of unloading and reloading can be superimposed

over a static force on the plate. It was sometimes difficult, however, to carefully

apply a small increment of load. The loading mechanism occasionally exhibited a

“stick-slip” behavior which produced a jump of 1000 to 2000 lb (4.45 to 8.90 kN)

when a small adjustment was applied to the hydraulic pressure.

3.2.2 Load-Plate Details

A circular aluminum plate 1-ft (30.5 cm) in diameter and 1.5-in. (3.8 cm)

thick was used for the plate-load tests. For each test, the plate was seated in a thin

layer of hydrostone, which is a mixture of plaster of Paris, Portland cement, and

lime. The hydrostone was allowed to cure for about one hour before application

of the load. For tests at locations A and B, a 6-in. (15.2 cm) diameter plate was

stacked on top of the 12-in. (30.5 cm) diameter plate as shown in Fig 3.3. Tests at

location C were conducted with the 12-in. (30.5 cm) diameter plate alone due to

clearance limitations beneath the load cell. Because the test pad was ramped at

each end, the clearance below the load cell decreased as the front wheels of the

RDD truck began to descend the ramp. This did not happen when the back

wheels were near the ramp during testing at location A. It is shown in Fig. 3.2

that the load plate is closer to the back wheels of the RDD. The truck, therefore,

took less space on the pad behind the loading plate than in front of the loading

plate.

21

Fig. 3.3. Plate Setup Showing 6 in. (15.2 cm) Diameter Plate Stacked on Top of the 12 in. (30.5 cm) Diameter Plate

3.2.3 Load Cell

A Beowulf model 330 load cell was bolted to the structural cross brace on

the truck’s loading mechanism. An excitation voltage of 5.00 volts DC was

applied to the load cell with a Lambda LL-902-OV regulated power supply. The

output signal was amplified with a Neff model 128 DC signal amplifier with a

gain setting of 1000. The load cell was calibrated in the laboratory and verified to

have a calibration factor of 2-mV/volt excitation/100 kips.

22

3.2.4 Displacement Transducers

Displacement of the plate was measured using three Direct Current Linear

Variable Differential Transformers (DC-LVDTs) placed at third points around the

circumference of the plate. Trans-Tek 0243-0000 DC-LVDTs were used for

these tests. The sensors had a maximum range of +/- 0.750 in. (+/- 19.1 mm)

from the center or neutral position with a working range of +/- 0.5 in. (+/- 12.7

mm) from the center or neutral position.

DC-LVDTs are excited with a DC voltage and output a DC voltage which

varies with the position of an iron core traveling freely inside the sensor’s

magnetic field. A 5.00-volt excitation signal was applied to the sensors with a

Lambda LL-902-OV regulated power supply. Calibration factors for each sensor

were determined in the laboratory using a 5.00-volt excitation signal. The sensors

were operated in the linear range, and the calibration factors are given in Table

3.1. A complete discussion of the application of calibration factors and other

details of the data reduction is given in Section 3.3.

Table 3.1. Direct Current Linear Variable Differential Transformer (DC-LVDT) Calibration Factors

Sensor Calibration Factor (volts/in.) # 1 5.081 # 2 5.136 # 3 5.089

23

3.2.5 Support Frame for DC-LVDTs

For small plate deflections to be measured precisely, the supports for the

DC-LVDT sensors had to be placed outside the deflection basin. In addition, the

instrumentation system had to provide clearance for the truck’s loading deck to

move freely throughout the test. This combination of constraints posed some

unique challenges in designing the support frame for the DC-LVDTs.

A framework of rectangular aluminum tubing and round aluminum bars

was used to hold the sensors in place during the test. A photograph of the test

frame erected under the truck is shown in Fig. 3.4. The frame consisted of two,

21-ft (6.4 m) lengths of 1-in. by 2-in. (25 mm by 51 mm) aluminum tubing

supported by photographic tripods. The tripods were chosen to support the

framework because the terrain was uneven, and the elevation around the test site

varied as much as 3 ft (0.9 m). The supports on one side of the test site were

required to be over 5 ft (1.5 m) tall, while the supports on the other side of the test

site needed only be 2 ft (0.6 m) tall. The tripods also offered flexibility for future

configurations of the measurement system.

24

Fig. 3.4. Measurement Frame Assembled for Plate-Load Testing

The camera attachment head was removed from the top of the tripods, and

replaced with an aluminum block machined to hold the aluminum tubing

(Fig. 3.5). Round aluminum bars, 5/8 in. (16 mm) in diameter were attached to

the tubing with machined blocks designed to provide flexibility in adjusting the

framework in the field (Fig 3.6). Laboratory apparatus clamps were used to

connect two aluminum bars at a 90-degree angle (Fig. 3.7). The DC-LVDTs were

attached to the round bars with mounting blocks as shown in Fig. 3.8. Hex-head

set screws were used to hold the components in place. All of the components of

the measurement frame were designed to be flexible and adjustable so that

changes to the configuration could be made easily in the field. This feature was

exploited numerous times during testing.

25

Fig. 3.5. Aluminum Adapter Block Attached to the Top of a Tripod

Fig. 3.6. Connection Between Rectangular Tubing and Round Bars [6-in. (15-cm) Ruler in Foreground]

26

Fig. 3.7. Connection of Round Bars [6-in. (15-cm) Ruler in Foreground]

Fig. 3.8. DC-LVDT Mounting Block with the Capability of Connecting In-Line With the Round Bar or at a 90-Degree Angle to the Round Bar

27

Unfortunately, it was uncovered during testing that the instrument frame

should have been stiffer in order to prevent vibrations from strong winds

experienced at the quarry. Noticeable vibration noise in the deflection data was

observed during periods of high wind. Plywood and corrugated metal wind

breaks were installed to reduce the effects of wind on the measurement system.

This proved very effective in reducing noise in the displacement data; however

stiffer cross braces would have decreased vibration noise even further.

3.2.6 Data Acquisition System

A computerized data acquisition system was employed to sample output

voltages from the load cell and DC-LVDTs. The computer and other data

acquisition hardware were mounted in the cab of the RDD. A Power Macintosh

7500/100 personal computer with a National Instruments data acquisition card

was coupled with a National Instruments Signal Conditioning extensions for

Instrumentation (SCXI) chassis to collect the data. National Instruments

LabVIEW software was used to sample the four data channels continuously at a

frequency of 32 Hz. Plots of the output voltages from the load cell and

displacement sensors were displayed in real time on the computer screen as the

test was running. This was particularly useful for selecting load and time

increments based on material response under the initial loading. The hardware

configuration is detailed in Fig. 3.9, and a photograph of the equipment installed

in the RDD is shown at Fig. 3.10.

28

LabVIEW DataAcquisition Software

SCXI Chassis

Iomega zip

Power Macintosh 7500/100

3 Channelsto DC-LVDTSensors

Neff Model 128InstrumentationAmplifier

1 Channelto Load Cell

Fig. 3.9. Schematic of Data Acquisition Hardware

Fig. 3.10. Data Acquisition System Installed in the Cab of the RDD

29

3.3 Data Reduction

Output voltages from the load cell and DC-LVDTs were recorded in

multiplex format during testing. Each channel was sampled at 32 Hz, and the

four voltages were recorded in a group of four binary numbers for each time

increment. To reduce the data, the binary stream was de-multiplexed into its four

channel components for each plate-load test.

Calibration factors were applied to the load cell data and to each

displacement transducer separately. The data were then zeroed based on the

initial offset load in the load cell and starting average displacement in the

displacement transducers.

For each test, the data were averaged over 31 points (1 second) to reduce

the effects of noise due to vibration in the measurement system. The force in the

load cell and the average displacement in the three sensors were plotted with

respect to time on separate plots. A continuous plot of displacement vs. load was

then plotted for each test.

3.4 Overview of Plate-Load Testing

Six tests were performed at three test locations on the base-course test

pad. The locations are labeled A, B, and C for reference (Fig. 2.6). Tests at a

given location were run consecutively, meaning for example, Test A2 was begun

after the completion of Test A1 without moving the plate or the truck. The suite

of tests included: (1) loading in the range of expected wheel loads in a pavement

structure; (2) loading well above the expected wheel loads in a pavement

structure; (3) traditional incremental loading over a long period of time (duration

30

of load ~ 2 hr); (4) cyclic loading over short periods of time (duration of load ~ 1

min.)

3.4.1 Modulus of Soil Reaction, ku’

The modulus of soil reaction, ku’ is used in rigid pavement design and is

defined by AASHTO T222 (1986) according to the following expression:

ku’ = 10 psi / (average deflection at 10 psi) ......................... (3.1)

The units of ku’ are lb/in.2/in. The value as obtained by plate load tests is

normally corrected for seasonal variation, composite effect due to layering, depth

to bedrock, and loss of sub-base support due to pumping. Typical values for the

composite k range from 50 lb/in.2/in. to 2000 lb/in.2/in. (13.6 kPa/mm to 543

kPa/mm) according to the AASHTO Guide for Design of Pavement Structures

(AASHTO, 1993).

The modulus of soil reaction has been linearly correlated to resilient

modulus, MR according to AASHTO (1993) as shown below for reference.

k = MR / 19.4 ........................................................ (3.2)

Equation 3.2 is not dimensionally correct, so care must be taken in using it. The

units of MR in equation 3.2 are lb/in.2.

31

3.4.2 Material Equivalent Spring Constant (Stiffness)

Material stiffness in the following discussion is defined in terms of an

equivalent spring constant keff. The spring constant is expressed as the amount of

displacement experienced under any given load. The equivalent stiffness was

determined at various points of interest in the plate load tests and is expressed in

units of lb/in. The stiffness was typically determined using one of two methods:

(1) taking the slope of a linear curve fit to a section of the loading or unloading

curve; or (2) taking the slope of the tangent of a polynomial curve fit to a section

of the curve.

3.4.3 Summary of Plate-Load Tests Performed

Tests at location A were performed to determine the capabilities of the

loading mechanism and measurement system as well as to determine the general

characteristics of the material. No moisture was added to the material before

testing, and these tests are referred to as “drier” tests (w ~ 4.7 %). The following

four tests were conducted at this location: (1) loading to a maximum force of 5

kips (22.2 kN) using traditional incremental loading procedures and performing

one cycle of unloading-reloading midway through the loading cycle; (2) quick

loading to 11 kips (48.9 kN) and quick unloading; (3) 32 cycles of loading-

unloading of 0 to 9 kips (40.0 kN); (4) quick loading to 40 kips (178 kN)

followed by quick unloading back to zero load.

One test was performed at location B. No moisture was added to the test

pad before testing, and these tests are also referred to as the “drier” tests (moisture

content ~ 4.7 %). Traditional incremental loading to 8.3 kips (36.9 kN) was

32

applied to the plate, and four unloading-reloading loops were applied at various

points during the test.

Two plate-load tests were performed at location C. Moisture was

periodically added to the surface of the test pad for two days prior to the plate-

load tests. The tests at location C are referred to the “wetter” tests (moisture

content ~ 6.1 %). The first test at location C was similar to the test at location B

except for the fact that the material was wetted before the test, and the maximum

load was approximately 8.6 kips (38.3 kN) as opposed to 8.3 kips (36.9 kN). The

second test at location C was similar to third test at location A where 32 cycles of

a 9-kip (40.0 kN) load were applied, except that the material was wetted and only

5 cycles of an 8-kip (35.6 kN) load were applied. The tests are described in detail

in Chapters 4-6.

33

CHAPTER FOUR

PRELIMINARY PLATE-LOAD TESTS AT

LOCATION A ON THE TEST PAD

4.1 Introduction

To initiate the field study and to learn about plate-load testing with the

Rolling Dynamic Deflectometer (RDD), four preliminary tests were performed at

location A on the Georgetown test pad (Figure 2.6). The plate-load tests were

performed on the top of the second lift for a total thickness of aggregate base

under the plate of 35 in. (89 cm). These were the first tests performed with the

RDD on the test pad, so they were used to evaluate the loading capabilities of the

RDD as well as the load-displacement behavior of the test pad under a variety of

loading conditions. No moisture was added to the surface of the test pad before

the plate-load tests were conducted. The surface moisture content was determined

to be 4.7%. These initial plate-load tests are discussed in detail below.

It should be noted that the effective plate area for tests at location A was

107 in.2 (690 cm2), as opposed to 113 in.2 (729 cm2) as would be calculated for a

12 in. (30.5 cm) diameter plate. The reason for this reduction in effective area is

that the hydrostone used to seat the loading plate did not flow completely to the

edge of the plate during seating. Because the loading plate was placed after the

truck was in position, the clearance did not allow placement in such a way that the

hydrostone would flow out around the entire circumference of the plate. This

35

problem was corrected in subsequent tests by seating the plate in hydrostone

before positioning the truck over the test location.

4.2 Test A1 - Incremental Plate-Load Test from 0 to 5 kips (22.2

kN)

The first plate-load test performed on the test pad was used to evaluate the

approximate magnitude of displacement under a typical working load and to

determine the capabilities of the loading system and data acquisition system. An

estimate of the typical range of working loads was made using a chart showing

the interface stresses in a two-layer pavement system presented by Huang (1969).

The estimates are presented in Figure 4.1. The typical working load was taken as

that stress level which would be applied by an 18 kip (80 kN) equivalent single

axle load (ESAL) on a 3 to 4 in. (7.6 to 10.2 cm) asphalt concrete pavement over

compacted aggregate base. This level was estimated to be in the range of 5 kips

(22.2 kN) and represented an average vertical stress under the loading plate of 47

psi (324 kPa).

36

This test is not discussed in detail because of its preliminary nature.

However, it was observed that the load plate displaced approximately 48 mils

(1.22 mm) vertically under the 5 kip (22.2 kN) load. A permanent (un-recovered)

displacement of 32 mils (0.81 mm) was also recorded. The variations in force and

displacement with time are shown in Figs. 4.2 and 4.3, respectively. The

continuous load-settlement curve is shown in Figure 4.4.

37

2.5 in. (6.4 cm) ACC 4 in. (10.2 cm) ACC6 in. (15.2 cm) ACC

8 in. (20.3 cm) Aggregate Base

10 in. (25.4 cm) Aggregate Base

12 in. 30.5 cm) Aggregate Base

Material Properties

Asphalt Cement Concrete (ACC), E1 = 500 ksi (3.4 X 106 kN/m2)

Aggregate Base, E2 = 33 ksi (2.3 x 105 kN/m2)

σc

σcσc

σc = 56 psi (386 kPa)

σc = 21 psi (145 kPa)

σc = 36 psi (248 kPa)

9000 lb (40.0 kN) 9000 lb (40.0 kN)9000 lb (40.0 kN)6 in.(15.3 cm)

6 in.(15.3 cm)

6 in.(15.3 cm)

Fig. 4.1. Estimates of Vertical Stress Delivered to the Top of a Pavement Base Course Beneath the Center of a 12 in. (30.5 cm) Diameter Circular Plate Under a 9000 lb (40.0 kN) Load

7

6

5

4

3

2

1

0

30

25

20

15

10

5

0140120100806040200

Elapsed Time (minutes)

Plate Diameter: 12 in. (305 mm)Effective Plate Area: 107 in.2 (690 cm2)

Fig. 4.2. Variation of Force with Time; Test A1 - Incremental Pate-Load Test from 0 to 5 kips (22.2 kN)

70

60

50

40

30

20

10

0

1.5

1.0

0.5

0.0

140120100806040200Elapsed Time (minutes)


Fig. 4.3. Variation of Displacement with Time; Test A1 - Incremental Plate-Load Test from 0 to 5 kips (22.2 kN)

70

60

50

40

30

20

10

06420

Force (kips)

1.5

1.0

0.5

0.0

302520151050Force (kN)


Fig. 4.4. Continuous Load-Settlement Curve from Test A1 - Incremental Plate-Load Test from 0 to 5 kips (22.2 kN)

4.3 Test A2 - Loading from 0 to 11 kips (48.9 kN) in 36 Seconds

Followed by Complete Unloading

The second test at location A on the Georgetown test pad was performed

to determine the effect of the duration of loading on the response of the unbound

aggregate base. A load of 11 kips (48.9 kN) was applied to the loading plate in 36

seconds. This load was maintained on the loading plate for 10 minutes. The load

was then removed in 4 seconds and the material was allowed to rebound for 11

minutes. The displacement under the 11 kip (48.9 kN) load was 42 mils (1.07

mm) after the 36-second load application. The maximum displacement under the

11 kip (48.9 kN) load increased to 60 mils (1.52 mm) after maintaining the load

for 10 minutes, showing significant creep in the unbound base material.

The variations of force and displacement with time for Test A2 are shown

in Figure 4.5 and 4.6, respectively. A decreasing rate of displacement with time

can be observed from the displacement-time record shown in Figure 4.5. The

average rate of displacement during the final 30 seconds of the loading was

observed to be nearly zero. In fact, the material was rebounding slightly due to the

fact that the load was gradually decreasing. The results of Test A2 allowed the

selection of appropriate load duration for future tests. Load duration of 15

minutes was selected in many of the subsequent tests to ensure that the rate of

displacement at the end point was nearly zero.

38

12

10

8

6

4

2

0

2520151050

50

40

30

20

10

02520151050



Fig. 4.5. Variation of Force with Time; Test A2 - Loading from 0 to 11 kips (48.9 kN) in 36 Seconds Followed by Complete Unloading

60

50

40

30

20

10

0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0



Fig. 4.6. Variation of Displacement with Time; Test A2 - Loading from 0 to 11 kips (48.9 kN) in 36 Seconds Followed by Complete Unloading

The permanent (un-recovered) displacement was measured 11 minutes

after the load was removed and found to be 38 mils (0.97 mm). The continuous

load-settlement curve for Test A2 is given in Figure 4.7.

4.4 Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes

Cyclic loading was applied to the test pad in Test A3 to better simulate the

loading conditions induced by traffic. Thirty-two cycles of loading and unloading

with a maximum load of approximately 9 kips (40.0 kN) were applied to the

plate. The average duration of each cycle was 1.5 minutes. Time plots of force

and displacement are shown in Figs. 4.8 and 4.9, respectively. The continuous

load-settlement curve showing all 32 cycles is given in Figure 4.10. It can be seen

that there is a general increase in permanent deformation with increasing number

of cycles. The deformation became approximately constant, and the cyclic

unload-reload loops were observed to “close” or plot on top of one another after 7

or 8 cycles. The displacement then increased for a few cycles, and the loops

began to close again after about 15 cycles. The displacement started increasing

again when the load was accidentally increased from 9 kips (40 kN) to 11 kips (49

kN) for cycle number 25.

39

60

50

40

30

20

10

0121086420

Force (kips)

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

50403020100Force (kN)


Time-DependentDisplacementUnder a SustainedConstant Load

Fig. 4.7. Continuous Load-Settlement Curve from Test A2 - Loading from 0 to 11 kips (48.9 kN) in 36 Seconds and Unloading to Zero

12

10

8

6

4

2

0

50

40

30

20

10

03020100



Fig. 4.8. Variation of Force with Time; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

30

25

20

15

10

5

0

0.8

0.6

0.4

0.2

0.0



Fig. 4.9. Variation of Displacement with Time; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

30

25

20

15

10

5

0121086420

Force (kips)

0.8

0.6

0.4

0.2

0.0

50403020100Force (kN)


Loop 25

Loop 30

Fig. 4.10. Continuous Cyclic Load-Settlement Curve; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

4.4.1 Determining Stiffness During Loading

The material equivalent spring constant, keff, is defined as the force

divided by the displacement at a given point in the loading cycle as discussed in

Section 3.4.2. The keff, is the inverse of the slope of the load-settlement curve at a

given point during loading or unloading. This equivalent spring constant is

referred to as the “stiffness” of the material.

Each of the 32 load cycles was plotted separately, and the stiffnesses of

the material at various points along the load-displacement plots were determined.

The slope of the loading curves was observed to be approximately bi-linear for

most loops as illustrated conceptually in Figure 4.11. The two stiffnesses were

determined by performing a linear curve fit to the data above and below the

observed break as shown by the dashed lines in Figure 4.11. Because loop 25 far

exceeded the average value of the maximum load in the preceding cycles, the

stiffness values for only the first 24 loops were recorded. The stiffness values for

the two sections of loading for curves 1-24 are shown in Figure 4.12.

40

Force

k eff-initial loading

k eff-final loading

Figure 4.11. Illustration of Bi-Linear Stiffness Behavior Upon Loading; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes

41

It can be observed in Figure 4.12 that the initial stiffness increased

approximately 25 % over 24 loading cycles. The stiffness during the second

portion of the loading cycle increased by 50% suddenly, after 12 cycles, and

remained relatively constant for the remaining 12 cycles. It is not clear why the

material became suddenly stiffer. No significant load spike was experienced

before cycle 25, and no unusual disturbance was observed during testing. The

individual loading loops annotated with the bi-linear curve fits are included in

Appendix B.

4.4.2 Determining Stiffness During Unloading

The stiffness of the test pad material during unloading was also evaluated

and recorded at three points on the unloading curve for each large-amplitude

unload-reload loop as illustrated conceptually in Figure 4.13. The rebound curves

were divided into thirds, and a polynomial curve-fit was performed on each third

of each loop. The derivative of each polynomial was evaluated at the center of

each segment to determine the slope of the tangent to the curve at the center. The

stiffness values for the three load ranges on the unloading loops are shown in

individual plots in Figure 4.14 and together in Figure 4.15.

It should be noted that the slope of the rebound curve is very slightly

negative at the beginning of rebound. This would suggest that the displacement

continued to increase after the load was released. This is counter-intuitive;

however, no investigation was made in this study to attempt to explain this

behavior.

42

1.6x106

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.024222018161412108642

Cycle Number

0.250x106

0.200

0.150

0.100

0.050

0.000

Stiffness During Initial Loading Stiffness During Final Loading

Fig. 4.12. Bi-Linear Stiffnesses of Each Cyclic Loop in the Loading Section; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

Force

k eff-beginning of rebound

k eff-middle of rebound

k eff-end of rebound

Figure 4.13. Illustration of Stiffness Behavior Upon Unloading; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes

The stiffnesses measured at the beginning of rebound were greater than

the stiffnesses measured during loading or at any other point on the rebound

curve. It was observed that the stiffnesses measured during unloading showed

significantly more scatter than the stiffnesses measured during loading.

43

-12x106

-8

-4

0

2015105Cycle Number

-60x106

-40

-20

0

stiffness at beginning of rebound

0.10x106 0.080.060.040.020.00

2015105Cycle Number

0.60x106 0.500.400.300.200.100.00

stiffness at end of rebound

2.5x106 2.01.51.00.50.0

2015105Cycle Number

16x106

12

8

4

0

stiffness at middle of rebound

Figure 4.14. Stiffnesses of Individual Loops During Rebound; Test A3 - 32 Loading Cycles from 0 to 9 kips on Drier Material (w ~ 4.7 %)

44

-60x106

-40

-20

0

242220181614121086420Cycle Number

-12x106

-10

-8

-6

-4

-2

0

2

stiffness at beginning of rebound stiffness at middle of rebound stiffnes at end of rebound

Fig. 4.15. Variation of Force with Time; Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

4.4.3 Measurement of Permanent Displacement

After having undergone 70 mils (1.78 mm) of permanent displacement

from Tests A1 and A2, the plate displaced a maximum of 30 mils (0.76 mm) as a

result of the 32 cycles of approximately 9 kips (40.0 kN) each. Of those 30 mils

(0.76 mm), the additional permanent displacement after test A3 was

approximately 15 mils (0.38 mm). This resulted in a total permanent displacement

of 85 mils (2.16 mm) for the three plate-load tests (A1, A2, and A3).

4.5 Test A4 - Loading from 0 to 40 kips (178 kN) in 6 Minutes

Followed by Complete Unloading

The last test at location A involved loading to 40 kips (178 kN) followed

by complete unloading. The 40-kip (178 kN) load was applied in approximately 6

minutes and held for 2 minutes. The force versus time and displacement versus

time plots are shown in Figs. 4.16 and 4.17, respectively. It should be noted that

this test began after the material had already sustained 85 mils (2.16 mm) of

permanent displacement from the previous tests. Only 58 mils (1.47 mm) of

displacement were observed before the load was released as can been seen in the

load-settlement curve shown in Figure 4.18. The load was released in 2 minutes,

and the material was allowed to recover for 2 minutes. A permanent displacement

of approximately 38 mils (0.97 mm) was recorded at the conclusion of the test. A

cumulative permanent displacement of 123 mils (3.12 mm) was recorded from all

four tests at location A.

45

40

30

20

10

0

200

150

100

50

014121086420



Fig. 4.16. Variation of Force with Time; Test A4 - Loading to 40 kips (178 kN) in 6 Minutes Followed by Complete Unloading on Drier Material (Moisture Content ~ 4.7 %)

160

140

120

100

80

60

40

20

0

4

3

2

1

0



Fig. 4.17. Variation of Displacement with Time; Test A4 - Loading to 40 kips (178 kN) in 6 Minutes Followed by Complete Unloading on Drier Material (Moisture Content ~ 4.7 %)

160

140

120

100

80

60

40

20

0403020100

Force (kips)

4

3

2

1

0

200150100500Force (kN)


Fig. 4.18. Continuous Load-Settlement Curve from Test A4 - Loading to 40 kips (178 kN) in 6 Minutes Followed by Complete Unloading on Drier Material (Moisture Content ~ 4.7 %)

The effective stiffness, keff, was determined over three load ranges on the

loading portion of the load-settlement curve (AA4-CA4) and over two load ranges

on the unloading portion of the load-settlement curve (zones DA4-EA4) as shown

in Figure 4.19 and 4.20, respectively. It is postulated that the initial stiffness of

the material (AA4) was dominated by soil stiffening due to compression.

Secondary stiffness in the load range over which BA4 was determined, which was

between 5 and 18 kips (22.2 kN and 66.7 kN), was greater than that determined

during the load range for AA4. The material stiffness decreased with increased

loading [above 18 kips (66.7 kN)] and increased again upon initial unloading

(DA4) due to the presence of locked in horizontal stresses. Once the load was

completely released from the plate, the material stiffness again decreased (EA4). It

is thought that the material began to fail in shear as the vertical stress was

decreased, and the passive pressure exerted on the material by the locked-in

horizontal stresses exceeded the material shear strength.

46

160

140

120

100

80

60

40

20

0403020100

Force (kips)

4

3

2

1

0

200150100500Force (kN)

Plate Diameter: 12 in. (305 mm)Effective Plate Area: 107 in.2

k =0.218 X 106lb/in.(0.038 X 106 kN/m)

AA4

BA4

CA4

Fig. 4.19. Load-Settlement Curve Annotated with Values of Relative Stiffness During Loading; Test A4 - Loading to 40 kips (178 kN) in 6 Minutes and Unloading to Zero on Drier Material (Moisture Content ~ 4.7 %)

160

140

120

100

80

60

40

20

0403020100

Force (kips)

4

3

2

1

0

200150100500Force (kN)


keff = 0.428 X 106lb/in.(0.075 X 106 kN/m)

keff = 5.05 X 106lb/in.(0.88 X 106 kN/m)EA4

DA4

Fig. 4.20. Load-Settlement Curve Annotated with Values of Relative Stiffness During Unloading; Test A4 - Loading to 40 kips (178 kN) in 6 Minutes and Unloading to Zero on Drier Material (Moisture Content ~ 4.7 %)

4.6 Summary of Observations From Preliminary Plate-Load Tests

at Location A

Five observations were made from plate-load tests at location A which

were useful in establishing the test conditions for plate-load tests at locations B

and C. First, it was observed that although the crushed limestone material used to

construct the test pad is a granular material with non-plastic fines, it exhibited a

significant time-dependent deformation behavior under sustained load. This

observation aided in selecting the time increment for subsequent incremental

plate-load tests. Secondly, the material was observed to increase in stiffness after

a load of 1 to 3 kips (4.4 to 13.3 kN) was applied. This corresponds to a pressure

of 9.3 to 28 psi (64 to 193 kPa) under a 107 in.2 (690 cm2) plate. This is believed

to be the result of initial compression in the near-surface material. Thirdly,

softening of the material was observed at pressures exceeding 159 psi (1096 kPa).

Such pressures are above those normally experienced in pavement base courses. It

is expected that the material began to experience shear deformation at pressures

greater than 159 psi (1096 kPa). For this reason, the pressures selected for plate-

load tests at locations B and C were well below 159 psi (1096 kPa). The fourth

observation was that the slope of the unloading curve was nearly flat at the

beginning of unloading, but increased sharply once the load was almost

completely released. It is postulated that the material began to fail in shear under

passive pressures from locked in horizontal stresses. Finally it was observed that

the material began to stabilize with respect to displacement under cyclic load after

only 7 or 8 cycles of loading. It was observed, however, that the displacement

47

increased significantly with small increases in the applied load. The loading plate

continued to displace at a higher rate after only one cycle of increased load

magnitude was applied, followed by subsequent cycles of the previous smaller

load.

48

CHAPTER FIVE

PLATE-LOAD TESTS ON DRIER MATERIAL AT

LOCATION B ON THE TEST PAD

5.1 Introduction

One plate-load test was performed at location B on top of the second

lift on the Georgetown test pad (Fig. 2.6). The thickness of the aggregate base

under the test plate was 35 in. (89 cm). This test was a traditional,

incremental plate-load test with one cycle of loading to a maximum load

followed by unloading to zero. However, four subsets of unload-reload cycles

at various points within the overall cycle were also applied. No moisture was

added to the surface of the test pad before plate-load testing at location B, so

the moisture content was approximately 4.7 % as in the tests at location A.

5.2 Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips

(36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

Five increments of loading and six increments of unloading were

applied to the plate over approximately 180 minutes at test location B. The

maximum load during the test was approximately 8.3 kips (36.9 kN). The

load versus time plot and the displacement versus time plot are shown in Figs.

5.1 and 5.2, respectively. A total displacement of approximately 50 mils (1.27

65

mm) was observed at the end of the loading sequence. A permanent

displacement of 35 mils (0.89 mm) was observed at the end of the test.

It can be seen in Fig. 5.1 that, at four different times during the test,

five cycles of unloading and reloading were delivered to the plate. The load

was decreased approximately 1 kip (4.4 kN) for each unloading cycle, and

increased 1 kip (4.4 kN) for each reloading cycle. The resulting continuous

load-settlement curve is shown in Fig. 5.3.

5.2.1 Adjusted Load-Settlement Curve from Test B1

The adjusted load-settlement curve shown in Fig. 5.4 was constructed

by determining the end-point displacement under each increment of loading

and unloading, and plotting this displacement against the average load for that

increment.

A seating load of approximately 1 kip (4.4 kN) was applied to the

loading plate and released to zero before the overall load sequence was started

as called for in AASHTO T222-81 (AASHTO, 1986). When, the seating load

was released, no additional seating load was applied before the first load

increment. This is a slight variation from the AASHTO procedure in that the

AASHTO procedure calls for application of one half of the original seating

load before zeroing the displacement sensors. For this test, the displacement

sensors were not zeroed. The complete load-settlement behavior, including

the seating load and resulting deformation, are included on the figures plotted

for this test.

66

10

8

6

4

2

0

40

30

20

10

0200150100500


Plate Diameter: 12 in. (305 mm)

Fig. 5.1. Variation of Force with Time; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

60

50

40

30

20

10

0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0



Fig. 5.2. Variation of Displacement with Time; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

60

50

40

30

20

10

01086420

Force (kips)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

403020100Force (kN)


Fig. 5.3. Continuous Load-Settlement Curve; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

50

40

30

20

10

01086420

Force (kips)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

403020100

Force (kN)


After Seating Sequence Plotted at Ave. Load

Using the Displ. After 15 Min. for Each Load Increment

Fig. 5.4. Adjusted Load-Settlement Curve; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

It can be seen in Fig. 5.4 that the seating sequence is denoted by a different

marker than the rest of the overall plate-load test.

It was observed that the load decreased slightly during the application

of each load increment as shown in Fig. 5.1. This was also previously

observed for tests at Location A. The loading mechanism of the RDD is

controlled by the pressure in the loading cylinders, and is capable of

compensating for creep in the test material. However, the feedback system

was not sensitive enough to compensate for the slight decreases in load

observed during plate-load testing. The loading mechanism also periodically

delivered a spike at the beginning of the load increment which affected the

instantaneous displacement of the plate. It is known that the duration of

load and the magnitude of the maximum load are both important in

determining the final displacement. However, by loading in nearly equal time

increments and taking the average load during each increment, it was hoped

that the effects of time and changing load magnitude could be minimized.

5.2.2 Plotting the Unload-Reload Loops on the Adjusted Load-

Settlement Curve from Test B1

Four groups of unload-reload loops were delivered to the loading plate

at different times during the overall plate-load test. Each loop was plotted

separately, and a linear curve-fit was performed on each loop. The average of

the slopes of the fitted lines was used to plot the unload-reload behavior for

each group on the adjusted load-settlement curve in Fig. 5.5. A straight-line

67

50

40

30

20

10

01086420

Force (kips)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

403020100

Force (kN)


A B1

B B1

C B1

D B1



Fig. 5.5. Adjusted Load-Settlement Curve With Unload-Reload Loops Plotted; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

idealization of the unload-reload behavior was used to illustrate the load-

settlement behavior in the test pad material during application of the unload-

reload loops. The average stiffnesses are presented in Table 5.1. The

individual unload-reload loops with the linear curve-fits are plotted in

Appendix C along with the values used to calculate the average stiffness for

each group of unload-reload loops.

Table 5.1. Average Stiffnesses Measured at Various Loading Conditions for Cyclic Plate-Load Tests on Drier Material (Moisture Content ~ 4.7 %)

Unload-Reload Group 1

Mean Pressure Directly Under the Plate

Equivalent Spring Constant (Stiffness), keff , Under Cyclic

Unloading-Reloading (psi) (kPa) lb/in. kN/m

AB1 13.3 92 1.8 X 106 0.32 X 106 BB1 32.7 225 2.2 X 106 0.39 X 106 CB1 28.3 195 2.5 X 106 0.44 X 106 DB1 14.1 97 1.2 X106 0.21 X 106

1 From Fig. 5.5.

The linear idealizations of each of the four unload-reload groups as

described above are labeled AB1, BB1, CB1, and DB1 in Fig. 5.5. It can be

observed that two of the unload-reload groups were applied during the loading

phase of the plate-load test, and the other two unload-reload groups were

applied during the unloading phase of the test. Group CB1 was intentionally

applied near the load range in which BB1 was applied. Similarly, group DB1

was applied in the load range in which AB1 was applied. The load range on

68

the plate was approximately the same for the coupled groupings (AB1 and DB1

for one coupled grouping; and BB1 and CB1 for the other coupled grouping).

However, the state of stress in the material was higher upon unloading than

during loading for a given pressure on the loading plate. This increase in the

state of stress is due to horizontal stresses which remained locked-in the

material upon unloading. Careful selection of the load ranges for the

application of the unload-reload loops allowed observation of the change in

stiffness due to locked-in stresses.

The stiffnesses determined during the cyclic unload-reload groups can

be seen to increase slightly as the static load level increases. The average

stiffness of the test pad material under group CB1 is the largest stiffness

observed among the unload-reload groups. This is presumably because the

highest locked-in horizontal stresses were present in the material at this

loading condition.

It can be seen that cyclic stiffnesses determined during unloading are

greater than those determined during loading for coupled groups BB1 and CB1

which were applied at similar load ranges. This is the expected result of

locking in horizontal stresses during loading. The same behavior was not

observed for coupled groups AB1 and DB1 as would be expected. The

observation is offered without explanation because there seems to be no

evidence as to why this behavior is observed. It will be shown that in

subsequent tests at location C the material showed the expected behavior.

69

5.2.3 Stiffness of the Test Pad Material as Determined from the

Overall Plate-Load Test

The adjusted load-settlement curve from Fig. 5.4 is re-plotted in Fig.

5.6 with annotations of three relative stiffness measurements which are

labeled EB1, FB1, and GB1. These stiffness measurements may be considered

the “long-term” stiffness of the material, while the stiffnesses determined with

unloading-reloading loops AB1 through DB1 may be considered the “short-

term” stiffness of the material. The measured overall stiffness values are

presented in Table 5.2.

Table 5.2. Overall Stiffnesses Measured Over Various Ranges of Loading for Static Plate-Load Tests on Drier Material (Moisture Content ~ 4.7 %)

Range for Determining Overall Stiffness 2

Overall Equivalent Spring Constant (Stiffness), keff, Under Static Loading and Unloading

lb/in. kN/m EB1 0.184 X 106 0.032 X 106 FB1 2.15 X 106 0.38 X 106 GB1 0.188 X 106 0.033 X 106

2 As shown in Fig. 5.6.

It was observed that the overall stiffness upon initial unloading (FB1)

was greater than for any other part of the overall loading or unloading curves

(EB1 or GB1). The stiffness upon initial unloading is similar to the stiffnesses

70

50

40

30

20

10

01086420

Force (kips)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

403020100

Force (kN)


E B1

F B1

G B1



Fig. 5.6. Adjusted Load-Settlement Curve With Relative Stiffnesses During Loading and Unloading Annotated; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7

measured under the cyclic unloading-reloading groups. This is the expected

behavior resulting from locked-in horizontal stresses.

The stiffness upon final unloading (GB1) was the lowest overall

stiffnesses recorded. It is surprising that the material became softer during

unloading (GB1) than it had been during initial loading (EB1) after having been

compressed under a load of 8.3 kips (36.9 kN). It is hypothesized that

horizontal stresses are locked into the material during loading, and that when

the vertical stress due to load on the plate is decreased sufficiently, the

material begins to fail under the passive pressure from the locked-in

horizontal stresses. As the material particles begin to rearrange, shear

deformation occurs, and the apparent stiffness of the material decreases.

5.2.4 Presentation of Cyclic Stiffness and Overall Stiffness

Measurements

The average stiffnesses at each load condition at which unload-reload

cycles were applied in the incremental plate-load test are shown on a bar chart

in Fig. 5.7. The overall (“long-term”) stiffnesses during loading and

unloading are also shown with the cyclic stiffness values on the bar chart in

Fig. 5.7.

It can be seen that the small-strain cyclic stiffnesses measured from the

plate-load tests are greater than the overall stiffnesses. This is partially the

result of having greater strain amplitude in the overall test than in the small-

amplitude cyclic unload-reload loops. However, the time-dependent

71

2.5x106

2.0

1.5

1.0

0.5

0.01.5 3.7 3.2 1.6

Force on Plate, kips (kN)

400x103

300

200

100

0

AB1

BB1

CB1

DB1

Loading

LoadingUnloading

Unloading

(6.7) (16.5) (14.2) (7.1)

EB1

FB1

GB1Loading

Unloading

Unloading

Fig. 5.7. Representation of Stiffness with Varying Degrees of Load on the Loading Plate; Test B1 - Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

deflection behavior of the unbound base course was a more significant factor

in determining the stiffness during plate-load testing than the strain amplitude

was.

5.2.5 Calculation of Modulus of Soil Reaction, ku’, for Drier

Material (Moisture Content ~ 4.7 %)

The modulus of soil reaction, ku’ is determined from plate-load tests

according to AASHTO (1986) by measuring the displacement of the loading

plate under a loading pressure of 10 psi (69.0 kPa) and dividing the loading

pressure by the deflection. This procedure is described in Section 3.4.1. It

should be noted that the AASHTO procedure requires that the instruments for

measuring displacement be zeroed after application of the seating load.

Therefore, in this test, the seating load displacement was subtracted from the

displacement measured at 10 psi (69.0 kPa) in Fig. 6.4. The modulus of soil

reaction, ku’, for the drier material (w ~ 4.7 %) in Test B1 was determined to

be 2141 psi/in. (581 kPa/mm).

5.3 Summary of Observations from Incremental Plate-Load

Test on Drier Material (Moisture Content ~ 4.7 %) at

Location B

It can be observed from the results of cyclic plate-load tests at location

B that the cyclic stiffness of the material increases significantly with

increasing pressure on the loading plate. However, the overall static stiffness

72

73

of the material remained relatively constant throughout the loading cycle. It is

believed that increasing the loading pressure increases the degree to which

horizontal stresses are locked into the material. The effect of locking in

horizontal stresses on the overall stiffness of the test pad material was also

observed upon static unloading. It can be seen that the overall stiffness at the

beginning of rebound (FB1) is much greater than the stiffness during loading

(EB1) at the same pressure. The same result was observed to be true for both

the cyclic unload-reload stiffnesses (AB1 through DB1) and for the overall

static stiffness upon unloading (FB1).

These results restate the importance of maintaining confining pressure

on unbound granular materials during loading. In a pavement system, this is

accomplished through the use of shoulders on the edge of the pavement which

help maintain a higher horizontal stress state. The significance of locking in

horizontal stresses during loading or during compaction is illustrated in these

observations.

CHAPTER SIX

PLATE-LOAD TESTS ON WETTER MATERIAL AT

LOCATION C ON THE TEST PAD

6.1 Introduction

As with plate-load tests described in Chapters 4 and 5, plate-load tests

were conducted at location C on top of lift two of the Georgetown test pad. The

thickness of aggregate base under the test plate was 35 in. (89 cm). The base

course material was wetted from the surface before seismic or plate-load tests

were conducted at location C by first placing a 48 in. (122 cm) diameter by 24 in.

(61 cm) high steel ring on the pad over the test site and sealing around the bottom

with hydrostone. The ring was filled with water and the surrounding area was

soaked three to four times over the two-day period preceding the plate-load and

seismic tests. Surface moisture content measurements made on specimens

collected before and after wetting indicated a 1.5 % increase in moisture content

in the top 3-4 in. (8-10 cm) of the material due to wetting from the surface. The

near-surface moisture content was 6.1 % after wetting. This value was confirmed

by corrected nuclear density gauge (NDG) readings. A 1.2 % increase in moisture

content at a depth of 12 in. (31 cm) was observed (yielding w ~ 5.5 %) as

calculated from the corrected NDG readings previously reported in Tables 2.3 and

2.4.

81

Two tests were performed at location C to compare the deformation of a

wetter base course material to that of the same material in a drier state. The

loading cycles in the tests at location C were closely matched with the loading

cycles from two previous tests in order to observe the effects of wetting the

unbound base course. The loading cycle for Test C1 which was performed on the

wetter material (w ~ 6.1 %) was similar to the loading cycle for Test B1 which

was performed on the drier material (w ~ 4.7 %). The loading cycle for Test C2

(wetter material) was similar to the loading cycle for Test A3 (drier material).

6.2 Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3

kN) on Wetter Material (Moisture Content ~ 6.1%)

Five increments of loading and four increments of unloading were applied

to the loading plate over a period of 200 minutes in the incremental plate-load test

at location C. The maximum force applied to the plate during the test was

approximately 8.6 kips (38.3 kN). This loading sequence is shown in the force

versus time and displacement versus time plots in Figs. 6.1 and 6.2, respectively.

A displacement of 90 mils (2.29 mm) was observed at the conclusion of the

loading cycle. A permanent displacement of 68 mils (1.73 mm) was recorded 15

minutes after the load was decreased to zero.

It can be seen in Fig. 6.1 that groups of unload-reload cycles were applied

to the loading plate at five different times during the overall plate-load test. Five

unload-reload cycles were applied for each group. The load was decreased

approximately 1 kip (4.4 kN) for each unloading cycle and increased 1 kip (4.4

82

10

8

6

4

2

0

40

30

20

10

0200150100500



Fig. 6.1. Variation of Force with Time; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

100

80

60

40

20

0

2.5

2.0

1.5

1.0

0.5

0.0



Fig. 6.2. Variation of Displacement with Time; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

kN) for each reloading cycle. The resulting continuous load-settlement curve is

shown in Fig. 6.3.

6.2.1 Adjusted Load-Settlement Curve from Test C1

The end-point displacement for each load increment was plotted against

the average load for the increment as described in Section 5.2.1 for Test B1. This

approach simply “adjusts for” or minimizes the effect of creep on the load-

settlement curve. It can be seen from both the force versus time plot in Fig. 6.1

and the continuous load-settlement plot in Fig. 6.3 that the load decreased slightly

during each increment of loading. This behavior is the same behavior observed in

the previous tests. The adjusted load-settlement curve plotted with the average

load values is shown in Fig. 6.4.

As in Test B1, a seating load of approximately 1000 lb (4.4 kN) was

applied to the loading plate and released before the overall load sequence was

started as called for in AASHTO T222-81 (1986). The seating load was

completely released, and no additional seating load was applied before the first

load increment. This is a slight variation from the AASHTO procedure in that the

AASHTO procedure calls for application of one half of the original seating load

before zeroing the displacement sensors. For this test, the displacement sensors

were not zeroed. The complete load-settlement behavior, including the seating

load and resulting deformation, are included on the figures plotted for this test. It

can be seen in Fig. 6.4 that the seating sequence is denoted by a different marker

than the rest of the overall plate-load test.

83

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100Force (kN)


Fig. 6.3. Continuous Load-Settlement Curve; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100

Force (kN)

Plate Diameter 12 in. (305 mm)



Fig. 6.4. Adjusted Load-Settlement Curve; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

6.2.2 Plotting the Unload-Reload Loops on the Adjusted Load-

Settlement Curve from Test C1

Five groups of unload-reload cycles were delivered to the plate at different

load conditions during the overall plate-load test. Three of the five groups were

applied during the loading phase of the overall plate-load test, and the other two

groups were applied during the unloading phase of the overall plate-load test. For

each unload-reload cycle, the load was decreased or increased approximately

1000 lb (4.4 kN).

The loops were plotted individually, and a linear curve-fit was performed

on each loop. The average of the slopes of the fitted lines was used to plot the

unload-reload behavior for each group on the adjusted load-settlement curve in

Fig. 6.5. A straight-line idealization of the unload-reload behavior was used to

illustrate the load-settlement behavior in the test pad material during application

of the unload-reload loops. The unload-reload groups are labeled AC1, BC1, CC1,

DC1, and EC1 on the adjusted load-settlement curve in Fig. 6.5. The stiffnesses for

each group of unload-reload loops are presented in Table 6.1. The individual

loops are shown in Appendix D. The stiffness values for each individual loop are

also presented in tabular form in Appendix D.

84

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100

Force (kN)

BC1

AC1

CC1DC1EC1

Plate Diameter 12 in. (30.5 cm)



Fig. 6.5. Adjusted Load-Settlement Curve With Unload-Reload Loops Plotted; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

Table 6.1. Average Stiffnesses Measured During Small Amplitude Unload-Reload Loops at Various Loading Conditions for Cyclic Plate-Load Tests on Wetter Material (Moisture Content ~ 6.1 %)

Unload-Reload Group 1

Mean Pressure Directly Under the Plate

Equivalent Spring Constant (Stiffness), keff, Under Cyclic

Unloading-Reloading (psi) (kPa) lb/in. kN/m

AC1 19.5 134 0.90 X 106 0.16 X 106 BC1 32.7 225 2.3 X106 0.40 X 106 CC1 71.6 494 14 X 106 2.5 X 106 DC1 31.8 219 3.0 X 106 0.53 X 106 EC1 20.3 140 2.7 X 106 0.47 X 106

1 From Fig. 6.5.

The unload-reload groups in this test were coupled as described in Section

5.2.2 for Test B1. Group EC1 was applied near the load range used for group AC1.

Group DC1 was applied near the load range used for group BC1. The load range on

the plate was approximately the same for the coupled groups (AC1 and EC1 for one

coupled group; and BC1 and DC1 for the other coupled group). However, the state

of stress in the material was higher upon unloading than during loading for a

given pressure on the loading plate due to the stiffening effect of locked-in

horizontal stresses. It can be seen that cyclic stiffnesses determined during

unloading are greater than those determined during loading despite the fact that

the load on the plate is approximately the same for a coupled group. This has been

observed for coupled groups AC1 and EC1 and for coupled groups BC1 and DC1 and

is the expected result of stiffening due to locked-in horizontal stresses.

85

The stiffnesses determined during the application of cyclic unload-reload

groups increased slightly as the static load level increased which was the same

behavior observed during Test B1. The average stiffness of the test pad material

under cyclic unload-reload group CB1 is significantly larger than for any other

group. It is presumed that the highest locked-in horizontal stresses were present in

the material at this loading condition. However, it is not apparent why the

stiffness at this loading condition is almost 5 times greater than the next largest

stiffness which was measured from group DC1.

6.2.3 Stiffness of the Test Pad Material as Determined from the

Overall Plate-Load Test

The adjusted overall load-settlement curve from Fig. 6.4 is re-plotted with

annotations of four relative stiffness measurements which are labeled FC1, GC1,

HC1, and IC1 in Fig. 6.6. These stiffness measurements may be considered the

“long-term” stiffness of the material, while the stiffnesses determined from

unload-reload loops AC1 through EC1 may be considered the “short-term” stiffness

of the material. The overall stiffness values are presented in Table 6.2.

86

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100

Force (kN)

FC1

GC1

HC1

IC1




Fig. 6.6. Adjusted Load-Settlement Curve with Relative Stiffnesses During Loading and Unloading Annotated; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

Table 6.2. Overall Stiffnesses Measured Over Various Ranges of Loading for Static Plate-Load Tests on Wetter Material (Moisture Content ~ 6.1 %)

Range for Determining Overall Stiffness 2

Overall Equivalent Spring Constant (Stiffness), keff, Under Static Loading and Unloading

lb/in. kN/m FC1 0.092 X 106 0.016 X 106 GC1 0.14 X 106 0.025 X 106 HC1 1.2 X 106 0.21 X 106 IC1 0.10 X 106 0.018 X 106

2 As shown in Fig. 6.6.

It was also observed that the overall stiffness upon initial unloading (HC1)

was greater than for any other part of the overall loading or unloading curves

(FC1, GC1, or IC1). The stiffness upon initial unloading is similar to the stiffnesses

measured under the cyclic unload-reload groups. This is, again, the expected

behavior resulting from locked-in horizontal stresses.

The stiffness upon initial loading (FC1) and the stiffness upon final

unloading (IC1) were the lowest overall stiffnesses recorded. The initial loading

(FC1) and final unloading (IC1) curves can be seen to be approximately parallel in

Fig. 6.6 indicating equal stiffness under these two loading conditions.

6.2.4 Presentation of Cyclic Stiffness and Overall Stiffness

Measurements

The average stiffnesses at each load condition at which unload-reload

cycles were applied in the incremental plate-load test are shown on a bar chart in

87

Fig. 6.7. The overall (“long-term”) stiffnesses during loading and unloading are

shown with the cyclic stiffness values on the bar chart in Fig. 6.7.

It was observed in Test B1 and again here in Test C1 that the small-strain

cyclic stiffnesses measured from the plate-load tests are greater than the overall

stiffnesses. This result can be partially accounted for because the strain amplitude

in the overall test is greater than that in the small-amplitude cyclic unload-reload

loops. However, the time-dependent deflection behavior of the unbound base

course appears to have a more significant effect on the stiffness of the material

than the strain amplitude does.

6.2.5 Calculation of Modulus of Soil Reaction, ku’, for Wetter

Material (Moisture Content ~ 6.1 %)

The modulus of soil reaction, ku’, is determined from plate-load tests

according to AASHTO (1986) by measuring the displacement of the loading plate

under a loading pressure of 10 psi (69.0 kPa) and dividing the loading pressure by

the deflection. This procedure is described in Section 3.4.1. It should be noted

that the AASHTO procedure requires that instruments for measuring

displacement are zeroed after application of the seating load. Therefore, in this

test, the seating load displacement was subtracted from the displacement

measured at 10 psi (69.0 kPa) in Fig. 6.4. The modulus of soil reaction, ku’, for

the wetter material (w ~ 6.1 %) in Test C1 was determined to be 635 psi/in. (172

kPa/mm).

88

16x106

14

12

10

8

6

4

2

02.2 3.9 8.1 3.6 2.3

Force on Plate, kips (kN)

2.5x106

2.0

1.5

1.0

0.5

0.0

AC1

BC1

CC1

DC1

EC1

Loading

Loading

Loading

UnloadingUnloading

FC1 GC1

HC1IC1

Loading Loading UnloadingUnloading

(9.8) (17.3) (36.0) (16.0) (10.2)

Fig. 6.7. Representation of Stiffness with Varying Degrees of Load on the Loading Plate; Test C1 - Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

6.3. Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 7 Minutes on

Wetter Material (Moisture Content ~ 6.1%)

Five cycles of approximately 8 kips (35.6 kN) each were applied to the

plate during the second test at location C. Each complete cycle was applied and

released over a period of 1.5 to 2 minutes as shown on the force versus time and

displacement versus time plots in Fig 6.8 and 6.9, respectively. This loading

sequence was similar to that of plate-load Test A3 in which 32 cycles of load

were applied to the plate in 20 minutes. As with the previous tests, load-

settlement data were collected continuously and are plotted in Fig. 6.10. A bi-

linear stiffness relationship was observed on four of the five loops similar to that

seen in Test A3. Each loop was plotted separately, and the stiffness was recorded

for the initial part of the loading curve and for the final part of the loading curve.

The stiffness values are plotted for comparison in Fig. 6.11, and the individual

loops are shown in Appendix E.

It was observed that the deformational behavior under cyclic loading

stabilized after a relatively few number of cycles. The cyclic loops began to plot

on top of one another when the load range and rate of loading were nearly

constant. Loops 4 and 5 from Test C2 are plotted in Fig. 6.12 to illustrate this

observation. This is an important finding, because it indicates that far less

permanent deformation due to volume change is occurring once the material is

conditioned (from previous loading during Test C1). No stiffnesses during

unloading are presented for Test C2. However, general behavior similar to that

observed in Test A3 was observed for Test C2.

89

10

8

6

4

2

0

40

30

20

10

0121086420



Fig. 6.8. Variation of Force with Time; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

30

25

20

15

10

5

0

0.8

0.6

0.4

0.2

0.0



Fig. 6.9. Variation of Displacement with Time; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

30

25

20

15

10

5

01086420

Force (kips)

0.8

0.6

0.4

0.2

0.0

403020100Force (kN)


Fig. 6.10. Continuous Load-Settlement Curve; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

700x103

600

500

400

300

200

10054321

Cycle Number

120x103

100

80

60

40

20

Stiffness During Initial Loading Stiffness During Final Loading

Fig. 6.11. Bi-Linear Stiffnesses of Each Cyclic Loop in the Loading Section; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

Loop 4

Loop 5

Fig. 6.12. Illustration of Stabilization or Closing of Cyclic Loops; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

6.4 Summary of Observations from Incremental Plate-Load Tests

on Wetter Material (Moisture Content ~ 6.1 %) at Location C

It was observed that increasing the moisture content in situ had significant

softening effects on the granular base course material used to construct the test

pad. The average near-surface moisture content increased from 4.7% (drier

material) to 6.1% (wetter material) as a result of wetting the test pad material

prior to performing these tests.

A 70% increase in the total displacement was observed between the

incremental plate-load test on the drier material (Test B1) and the incremental

plate-load test on the wetter material (Test C1) after wetting the test pad from the

surface. This increase was determined by taking the difference between the

observed displacement in Test B1 on the drier material (w ~ 4.7 %) and Test C1

on the wetter material (w ~ 6.1 %). The load on the plate was approximately equal

for the two tests [8.3 kips (36.9 kN) for Test B1 on the drier material and 8.6 kips

(38.3 kN) for Test C1 on the wetter material].

The most significant decrease in overall stiffness due to wetting the

material was observed during the incremental plate-load test with low load on the

plate. The stiffness of the drier material was approximately two times the stiffness

of the wetter material at loads below 2.7 kips (12.0 kN) as can be observed from

the slope of the initial loading curves for incremental plate-load Tests B1 and C1

(Figs. 5.4 and 6.4 respectively). The initial overall stiffness for Test B1 (EB1, drier

material, w ~ 4.7%) was observed to be 0.184 X 106 lb/in. (0.032 X 106 kN/m).

The initial overall stiffness for Test C1 (FC1, wetter material, w = 6.1%) was

90

observed to be 0.092 X 106 lb/in. (0.016 X 106 kN/m) which is half of the

stiffness observed for the drier material. Once the load increased above 2.7 kips

(12.0 kN), the difference in stiffness between the wetter material and the drier

material was less significant. The overall stiffness upon initial unloading also

differed by a factor of two between the wetter test and the drier test.

It was observed that the stiffness during initial loading (FC1) and the

stiffness during final unloading (IC1) were similar. However, at higher loads on

the plate, the stiffness during unloading (GC1) was greater than the stiffness

during loading (HC1).

Decreases in stiffness were also observed under cyclic loading between

Test A3 (drier material, w ~ 4.7 %) and Test C2 (wetter material, w ~ 6.1 %).

Detailed comparison of the stiffness measurements among the tests is included in

Chapter 8.

91

CHAPTER SEVEN

SEISMIC TESTING AND STIFFNESS EVALUATIONS

7.1 Overview

Seismic testing was conducted to determine the small-strain stiffness

of the test pad material in-situ. The Spectral-Analysis-of-Surface-Waves

(SASW) technique was chosen because it is non-invasive and allows

characterization of the material at varying depths. SASW testing produces a

profile of shear wave velocity with depth which can then be related to the

shear modulus and Young’s modulus (which equals resilient modulus at small

strains), and equivalent stiffness of the material as discussed below.

7.2 SASW Testing

SASW testing was performed by applying a transient excitation to the

surface of the aggregate test pad with a hammer. Receivers placed on the

surface of the test pad in a linear array were used to measure the velocity of

Raleigh-type surface waves passing between the receivers. This general

arrangement is illustrated in Fig. 7.1. The tests were conducted with the

receivers placed at various spacings in the linear array which allowed

sampling of a range of depths in the test pad. Through a forward modeling

procedure, a profile of shear wave velocity with depth was obtained. A

105

complete discussion of the SASW procedure for use in characterizing

geotechnical materials can be found in Stokoe et al. (1994).

RecordingEquipment

Receiver 2Receiver 1

ActiveSource

d1

d2

Fig. 7.1. Basic Configuration for SASW Testing (from Stokoe et al. 1994)

7.3 Non-Linear Moduli

It can be shown that material stiffness in shear decreases with

increasing strain amplitude as illustrated in Figs. 7.2 and 7.3. Small-strain

shear moduli below 0.001 % strain are considered to be independent of strain

amplitude. These shear moduli are typically denoted as Gmax or G0. Because

106

the SASW procedure produces strains in the material less than 0.001 %, the

shear modulus obtained from these tests is independent of strain amplitude.

.

Shea

ring

Stre

ss, t

Shearing Strain, γ, %

GmaxG1 G2 G3 G4

MonotonicLoading Curve

00.05 0.10 0.150

Fig. 7.2. Relationship between the Monotonic Loading Curve and Material Stiffness in Shear of a Geotechnical Material

107

Gmax

10-3 10-2 10-110-40

gte=

G2

G3G4

G1

1

Nc = 1 cycleso = Constant

threshold strain below which Young's modulus is constant

10-5

Strains Generatedin Field Seismic Tests

Shearing Strain, γ, percent

Shea

r Mod

ulus

, G

Fig. 7.3. Variation in Material Stiffness in Shear with Increasing Strain Amplitude as Determined from Fig. 7.2

This range of strain amplitude (< 0.001%) is referred to as the linear or small-

amplitude range. There is evidence to suggest that, many times, strains under

working loads in geotechnical materials are also within the small-strain range

(Burland 1989). Of course, the strain amplitude depends on the conditions of

loading and the proximity to the load point or loaded area. It is important to

have a good idea of the strain amplitude being produced in the material under

observation so that the appropriate modulus can be used in characterizing the

material. A discussion of the relative magnitude of strains induced in the

seismic tests and the plate-load tests in the Georgetown, Texas test pad is

included in Chapter 8.

108

7.4 Determination of Small-Strain Shear Modulus

Shear modulus is defined as the shearing stress divided by the shearing

strain. The small-strain shear modulus, Gmax, may be determined from shear

wave velocity measurements according to the following relationship:

Gmax = ρVs2 .............................................................. (7.1)

where Gmax = small-strain shear modulus,

ρ = mass density (total density divided by acceleration due to

gravity), and

Vs = shear wave velocity.

7.5 Determination of Small-Strain Equivalent Spring Constant

The stiffness of the material may be described in terms of an

equivalent spring constant, keff, which is represented in units of force divided

by units of length (displacement). The theoretical equivalent spring constant

(stiffness) of a geotechnical material experiencing a uniform load under a

circular plate is defined in terms of the shear modulus as shown below:

109

keff = 4Gro/(1-ν) ........................................................ (7.2)

where keff = equivalent spring constant,

G = shear modulus,

ro = radius of plate, and

ν = Poisson’s ratio.

If the strains in the material beneath the plate are in the small-strain

range, then G in Eq. 7.2 is simply Gmax. The relationship between Vs (hence

Gmax) and keff expressed by Eq. 7.2 is presented graphically in Fig 7.4 for an

average total unit weight of 138 lb/ft3 (21.7 kN/m3), a plate radius of 0.5 ft

(15.2 cm) and a small-strain Poisson’s ratio of the material (granular base) of

0.25.

110

8

7

6

5

4

3

2

1

02500200015001000500

Shear Wave Velocity (VS), ft/sec

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

800600400200

Shear Wave Velocity (VS), m/sec

γ = 138 lb/ft 3

ro = 0.5 ftν = 0.25

Fig. 7.4. Equivalent Spring Constant, keff, Determined from Shear Wave Velocity for Strain Amplitudes in the Small-Strain Range

7.6 Shear Wave Velocity Profiles

SASW testing and the associated data reduction produces a profile of

shear wave velocity with depth in the material being tested. Two such velocity

profiles for the Georgetown test pad at the plate-load test locations are shown

in Figs. 7.5 and 7.6. The shear wave velocity profile for the drier material (w ~

4.7 %) is shown in Fig. 7.5 and the shear wave velocity profile for the wetter

material (w ~ 6.1 %) is shown in Fig. 7.6.

111

30

20

10

0

-10

-20

150010005000

Shear Wave Velocity, Vs, ft./sec

50

40

30

20

10

0

5004003002001000

Shear Wave Velocity, Vs, m/sec

Subgrade

Lift 1

Lift 2

Fig. 7.5. Shear Wave Velocity Profile of the Test Pad for the Drier Material (w ~ 4.7 %)

30

20

10

0

-10

-20

150010005000

Shear Wave Velocity, Vs, ft./sec

50

40

30

20

10

0

5004003002001000

Shear Wave Velocity, Vs, m/sec

Lift 1

Subgrade

Lift 2

Fig. 7.6. Shear Wave Velocity Profile of the Test Pad for the Wetter Material (w ~ 6.1 %)

112

The near-surface material [depth < 5 in. (12.7 cm)] in a wetter state

can be seen to be much softer than the near-surface material in a drier state as

shown in Figs. 7.5 and 7.6. The wetter material at a depth of 5 to 12 in. (12.7

to 30.5 cm) is only slightly softer than the drier material at the same depth.

Below a depth of 12 in. (30.5 cm), the small-strain shear stiffness is the same

for both materials (wetter state and drier state).

It can be observed from Fig. 7.6 that the near surface material in the

wetter state [depth < 5 in. (12.7 cm)] is considerably less stiff than the deeper

material. This is not true of the material in a drier state as observed in Fig. 7.5.

It is believed that the negative capillary stresses in the drier material had a

stiffening effect on the material. It was also observed that the increase in

stiffness due to negative capillary stresses causes the stiffness of the near-

surface material to be even greater than the stiffness of the material at a depth

of 5 in. (12.7 cm) for the drier material.

113

7.7 Measured Small-Strain Stiffness

The small-strain equivalent spring constant was calculated from the

shear wave velocity for both the drier and wetter material under the condition

of no external (surface) load. The stiffness was calculated using the average

shear wave velocity in the top 12 in. (30.5 cm) (one plate diameter) of the test

pad. The small-strain equivalent spring constants are presented in Table 7.1. Table 7.1. Average small-strain stiffness for an effective depth of 12 in. (30.5 cm) (one plate diameter)

Material Moisture Condition

Average Shear Wave Velocity, Vs

Small-Strain Equivalent Spring Constant, k

(Stiffness) ft/sec m/sec lb/in. kN/m

Drier (w ~ 4.7 %) 1300 396 1.7 X 106 0.30 X 106 Wetter (w ~ 6.1 %) 900 274 0.79 X 106 0.14 X 106

The material near the surface exhibited more than a 50% reduction in

stiffness due to the wetting as shown by the SASW data. This reduction in

stiffness was not detected by changes in density. The average value of the

density for the drier material is 131 pcf (20.6 kN/m3), and the average density

of the wetter material is 135 pcf (21.2 kN/m3). This change in density is small

compared to the change in seismic stiffness. In addition, the measurement of

dry density depends on measurement of the moisture content which is only a

rough estimate of the actual moisture content of the material when measured

with the NDG.

114

115

7.8 Adjustment to the Small-Strain Stiffness for Variations in

the State of Stress

The state of stress in compacted granular materials depends on a

number of factors which include the effective horizontal state of stress and the

effect of negative capillary stresses. The development of negative capillary

stresses produces an all-around increase in the state of stress; the process of

compaction locks in horizontal stresses of unknown magnitude; and finally,

increases in the vertical load produce increases in the state of stress.

The only variable mentioned above which was measured in this study

was the vertical load. Quantifying increases in the state of stress due to

locked-in horizontal stresses is difficult and was not attempted in this study. It

has been shown, however, that locked-in horizontal stresses can exceed the

vertical overburden stress. Negative capillary stresses can also have a very

significant impact on the state of stress at low levels of vertical effective

stress. However, negative capillary stresses were also not evaluated. The

application of a vertical load on a circular plate also produces a stress

distribution under the plate which varies with depth from the surface which

was only estimated in this study.

CHAPTER EIGHT

COMPARISON OF STIFFNESS VALUES FROM PLATE-

LOAD AND SEISMIC TESTS

8.1 Introduction

The stiffness measurements made in plate-load and seismic tests are

compared and discussed in this chapter. This discussion is divided into two

major categories: small-strain comparisons and large strain comparisons. A

method for determining strain amplitude, and an explanation of the reduction

in shear modulus for increasing strain amplitude are discussed before the

small and large strain comparisons are presented.

8.1.1 Estimation of Strain Amplitude

Comparison of seismic and working-load stiffness measurements for

the purpose of predicting displacement requires evaluating the strain

amplitudes generated in SASW and plate-load testing. The shearing strain

produced during seismic testing is less than 0.001% as discussed in Chapter 7.

The modulus of the material is well within the linear range at these levels of

strain. No measurement of internal displacements were made below the plate

during plate-load tests. However, vertical strains can be estimated from

115

measured vertical displacements of the plate if an effective depth of influence

is assumed.

A procedure for estimating the strain distribution in granular material

under the center of a loaded circular plate was developed by Schmertmann

(1970). The strain estimation consists of a bi-linear idealization of the

distribution of strain with depth as shown in Fig. 8.1. The effective depth is

assumed to be two plate diameters, with the maximum strain occurring at a

depth of 0.5 diameter. The strain influence factor Iz from Fig. 8.1 is used in

Eq. 8.1 to estimate vertical strain at any depth under the center of the plate:

εv = Iz (q/Es) ..............................................................(8.1)

where:

εv = vertical normal strain,

Iz = influence factor,

q = applied pressure at the surface, and

Es = Young’s modulus of the granular base material.

116

2.0

1.5

1.0

0.5

0.0

0.70.60.50.40.30.20.10.0

Vertical Strain Influence Factor, IZ Fig. 8.1. Variation of Vertical Normal Strain with Depth Beneath a Circular Plate on Sand after Schmertmann (1970)

8.1.2 Adjustments to Small-Strain Shear Modului for Increase

in Strain Amplitude and Increase in State of Stress

Young’s modulus of the material is estimated from the stiffness

measurements back-calculated from the plate-load tests. The equivalent

spring constant, keff, at small strains, is evaluated from the average slope of

the unload-reload loops at each general load zone. The equivalent spring

117

constant is related to the shear modulus, G, by the expression shown in Eq.

7.2. Young’s modulus can then be determined from the shear modulus as

shown in Eq. 8.2.

E = k(1+ν)(1-ν)/2r0 ................................................... (8.2)

where:

E = Young’s modulus,

k = equivalent spring constant,

ν = Poisson’s ratio, and

r0 = radius of the plate.

An average curve illustrating the reduction in normalized shear

modulus with increasing shearing strain amplitude for sands (Seed et al.,

1987) is shown in Fig. 8.2. It is recognized from Fig. 8.2 that stiffnesses

evaluated by seismic tests may be slightly greater than the stiffness of a

granular base course material under working loads due to the fact that

working load strains may be somewhat above the boundary which defines the

linear range of stiffness (γ = 0.001%).

118

1.2

1.0

0.8

0.6

0.4

0.2

0.010-5 10-4 10-3 10-2 10-1 100


Seed et al. (1986) (For Sands) Range Mean

Fig. 8.2. Illustration of the Reduction in Shear Modulus with Increases in Strain Amplitude

A second factor affecting the measured stiffness between the seismic

tests and the plate-load tests is that they are determined at different states of

stress. The initial state of stress during seismic testing is due to the weight of

the material, locked-in horizontal stresses, and negative capillary stresses.

The initial state of stress in the plate-load test includes a significant additional

vertical stress from loading of the plate. As a result, the material becomes

stiffer under plate-loading due to the increased confinement of the material.

119

The effects of strain amplitude and vertical effective stress counteract

one another in this case, but they do not have equal influences. For this

reason, each factor must be accounted for separately.

8.2 Small-Strain Stiffness Comparisons

Spectral-Analysis-of-Surface-Waves (SASW) tests yielded profiles of

shear wave velocity with depth for the aggregate base course. Profiles of

shear modulus or Young’s modulus with depth were evaluated from these

seismic measurements. The strains induced during seismic testing (SASW)

are sufficiently low that the calculated stiffness values are within the linear

range. The same is not true for stiffness values calculated during plate-load

testing, because strains induced in the aggregate base can be in the linear or

nonlinear range, depending upon the applied load and proximity to the load

plate. It is convenient to first evaluate those plate-load stiffness values which

were measured within or very near the linear range. The nine groups of small

unload-reload loops [four in Test B1(drier test) and five in Test C1(wetter

test)] meet this criterion and are discussed below. The adjusted load-

settlement curves are included again for convience in Figs. 8.3 and 8.4 for the

drier and wetter aggregate base conditions, respectively.

120

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100Force (kN)

AB1BB1

CB1DB1


Fig. 8. 3. Adjusted Load-Settlement Curve with Unload-Reload Loops for Test B1; Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7 %)

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100Force (kN)

BC1

AC1

CC1DC1EC1


Fig. 8.4. Adjusted Load-Settlement Curve with Unload-Reload Loops for Test C1; Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~ 6.1 %)

121

8.2.1 Adjustment to Small-Strain Stiffnesses for Increased

Strain Amplitude

The maximum and average strains were calculated for each group of

unload-reload loops in Test B1 (drier base course) and Test C1 (wetter base

course) assuming a value of 0.25 for Poisson’s ratio. An influence factor of

0.6 was selected from Fig. 8.1 for the maximum strain in each group of

unload-reload loops, while an average influence factor of 0.325 was used over

an effective depth of two plate diameters to estimate the average strain for

each group of unload-reload loops in the plate-load tests. Strains experienced

during initial loading are not included in this discussion, because they are well

outside the small-strain range due, in large part, to the high degree of volume

change during compression. The average and maximum strain amplitudes for

each group of unload-reload loops are presented in Table 8.1.

122

Table 8.1. Estimated Strain Amplitudes for Small-Strain, Unload-Reload Loops in Tests B1 and C1

Moisture Condition of Base Course

Unloading-Reloading Loops

Maximum Vertical Normal

Strain, εv-max

Average Vertical Normal

Strain, εv-ave % %

Drier AB1 0.0041 0.0022 (w ~ 4.7 %) BB1 0.0035 0.0019

CB1 0.0028 0.0015 DB1 0.0060 0.0032

Wetter AC1 0.0084 0.0046 (w ~ 6.1 %) BC1 0.0033 0.0018

CC1 0.0005 0.0003 DC1 0.0024 0.0013 EC1 0.0031 0.0017

The estimated strain ranges induced during the application of the

cyclic unload-reload loops in the plate-load tests have plotted with the Seed et

al. (1986) curve for both the wetter base course material and the drier base

course material in Fig. 8.5. It should be noted that the shearing strain, γ, was

determined from the axial strain following Eq. 8.3.

γ = (1 + ν)εv .............................................................(8.3)

where:

γ = shearing strain,

ν = Poisson’s ratio (0.25 for this case), and

εv = vertical normal strain.

123

1.2

1.0

0.8

0.6

0.4

0.2

0.010-5 10-4 10-3

γ10-2 10-1 100

Shearing Strain, , %

Seed et al. (1986) (For Sands) Range Mean

10-5 10-4 10-3 10-2 10-1 100


Drier Material - Max. Strain

Drier Material - Ave. Strain

Wetter Material - Max. Strain

Wetter Material - Ave. Strain

Fig. 8.5. Illustration of Strain Ranges Observed from Multiple Series of Cyclic Unload-Reload Loops During Plate Load Testing

124

For these tests, the magnitude of the stiffness reduction observed for

the granular base course material varied from about 0-27 % depending on

whether the maximum strain or the average strain was considered. There is

little variation in the value of this reduction between the tests on drier material

and the tests on wetter material.

8.2.2 Adjustment to Small-Strain Stiffnesses for Increased

State of Stress due to Wetting and due to Increasing the Load

on the Plate

The Young’s modulus values calculated from plate-load tests and

seismic tests are presented in Table 8.2. It can be observed that the base

course material became softer as a result of wetting the test pad from the

surface. However, the softening effect was observed only when there were

relatively low loads on the plate [< 2 or 3 kips (8.9 to 13.3 kN)]. This

corresponds to an average pressure directly under the plate of 17.7 to 26.5 psi

(122 to 183 kPa). This result was observed for unload-reload groups applied

during the overall loading cycle and the during overall unloading cycle. The

softening effect was diminished when the load on the plate was increased.

125

Table 8.2. Young’s Modulus Values Calculated Directly from Unload-Reload Loops in Plate-Load Tests and from Small-Strain Seismic Tests

Moisture Condition of

the Base Course

Test Number

Average Load on Plate

During Cyclic Unloading-Reloading

Young’s Modulus, E 1

kips kN (psi) (kPa) Drier AB1 1.5 6.7 1.38 X105 9.51 X 105

(w = 4.7 %) BB1 3.7 16.5 1.68 X 105 11.6 X 105 CB1 3.2 14.2 1.96 X 105 13.5 X 105 DB1 1.6 7.1 0.95 X 105 6.55 X 105 SASWB1 1.26 X105 8.69 X 105

Wetter AC1 2.2 9.8 0.70 X 105 4.83 X 105 (w = 6.1 %) BC1 3.9 17.3 1.80 X 105 12.4 X 105

CC1 8.1 36.0 11.0 X105 75.8 X 105 DC1 3.6 16.0 2.32 X105 16.0 X 105 EC1 2.3 10.2 2.08 X 105 14.3 X 105 SASWC1 0.60 X 105 4.14 X 105

1 Young’s moduli calculated from seismic tests have not been adjusted for strain amplitude and stress-state effects.

The stiffnesses determined from the unload-reload cycles in the plate-

load test are approximately equal for the the material in the drier state and in

the wetter state under loads larger than 2 to 3 kips (8.9 to 13.3 kN). In fact,

the stiffnesses of the wetter material appear to be slightly higher than the

stiffnesses of the drier material in some cases. For example, the stiffness

determined from unload-reload group BC1 (wetter base course) is slightly

higher that the stiffness determined from BB1 (drier base course) despite the

126

fact that they were determined at nearly the same load range. This is because

the load on the plate was slightly higher for unload-reload group BC1 (wetter

base course) than for group BB1 (drier base course). The same result can be

observed by comparing Groups CB1-DC1 and DB1-EC1. This indicates that the

load on the plate (or the confinement of the material) has a much more

significant effect on the stiffness of the material than the moisture content

does at these states of stress.

In an effort to adjust seismic stiffness measurements to account for the

increased state of stress during plate loading, the observed stiffnesses were

plotted against the pressure applied to the plate as shown in Fig. 8.6.

Reduction of the seismically determined moduli was performed using the

average Seed et al. (1987) curve before determining the increase in stiffness

due to increased confining pressure. Considerable scatter is observed over the

nine points for which small-strain stiffnesses were measured in the plate-load

test. A trend line through the data should cross the vertical axis at 1.0.

However , no convenient correlation was found to account for the effect of

increasing the confining pressure in this material. Additional data may better

define a trend between the applied pressure and the increase in seismically

predicted stiffnesses. Future tests should include seismic testing under the

loaded plate in order to better explain the change in material properties due to

changes in mean effective stress.

127

89

1

2

3

4

56789

10

2

3

10 2x101 3 4 5 6 7 8

Applied Pressure Under Plate (psi)

7 8 9100

2 3 4 5

Applied Pressure Under Plate (kPa)

Group Applied During Loading Group Applied During Unloading

AB1

BB1

CB1

DB1

AC1

BC1

CC1

DC1EC1

Fig. 8.6 Relationship Between Pressure Applied to the Plate and Observed Increase in Small-Strain Stiffness

8.2.3 Conclusions From Small-Strain Moduli Comparisons

Based on previous theoretical and experimental studies, the mean

effective stress in an unbound granular base course material has a significant

effect on the stiffness. The mean effective stress depends on the following

two factors.

128

(1) Horizontal stresses locked-in during compaction and

negative capillary stresses developed during drying are

significant in defining the initial stiffness of the material. This

initial state of stress in the material governs the measurement

of shear wave velocities in seismic tests.

(2) The increase in mean effective stress due to increased load

on the plate during plate-load testing depends on the

distribution of vertical stress with depth as well as the degree

to which horizontal stresses are maintained after the material is

unloaded.

Improving the determination of the state of stress at any point before,

during, or after loading will aid in quantifying the effects of stress state on

seismically determined moduli. Despite its limitations, seismic testing has

been used to show material behavior not previously observed in granular

bases such as the significant stiffening effect of negative capillary stresses.

Further research in determining small-strain stiffnesses under pavement

surfaces during loading will help to improve the prediction of working load

displacements with seismic tests.

8.3 Large-Strain Stiffness Comparisons

Stiffness measurements from plate-load tests were taken under a

variety of loading conditions. Throughout the tests, it was observed that the

measured equivalent spring constant, keff, was influenced by the magnitude of

129

the load, duration of loading, moisture content, stress history, and strain

amplitude. Although the material response discussed below involves strain

amplitude outside the linear range and is influenced by the aforementioned

factors simultaneously, the relative magnitude of the stiffnesses offer some

useful insight about the material behavior.

8.3.1 Short Duration Load to Failure

The deformational properties of the compacted material can be

observed through a range of loading states from the results of Test A4 in

which the material was subjected to a 40 kip (178 kN) load after having

undergone 85 mils (2.2 mm) of permanent deformation from the three

previous tests. This displacement corresponds to an average strain on the

order of 0.4 % when an effective depth of two plate diameters is assumed.

The stress history may be considered analogous to the “conditioning” imposed

by compaction of an asphalt cement concrete surface course over an unbound

base course.

The load was applied over a period of approximately 7 minutes and

released over a period of 3 minutes. The continuous load-settlement curve has

been replotted in Fig. 8.7 for convenience. For a loading range from 0 to

about 18 kips (0 to 80 kN), the material exhibited the same approximately bi-

linear stiffness behavior observed during loading in the large amplitude cyclic

tests on both the wetter and drier material (Tests A3 and C2). At a load of 18

kips (80 kN) which corresponds to a pressure of 159 psi (1096 kPa), the

130

material began to soften, and the plate displacements began to increase.

While it is not clear that this point should be called failure, it is a significant

point of interest in the deformational behavior of the material. The maximum

load was less than about 11 kips (49 kN) or 97 psi (669 kPa) in all of the other

plate-load tests performed on the Georgetown test pad. It should also be

noted that the pressure on the base course in a layered pavement system would

not be expected to reach 159 psi (1196 kPa) under even the most severe

loading as previously illustrated in Fig. 4.1. The increased displacement at

the end of the loading cycle was the result of maintaining the 40-kip (178 kN)

load on the plate for 2 minutes.

131

160

140

120

100

80

60

40

20

0403020100

Force (kips)

4

3

2

1

0

200150100500Force (kN)

AA4

BA4

CA4

Fig. 8.7. Illustration of Material Response Under Load to “Failure”; Test A4, Loading from 0 to 40 kips (0 to 178 kN) in 6 Minutes and Unloading to Zero

The failure load was also estimated using bearing capacity theory. For

an effective friction angle, φ’, of 45 degrees, the bearing capacity was

calculated to be about 132 psi (910 kPa) which corresponds to 15 kips (67 kN)

on a 1 ft (30.5 cm) diameter loading plate. This is near the load at which the

material began to soften during the plate-load test as shown in Fig. 8.7.

132

8.3.2 Stiffness Under Large-Amplitude, Short-Duration Cyclic

Loading

The base course material was tested under cyclic loads of 8 or 9 kips

(36 or 40 kN) in both the drier and the wetter state. The duration of loading

was varied from 11 seconds to 3.7, minutes with most cycles lasting less than

one minute. The continuous load-settlement curves from Test A3 (drier

material) and Test C2 (wetter material) are plotted again for convenience in

Figs. 8.8 and 8.9, respectively.

It was observed that the cyclic plate-load tests showed a 65-70 %

reduction in stiffness during loading from the dry test to the wet test. This is

shown graphically in Fig. 8.10 where loop 5 for the test on drier material (Test

A3) and loop 5 for the test on wetter material (Test C2) have been plotted

together. The reduction in stiffness was observed for both the initial and final

sections of the loading curve.

133

30

2520

15

105

0121086420

Force (kips)

0.8

0.6

0.4

0.2

0.0

50403020100Force (kN)

Plate Diameter:12 in. (305 mm)Effective Plate Area:107 in.2 (690 cm2)

Loop 30Loop 25

Fig. 8.8. Continuous Force-Displacement Plot from Test A3; 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

30

2520

15

105

0121086420

Force (kips)

0.8

0.6

0.4

0.2

0.0

50403020100Force (kN)


Fig. 8.9. Continuous Force-Displacement Plot from Test C2; 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

134

0.6

0.4

0.2

0.0

403020100Force (kN)

30

25

20

15

10

5

01086420

Force (kips)

Loop 5 - Test A3 - Drier Base Course

Loop 5 - Test C2 - Wetter Base Course

Fig. 8.10. Comparison of Stiffness Under Large Amplitude Cyclic Loading; Loop 5 from Tests A3 (Drier Material) and C2 (Wetter Material)

The seismically determined stiffnesses were compared to the

stiffnesses determined during the large-amplitude cyclic loading cycles in the

plate-load tests. The stiffness data for loop 5 from Test A3 (drier material)

and for loop 5 from Test C2 (wetter material) are presented in Table 8.3. The

seismically determined stiffness values are also presented graphically for

comparison in Figs. 8.11 and 8.12 for the drier base course and wetter base

course, respectively.

135

Table 8.3. Comparison of Stiffness Values Determined from Large Amplitude Plate-Load Tests and from Seismic Tests

Test/Moisture Condition Equivalent Spring Constant, keff, (Stiffnesses), lb/in.

Initial Portion of Loading

Curve From Plate-Load

Test 1

Final Portion of Loading Curve From Plate-Load

Test 1

SASW Adjusted for

Strain Amplitude 2

Test A3 - Drier Material 0.45 X 106 0.78 X 106 1.53 X 106 Test C2 - Wetter Material

0.16 X 106 0.65 X 106 0.67 X 106

Test/Moisture Condition Equivalent Spring Constant, keff, (Stiffnesses),

kN/m Initial

Portion of Loading

Curve From Plate-Load

Test 1

Final Portion of Loading Curve From Plate-Load

Test 1

SASW Testing Adjusted for

Strain Amplitude 2

Test A3 - Drier Material 0.078 X 106 0.14 X 106 0.27 X 106 Test C2 - Wetter Material

0.028 X 106 0.11 X 106 0.12 X 106

1 Working-load stiffness values were determined from large-amplitude cyclic plate-load tests. The bi-linear stiffness values from loop 5 in Test A3 (drier material) and from loop 5 in Test C2 (wetter material) were used for comparison. 2 Seismic stiffnesses were adjusted for strain amplitude, but were not adjusted for increased state of stress from loading of the plate.

136

0.6

0.4

0.2

0.0

403020100Force (kN)

30

25

20

15

10

5

01086420

Force (kips)

Slopes NearlyParallel

SeismicallyDeterminedStiffnesskeff = 1.53 X 106 lb/in.(0.27 X 106 kN/m)

Fig. 8.11. Comparison of Seismically Determined Stiffness with Plate-Load Stiffness for Large Amplitude Cyclic Plate-Load Tests; Loop 5, Test A3 - 32 Cycles of 0 to 9 kips (40.0 kN) in 20 Minutes on Drier Material (Moisture Content ~ 4.7 %)

0.6

0.4

0.2

0.0

403020100Force (kN)

30

25

20

15

10

5

01086420

Force (kips)

Slopes NearlyParallel

SeismicallyDeterminedStiffnesskeff = 0.67 X 106 lb/in.(0.12 X 106 kN/m)

Fig. 8.12. Comparison of Seismically Determined Stiffness with Plate-Load Stiffness for Large Amplitude Cyclic Plate-Load Tests; Loop 5, Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

Stiffness calculations from the unloading curves showed considerably

more scatter than those from the loading curves. It was therefore difficult to

make meaningful conclusions from observation of the stiffness during

unloading. It can be observed, however, that the material was stiffest at the

beginning of the rebound curve. This is presumed to be the result of locked-in

horizontal stresses from the loading portion of the cycle.

Finally, it was observed that the large-amplitude cyclic load-settlement

curves began to “close” or plot on top of one another after a short number of

cycles as shown in Fig. 8.13 for the drier material and in Fig. 8.14 for the

wetter material. However, the displacement began to increase more rapidly if

even a slight increase in loading was applied to the plate. It was observed in

Test A3 on the drier material that the displacement periodically stabilized for

a few cycles, and then increased for a few cycles. Loops 23 and 24 are plotted

in Fig. 8.15 to illustrate this point. The loops in Fig. 8.15 have “closed”,

however the maximum displacement is significantly greater than that

observed for the two loops in Fig. 8.13 which were recorded earlier in the

same test (Test A3).

139

30

25

20

15

10

5

01086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

Loop 5Loop 6

Loop 7

Fig. 8.13. Closure of Cyclic Loops From Plate-Load Test A3; 32 Cycles of 0 to 9 Kips on Drier Material (w ~ 4.7 %)

30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

Loop 4

Loop 5

Fig. 8.14. Closure of Cyclic Loops From Plate-Load Test C2; 5 Cycles of 0 to 8 Kips on Wetter Material (w ~ 6.1 %)

140

30

25

20

15

10

5

01086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

Loops 23 and 24

Fig. 8.15. Closure of Cyclic Loops From Plate-Load Test A3 After Accumulated Displacement from Application of Previous Loops; 32 Cycles of 0 to 9 Kips on Drier Material (w ~ 4.7 %)

8.3.3 Stiffnesses From Incremental Plate-Load Tests

Two nearly identical incremental plate-load tests were performed, one

on the drier material (B1) and one on the wetter material (C1). A maximum

force of 8.3 to 8.6 kips (36.9 to 38.3 kN) was applied to the plates over

approximately 3 to 3.5 hours. The maximum displacement with the drier

material was 51 mils (1.3 mm), and the maximum displacement with the

wetter material was 91 mils (2.3 mm). The permanent displacement from the

test on the drier material was approximately 67 % of the maximum

displacement, while the permanent displacement from the test on the wetter

material was 73 % of the maximum. The stiffness of the wetter material was

141

61 % less than that of the drier material at the beginning of loading, but only

20 % less during the latter part of the loading curve. This is slightly different

from the behavior observed in the short duration cyclic tests in which the

stiffness at every point during loading was reduced 65-70% due to wetting.

The adjusted load-settlement curves are plotted in Chapters 5 and 6

without zeroing the displacement measurement after application of the seating

load. Both curves are replotted in Figs. 8.16 and 8.17 without the small

unloading-reloading loops and with re-zeroing to the displacement measured

after the seating load was applied.

The overall stiffnesses for the incremental tests were the lowest

observed among all of the plate-load tests conducted on the test pad. These

overall stiffnesses are not particularly meaningful in terms of the displacement

expected from normal traffic loads, but may be relevant for predicting

displacement due to stationary pavement loads on parking areas and cargo

staging facilities. In addition, the moduli determined at a pressure of 10 psi

(69 kPa) in the incremental plate-load tests are the values used in rigid

pavement design procedures (AASHTO, 1986) as noted in Figs. 8.16 and

8.17. This pressure corresponds to a load of 1131 lb (5.0 kN) on the 1 ft (30.5

cm) diameter plate.

The modulus of the material at a pressure of 10 psi (69.0 kPa) is

referred to as the modulus of soil reaction, ku’. The modulus is determined

from the load-settlement plot after zeroing for the seating load. The values of

modulus of soil reaction as determined by the procedure given in AAHSTO

142

T-222 (1986) are recorded in Table 8.3 for reference. A 70% reduction in

modulus of soil reaction was observed as a result of wetting the near-surface

material. Table 8.4 AASHTO T-222 Modului of Soil Reaction for Georgetown Crushed Limestone Test Pad

Test Modulus of Soil Reaction, ku’ (lb/in3) (MN/m3)

B1, Drier Material 2141 581 C1, Wetter Material 635 172

8.3.4 Sensitivity of the Material to Duration of Loading

The stiffness of the unbound granular base material has been shown to

be dependent on the duration of loading. This is again illustrated in Fig. 8.18

for the drier material and in Fig. 8.19 for the wetter material. The initial

loading curves for the cyclic test and the incremental test were plotted

together on one graph for each moisture condition of the base course. The

plots show the time-dependent displacement behavior observed during plate-

load testing. It should be noted that the cyclic tests were conducted after other

plate-load tests had been conducted at the same location. The displacements

are, therefore, slightly less than those which would be expected had no

previous load been applied to the plate. It should be noted that the seismically

determined stiffnesses fit the end of the cyclic loading curves well especially

for the later loops after much of the initial volume change in the material had

taken place.

143

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100

Force (kN)


ku' determined at10 psi (69 kPa)1.1 kips (4.9 kN)

Fig. 8.16. Long-Term, Adjusted Load-Settlement Curve; Test (B1) on Drier Material (w ~ 4.7 %)

100

80

60

40

20

01086420

Force (kips)

2.5

2.0

1.5

1.0

0.5

0.0

403020100

Force (kN)


ku' determined at10 psi (69 kPa)1.1 kips (4.9 kN)

Fig. 8.17. Long-Term, Adjusted Load-Settlement Curve; Test (C1) on Wetter Material (w ~ 6.1 %)

144

50

40

30

20

10

01086420

Force (kips)

1.2

1.0

0.8

0.6

0.4

0.2

0.0

403020100Force (kN)


Duration of LoadingCycle = 0.94 minutes(Test A3 - cyclic loading)Duration of Loading

Cycle = 97 minutes(Test B1 - incremental test)

SeismciallyDeterminedStiffness 1

Fig. 8.18. Illustration of the Effect of Duration of Loading on Unbound Granular Base Course Material; Plate-Load Tests at Location C on Drier Material (Moisture Content ~ 4.7%) 1 Seismically Determined Stiffness Corrected for Strain Amplitude, but not for State of Stress

80

60

40

20

01086420

Force (kips)

2.0

1.5

1.0

0.5

0.0

403020100Force (kN)


Duration of LoadingCycle = 2.7 minutes(Test C2 - cyclic test)

Duration of LoadingCycle = 125 minutes(Test C1 - incremental test)

Seismically Determined Stiffness 1

Fig. 8.19. Illustration of the Effect of Duration of Loading on Unbound Granular Base Course Material; Plate-Load Tests at Location C on Wetter Material (Moisture Content ~ 6.1%) 1 Seismically Determined Stiffness Corrected for Strain Amplitude, but not for State of Stress

8.4 Summary of Stiffness Comparison

The results of plate-load and seismically-determined stiffness

measurements show unbound granular material to be sensitive to duration of

loading and moisture content. It was observed that the material was more

sensitive to moisture content at low loads [< 2 or 3 kips (8.9 to 13.3 kN] than

at high loads. This threshold corresponds to an average pressure directly

under the plate of 17.7 to 26.5 psi (122 to 183 kPa). Significant softening was

observed at low loads in both types of tests with increased wetting.

Seismic tests have the advantage of being able to show the softening

effects of wetting or surface disturbance without damaging the pavement.

Seismically determined stiffness measurements show promise in the

prediction of working load displacements if adjusted for increases in material

stiffness during loading. The importance of the adjustment may be minimized

significantly if seismic tests are performed on a layered pavement system,

because smaller pressures and smaller strains will be induced in the granular

material under a pavement wearing course.

147

CHAPTER NINE

SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

9.1 Summary of Testing Program

Plate-loading and seismic tests were performed on a test pad

constructed at the Capitol Aggregates quarry in Georgetown, Texas. The test

pad was constructed to determine whether or not the crushed limestone base

course material could be compacted in single lifts thicker than 6 to 8 in. (15.2

cm to 20.3 cm) while still achieving the same material properties as would be

achieved by compacting in thinner single lifts. Plate-load tests and seismic

tests were conducted on the surface of the Georgetown test pad to evaluate the

use of seismic stiffness measurements for predicting working-load

displacements.

The test pad was constructed of unbound crushed limestone which is

commonly used in the construction of pavement base courses. The material

had a nominal maximum particle size of 1.5 in. (3.8 cm), a modified Proctor

maximum density of 141 pcf (22.1 kN/m3), and an optimum moisture content

of 6.5 %. The fines were non-plastic. The portion of the test pad used for

plate-load and seismic testing was constructed in two thick lifts and was

compacted with a vibratory pad foot roller. The first lift was 12 in. (30.5 cm)

thick and the second lift was 23 in. (58.4 cm) thick. Nuclear density gauge

measurements were made before the plate-load and seismic tests, and

149

laboratory moisture content specimens were collected from beneath the plate

at the conclusion of each series of plate-load tests.

The plate-load tests were conducted using the mechanical loading

mechanism of the Rolling Dynamic Deflectometer (RDD) developed at The

University of Texas at Austin (Bay 1997). The RDD was used in a stationary

mode to deliver static loads to a plate on the surface of the test pad. The

loading mechanism of the RDD also allowed the application of short duration

cyclic loads to the plate, and was used to apply small amplitude [1 kip (4.4

kN)] and large amplitude [9 kip (40 kN)] cyclic loads with a loading duration

of less than one minute. The time-dependent deformational behavior of the

unbound crushed limestone was observed from the results of the plate-load

tests conducted with different loading rates.

Continuous measurements of load and displacement were made during

the plate-load tests using the computer-based data acquisition system installed

in the RDD. The load applied to the plate was measured with a load cell

mounted on the loading frame of the RDD. The displacement of the plate was

measured with Linear Variable Differential Transformers (LVDTs) which

were held in place with an aluminum framework constructed specifically for

this application.

Seismic surface wave tests were conducted on the surface of the test

pad directly before the plate-load tests were conducted. The Spectral-

Analysis-of-Surface-Waves (SASW) test was used to determine profiles of

150

shear wave velocity with depth. Shear stiffness and axial stiffness profiles

were calculated from the SASW results.

The softening effect of adding moisture to the unbound aggregate base

course was evaluated by performing plate-load and seismic tests on both drier

base course (moisture content ~ 4.7%) and wetter base course (moisture

content ~ 6.1%). Plate-load and seismic tests were first conducted on the test

pad in a drier state and later conducted near the same location on the test pad

after adding moisture to the test pad from the surface.

9.2 Summary of Observations

The unbound granular base material used to construct the test pad in

the Capitol Aggregate quarry exhibited time-dependent settlement under

sustained loading conditions. Although the granular base contained only 12%

material passing the #40 (0.425 mm) sieve, and the fines were non-plastic, the

loading plate continued to displace with time under a constant load. This

response under loading is more commonly associated with somewhat

cohesive-type material. This time-dependent behavior suggests significant

variation in the load-displacement performance depending on the duration of

loading.

It was observed that plate-load testing in the range of working loads

similar to those induced by 18 kip (80 kN) Equivalent Single Axle Loads

(ESALs) produced surface displacements which corresponded to strain

amplitudes in the unbound aggregate base course that fall in the nearly linear

151

to mildly non-linear range of stiffness. It is estimated that an adjustment to

the small-strain modulus determined by seismic tests is necessary to account

properly for the difference in strain amplitude between seismic tests and static

plate-load tests. This adjustment is made by multiplying the small-strain

(seismic) modulus by a factor which ranges from 0.75 to 0.95.

Factors such as natural cementation, negative capillary stresses,

locked-in horizontal stresses during compaction, and locked-in horizontal

stress during unloading had a stiffening effect on the unbound granular

material tested at the Georgetown, Texas test site. The addition of moisture

from the surface of the test pad was found to have a softening effect on the

unbound granular base material. This is presumably due mainly to the relief

of negative capillary stresses, but may also be due to reducing any

contribution from natural cementation. The softening effect when moisture

was added was observed in both the plate-load tests and the seismic surface

wave tests.

Seismic test results were also used to show that the material near the

surface of the compacted test pad was significantly softer than the material at

a depth of 6 in. (15 cm) or greater immediately after compaction. However,

the seismic tests also showed that this softer zone became just as stiff as (or

stiffer than) the underlying material once it dried. Compression of the softer,

near-surface material when the base course was tested in a wetter state is felt

to have contributed significantly to the displacement of the loading plate

during plate-load testing. It has been shown by Bueno et al. (1998) that the

152

disturbed layer near the surface of an unbound aggregate base course is later

compacted when another layer is compacted on top of the initially disturbed

material. For this reason, it is expected that displacements measured in the

plate-load test would be significantly smaller if the test pad had been paved

with asphalt concrete or Portland cement concrete wearing course, even if the

pressure delivered to the granular base course layer was the same as for the

plate-load tests on the unpaved test pad.

Paving the test pad would also have the effect of reducing the strain

amplitude in the granular base course layer. This would improve the

prediction of surface displacements using seismically determined stiffness

values.

9.3 Conclusions from Plate-Load and Seismic Tests

Seismic tests show potential for predicting pavement surface

displacements under working loads. However, fundamental differences exist

between seismic tests and plate-load tests which must be taken into account

when predicting settlement from the seismic test measurements. The strain

amplitudes induced by plate-load testing ranged from slightly greater to much

greater than those induced by seismic tests. Estimates of strain amplitudes

were made from small, unload-reload cycles [1 kip (4.4 kN)] during plate-load

tests on the unbound aggregate test pad. These strain amplitudes were found

to be slightly above the linear range. This increased strain amplitude

153

corresponded to a 5-25% reduction in stiffness between seismically measured

stiffnesses and stiffness under working traffic loads.

Larger amplitude unload-reload loops [9 kips (40.0 kN)] showed

greater surface displacement and thus greater strain amplitudes than the

smaller unload-reload loops [1 kip (4.4 kN)]. However, no constant vertical

load was applied to the material during the larger unload-reload cycles as was

the case for the smaller unload-reload loops. Compression of the less-stiff,

near-surface material accounted for a large part of the displacement under the

larger magnitude cyclic loading. It was observed that the softening effects of

wetting and the effect of the presence of a softer layer near the surface of the

test pad were both diminished significantly when the load on the plate was

increased above 2 or 3 kips (8.9 to 13.3 kN). This corresponds to a pressure

directly beneath the plate of about 18 to 27 psi (124 to 186 kPa).

Additional adjustment to the small-strain stiffnesses measured

seismically is necessary in this test pad material if displacements under

working traffic loads are to be predicted. The difference in the state of stress

in the material under load must be accounted for in the prediction.

Theoretical solutions for displacements under working loads will also have to

account for the effects of locked-in horizontal stresses, any natural

cementation, and negative capillary stresses.

Despite the fact that stiffnesses measured with seismic tests must be

adjusted for strain amplitude and state of stress, the SASW test has been

shown to be very effective in detecting the relative softening of granular

154

155

material due to wetting or disturbance near the surface. These changes in

stiffness could not be detected by field tests which measure density.

9.3 Recommendations for Future Work

Understanding of the behavior of unbound granular base courses under

working loads could be improved through in-situ seismic testing of complete

pavement systems. The state of stress under a compacted pavement is

expected to be higher than the state of stress in the granular base course test

pad built in Georgetown, Texas. Increasing the state of stress by adding a

confining layer of asphalt concrete or Portland cement concrete would

decrease displacements and strain amplitudes in the unbound granular base

course layer. It is expected that working loads applied to the pavement

wearing course would induce strains in the base course closer to the small-

strain range under which seismic tests are performed. This reduction in strain

amplitudes will improve the prediction of surface displacements using

seismically determined stiffness measurements.

Additional deflection testing under working loads would also add to

the understanding of the relationship between seismically determined

stiffnesses and working load displacements. Additional material types should

be tested to investigate the time-dependent characteristics under slow loading.

APPENDIX A

NUCLEAR DENSITY GAUGE DATA

155

Table A.1. Nuclear Density Gauge Data for Drier Material (w ~ 4.7 %); Collected by Gene Schlieker, Texas Transportation Institute Date: 11 Mar 98 Location A (drier test, moisture content ~ 4.7 %)

Station # (Location)

Depth DC MC Wet Density Dry Density M % Moisture

N#1 12 88 135.9 129.3 6.6 5.1(not used) 10 418 80 132.8 126.2 6.6 5.2

8 785 91 129.0 122.1 6.9 5.76 1345 88 123.4 112.8 6.6 5.74 2008 84 116.1 109.8 6.2 5.72 2414 90 104.5 97.7 6.8 7.0

Back Scatter N#1 12 217 91 135.6 128.7 6.9 5.4Second Test 10 402 89 134.2 127.4 6.7 5.3

8 793 91 128.6 121.7 6.9 5.76 1367 93 122.6 115.5 7.1 6.24 2045 95 115.1 107.7 7.3 6.82 2460 91 103.4 96.4 6.9 7.2

Back Scatter 720 96 101.4 95.0 6.4 6.8S#1 12 262 84 129.6 123.3 6.2 6.2

10 482 83 127.9 121.7 6.1 5.08 894 80 124.0 118.1 5.8 4.96 1499 85 118.5 112.2 6.3 5.64 2140 81 112.7 106.8 5.9 5.62 2479 87 102.9 96.4 6.5 6.8

Back Scatter 734 86 100.3 93.8 6.4 6.8

156

Table A.1. Continued

N#2 12 236 88 132.9 126.3 6.6 5.2

10 430 90 131.8 125.0 6.8 5.58 794 86 128.6 122.1 6.4 5.36 1362 85 122.8 116.5 6.3 5.44 2004 87 116.1 109.6 6.5 6.02 2431 89 104.1 97.4 6.7 6.9

Back Scatter 665 81 106.3 100.4 5.9 5.9S#2 12 248 89 131.3 124.6 6.7 5.4

10 436 87 131.3 124.8 6.5 5.28 785 92 129.0 122.0 7.0 5.76 1333 87 123.8 117.3 6.5 5.64 1956 88 117.4 110.8 6.6 6.02 2273 87 108.1 101.6 6.5 6.4

Back Scatter 681 87 104.8 98.1 6.7 6.8

Fig. 6.9. Variation of Displacement with Time; Test C2 - 5 Cycles of 0 to 8 kips (35.6 kN) in 9 Minutes on Wetter Material (Moisture Content ~ 6.1%)

157

158

Table A.2. Nuclear Density Gauge Data for Wetter Material (w ~ 6.1 %); Collected by Gene Schlieker, Texas Transportation

Institute

Date: 10 Apr 98 Location C (wetter test, laboratory moisture content ~ 6.1 %))

Station #

(Location) Depth DC MC Wet Density Dry Density M % Moisture

NW 12 226 149.0 139.6 9.4 6.7 10 398 150.1 140.0 10.2 7.3 8 729 149.0 138.5 10.5 7.5 6 1277 145.6 135.6 10.0 7.4 4 1907 141.7 131.8 9.9 7.5 2 2255 135.0 125.1 9.9 7.9 Back Scatter 660 135.3 125.6 9.7 7.7

SW 12 258 144.5 134.3 10.2 7.6 10 470 144.2 134.2 10.0 7.4 8 817 144.6 134.2 10.4 7.7 6 1368 142.5 132.8 9.7 7.3 4 2002 139.2 128.7 10.5 8.1 2 2309 133.6 123.7 9.9 8.0 Back Scatter 689 132.4 122.4 10.1 8.2

APPENDIX B

INDIVIDUAL LOADING CYCLE PLOTS FOR TEST A3

159

30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.267 X 106 lb/in.(0.0468 X 106 kN/m)

k = 0.611 X 106 lb/in.(0.107 X 106 kN/m)

Fig. B.1. Test A3; Loop 1 of 32 - 32 Loading Cycles from 0 to 9 kips on Drier Material (w ~ 4.7 %)

30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.864 X 106 lb/in.(0.151 X 106 kN/m)

k = 0.473 X 106 lb/in.(0.0828 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.709 X 106 lb/in.(0.124 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.356 X 106 lb/in.(0.0623 X 106 kN/m)

k = 0.723 X 106 lb/in.(0.127 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.452 X 106 lb/in.(0.0792 X 106 kN/m)

k = 0.785 X 106 lb/in.(0.137 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.399 X 106 lb/in.(0.0699 X 106 kN/m)

k = 0.868 X 106 lb/in.(0.152 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.495 X 106 lb/in.(0.0867 X 106 kN/m)

k = 0.923 X 106 lb/in.(0.162 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.451 X 106 lb/in.(0.0790 X 106 kN/m)

k = 0.726 X 106 lb/in.(0.127 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.336 X 106 lb/in.(0.0588 X 106 kN/m)

k = 0.846 X 106 lb/in.(0.148 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.556 X 106 lb/in.(0.0974 X 106 kN/m)

k = 0.843 X 106 lb/in.(0.146 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.458 X 106 lb/in.(0.0802 X 106 kN/m)

k = 0.886 X 106 lb/in.(0.155 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.440 X 106 lb/in.(0.0771 X 106 kN/m)

k = 0.827 X 106 lb/in.(0.145 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.618 X 106 lb/in.(0.108 X 106 kN/m)

k = 1.32 X 106 lb/in.(0.231 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.379 X 106 lb/in.(0.0664 X 106 kN/m)

k = 1.17 X 106 lb/in.(0.205 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.465 X 106 lb/in.(0.0814 X 106 kN/m)

k = 1.10 X 106 lb/in.(0.193 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.613 X 106 lb/in.(0.107 X 106 kN/m)

k = 1.23 X 106 lb/in.(0.215 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.634 X 106 lb/in.(0.111 X 106 kN/m)

k = 1.52 X 106 lb/in.(0.266 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 1.17 X 106 lb/in.(0.205 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.593 X 106 lb/in.(0.104 X 106 kN/m)

k = 1.29 X 106 lb/in.(0.226 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.501 X 106 lb/in.(0.0877 X 106 kN/m)

k = 1.17 X 106 lb/in.(0.205 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.386 X 106 lb/in.(0.0676 X 106 kN/m)

k = 1.19 X 106 lb/in.(0.208 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.633 X 106 lb/in.(0.111 X 106 kN/m)

k = 1.40 X 106 lb/in.(0.245 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.357 X 106 lb/in.(0.0625 X 106 kN/m)

k = 1.04 X 106 lb/in.(0.182 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.594 X 106 lb/in.(0.104 X 106 kN/m)

k = 1.21 X 106 lb/in.(0.212 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.595 X 106 lb/in.(0.104 X 106 kN/m)

k = 1.28 X 106 lb/in.(0.224 X 10 6 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.433 X 106 lb/in.(0.0758 X 106 kN/m)

k = 1.02 X 106 lb/in.(0.179 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.676 X 106 lb/in.(0.118 X 106 kN/m)

k = 0.939 X 106 lb/in.(0.164 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.633 X 106 lb/in.(0.111 X 106 kN/m)

k = 0.827 X 106 lb/in.(0.145 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.587 X 106 lb/in.(0.103 X 106 kN/m)

k = 1.14 X 106 lb/in.(0.200 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.676 X 106 lb/in.(0.118 X 106 kN/m)

k = 1.61 X 106 lb/in.(0.282 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 0.574 X 106 lb/in.(0.101 X 106 kN/m)

k = 1.09 X 106 lb/in.(0.191 X 106 kN/m)


30

25

20

15

10

5

0121086420

Force (kips)

0.6

0.4

0.2

0.0

50403020100Force (kN)

k = 1.22 X 106 lb/in.(0.214 X 106 kN/m)

k = 0.695 X 106 lb/in.(0.122 X 106 kN/m)


APPENDIX C

INDIVIDUAL UNLOAD-RELOAD LOOPS FOR TEST B1

16

14

2.22.01.81.61.41.21.0

Force (kips)

0.380.360.34

k = 0.82 X 106 lb./in.(0.14 X 106 kN/m)

16

14

0.380.360.34

k = 1.9 X 106 lb./in.(0.33 X 106 kN/m)

16

14

0.380.360.34

k = 1.6 X 106 lb./in.(0.28 X 106 kN/m)

16

14

0.380.360.34

k = 1.6 X 106 lb./in.(0.28 X 106 kN/m)

16

14

0.380.360.34

10987654

Force (kN)

k =2.2 X 106 lb./in.(0.39 X 106 kN/m)

Fig. C.1. Test B1, Individual Unload-Reload Loops, Group AB1

193

28

26

4.54.03.53.0

Force (kips)

0.72

0.70

0.68k = 4.5 X 106 lb./in.(0.79X 106 kN/m)

28

26

0.72

0.70

0.68k = 32.3 X 106 lb./in.(5.7 X 106 kN/m)

28

26

0.72

0.70

0.68k = 2.2 X 106 lb./in.(0.39 X 106 kN/m)

28

26

0.72

0.70

0.68k = 1.5 X 106 lb./in.(0.26 X 106 kN/m)

28

26

0.72

0.70

0.68

20181614

Force (kN)

k =1.0 X 106 lb./in.(0.18 X 106 kN/m)

Fig. C.2. Test B1, Individual Unload-Reload Loops, Group BB1

194

504846

4.03.83.63.43.23.02.82.62.4

Force (kips)

1.28

1.24k = 1.4 X 106 lb./in.(0.25 X 106 kN/m)

504846

1.28

1.24k = 2.2 X 106 lb./in.(0.39 X 106 kN/m)

504846

1.28

1.24k = 2.4 X 106 lb./in.(0.42 X 106 kN/m)

504846

1.28

1.24k = 2.8 X 106 lb./in.(0.49 X 106 kN/m)

504846

1.28

1.24

18161412

Force (kN)

k =2.8 X 106 lb./in.(0.49 X 106 kN/m)

Fig. C.3. Test B1, Individual Unload-Reload Loops, Group CB1

195

45

402.22.01.81.61.41.21.0

Force (kips)

1.16

1.12k = 1.2 X 106 lb./in.(0.21 X 106 kN/m)

45

40

1.16

1.12k = 1.2 X 106 lb./in.(0.21 X 106 kN/m)

45

40

1.16

1.12k = 1.3 X 106 lb./in.(0.23 X 106 kN/m)

45

40

1.16

1.12k = 0.86 X 106 lb./in.(0.15 X 106 kN/m)

45

40

1.16

1.12

1098765

Force (kN)

k =1.2 X 106 lb./in.(0.21 X 106 kN/m)

Fig. C.4. Test B1, Individual Unload-Reload Loops, Group DB1

196

Table C.1. Stiffnesses of Individual Small-Amplitude [1 kip (4.4 kN)] Unload-Reload Loops; Test B1, Incremental Plate-Load Test from 0 to 8.3 kips (36.9 kN) on Drier Material (Moisture Content ~ 4.7%)

Drier Material (moisture content ~ 4.7%)

ρ = 135 (lb/ft3) From nuclear density tests

ν = 0.25

ro = 0.5 (ft) Radius of plate

Cursors

Loop Number A B

Numpts in loop Slope k (from plt)

Average k for Each Group (from plt)

Relative Shear Wave Velocity, vsi (ft/sec)

k (from SASW)

Lbs/in Lbs/in k-4Gro/(1-ν)

First Set of Loops 1-1 75885 77691 1835 1.22 6.203+05

1-2 77491 79378 1887 0.53947 1.85E+06

1-3 79205 80927 1722 0.62856 1.59E+06

1-4 80715 82437 1722 0.643 1.56E+06

1-5 82074 83502 1428 0.446 2.24E+06 1.81 E+06 800 5.96 E+05

Second Set of Loops 2-1 114466 116186 1720 0.222 4.50E+06

2-2 115930 117449 1519 0.031 3.23E+07

2-3 117352 118605 1253 0.447 2.24E+06

2-4 118436 119912 1476 0.663 1.51E+06

2-5 119832 121112 1280 0.964 1.04E+06 8.31E+06 1100 1.13E+06

Third Set of Loops 3-1 247739 250068 2329 0.7337 1.36E+06

3-2 249871 251668 1797 0.45603 2.19E+06

3-3 251524 252869 1345 0.42034 2.38E+06

3-4 252727 254128 1401 0.35297 2.83E+06

3-5 254015 255081 1066 0.36267 2.76E+06 2.54E+06 #REF!

Fourth Set of Loops 4-1 316719 318471 1752 0.84551 1.18E+06

4-2 318282 319876 1594 0.824 1.21E+06

4-3 319638 321001 1363 0.788 1.27E+06

4-4 320777 322094 1317 1.17 8.65E+06

4-5 321903 323220 1317 0.847 1.18E+06 1.21E+06 #REF!

Note: Shaded boxes not used in the average

APPENDIX D

INDIVIDUAL UNLOAD-RELOAD LOOPS FOR TEST C1

50.049.048.0

3.02.82.62.42.22.01.81.6

Force (kips)

1.28

1.24

1.20

k = 1.0 X 106 lb./in.(0.18 X 106 kN/m)

50.049.048.0

1.28

1.24

1.20

k = 0.76 X 106 lb./in.(0.13 X 106 kN/m)

50.049.048.0

1.28

1.24

1.20

k = 0.95 X 106 lb./in.(0.17 X 106 kN/m)

50.049.048.0

1.28

1.24

1.20

k = 0.79 X 106 lb./in.(0.14 X 106 kN/m)

50.049.048.0

1.28

1.24

1.20

13121110987

Force (kN)

k = 1.1 X 106 lb./in.(0.19 X 106 kN/m)

Fig. D.1. Test C1, Individual Unload-Reload Loops, Group AC1

199

60.059.559.058.5

4.54.03.53.0

Force (kips)

1.52

1.48k = 2.8 X 106 lb./in.(0.49 X 106 kN/m)

60.059.559.058.5

1.52

1.48k = 1.5 X 106 lb./in.(0.26 X 106 kN/m)

60.059.559.058.5

1.52

1.48

k = 1.2 X 106 lb./in.(0.21 X 106 kN/m)

60.059.559.058.5

1.52

1.48

k = 2.8 X 106 lb./in.(0.49 X 106 kN/m)

60.059.559.058.5

1.52

1.48

20181614

Force (kN)

k = 2.9 X 106 lb./in.(0.51 X 106 kN/m)

Fig. D.2. Test C1, Individual Unload-Reload Loops, Group BC1

200

92.091.691.2

9.59.08.58.07.57.0

Force (kips)

2.352.342.332.32

k = 9.0 X 106 lb./in.(1.6 X 106 kN/m)

92.091.691.2

2.352.342.332.32

k = 32 X 106 lb./in.(5.5X 106 kN/m)

92.0

91.6

91.2

2.352.342.332.32k = 18.7 X 106 lb./in.

(3.3 X 106 kN/m)

92.091.691.2

2.352.342.332.32k = 11.5 X 106 lb./in.

(2.0 X 106 kN/m)

92.091.691.2

2.352.342.332.32

424038363432

Force (kN)

k = 3.7 X 106 lb./in.(0.65 X 106 kN/m)

Fig. D.3. Test C1, Individual Unload-Reload Loops, Group CC1

201

88.0

87.0

4.44.24.03.83.63.43.23.0

Force (kips)

2.232.222.212.20

k = 2.8 X 106 lb./in.(0.49 X 106 kN/m)

88.0

87.0

2.232.222.212.20

k = 12 X 106 lb./in.(2.1 X 106 kN/m)

88.0

87.0

2.232.222.212.20k = 3.1 X 106 lb./in.

(0.54 X 106 kN/m)

88.0

87.0

2.232.222.212.20

k = 3.3 X 106 lb./in.(0.58 X 106 kN/m)

88.0

87.0

2.232.222.212.20

20191817161514

Force (kN)

k = 2.8 X 106 lb./in.(0.49 X 106 kN/m)

Fig. D.4. Test C1, Individual Unload-Reload Loops, Group DC1

203

84.0

83.0

82.03.23.02.82.62.42.22.01.8

Force (kips)

2.12

2.10k = 1.5 X 106 lb./in.(0.26 X 106 kN/m)

84.0

83.0

82.0

2.12

2.10k = 2.9 X 106 lb./in.(0.51 X 106 kN/m)

84.0

83.0

82.0

2.12

2.10k = 2.1 X 106 lb./in.(0.37 X 106 kN/m)

84.0

83.0

82.0

2.12

2.10k = 2.4 X 106 lb./in.(0.42 X 106 kN/m)

84.0

83.0

82.0

2.12

2.10

141312111098

Force (kN)

k = 3.9 X 106 lb./in.(0.68 X 106 kN/m)

Fig. D.5. Test C1, Individual Unload-Reload Loops, Group EC1

204

Table D.1. Stiffness of Individual Small-Amplitude [1 kip (4.4 kN)] Unload-Reload Loops; Test C1, Incremental Plate-Load Test from 0 to 8.6 kips (38.3 kN) on Wetter Material (Moisture Content ~6.1%)

Wetter Material (moisture content ~ 6.1%)

ρ = 135 (lb/ft3) From nuclear density tests

ν = 0.25

ro = 0.5 (ft) Radius of plate

Cursors

Loop Number A B

Numpts in loop Slope k (from plt)

Average k for Each Group (from plt)

Relative Shear Wave Velocity, vsi (ft/sec) k-4Gro/(1-ν)

(lbs/in) (lbs/in) (lbs/in)

First Set of Loops 1-1 74032 75499 1467 1.002 9.98E+05

1-2 75407 76874 1467 1.32 7.58E+05

1-3 76740 77903 1163 1.0532 9.49E+05

1-4 77747 79305 1558 1.26 7.94E+05

1-5 79151 80597 1446 0.91 1.10E+06 9.20E+05 900 7.55E+05

Second Set of Loops 2-1 146504 147618 1114 0.35374 2.83E+06

2-2 147500 149771 2271 0.67153 1.49E+06

2-3 149513 151598 2085 0.82735 1.21E+06

2-4 151448 153114 1666 0.36158 2.77E+06

2-5 152962 154351 1389 0.34355 2.91E+06 2.50E+06 1100 1.13E+06

Third Set of Loops 3-1 241208 242314 1106 0.11127 8.99E+06

3-2 242169 244716 2547 0.0316 3.16E+07

3-3 244388 246109 1721 0.0535 1.87E+07

3-4 245982 247242 1260 0.00866 1.15E+07

3-5 247123 248521 1395 0.22416 3.65E+07 1.77E+07 0 0.00E+00

Fourth Set of Loops 4-1 313076 315398 2322 0.35754 2.80E+06

4-2 315153 316381 1228 0.083582 1.20E+07

4-3 316258 317850 1592 0.32177 3.11E+06

4-4 317689 318992 1303 0.29949 3.34E+06

4-5 318822 320553 1731 0.36424 2.75E+06 3.00E+06 1260 1.48E+06

Fifth Set of Loops 5-1 364570 366193 1623 0.666 1.50E+06

5-2 365988 368461 2473 0.344 2.91E+06

5-3 368355 370134 1779 0.47976 2.08E+06

5-4 370045 371369 1324 0.42574 2.35E+06

5-5 371270 372860 1590 0.25653 3.90E+06 2.81E+06 1030 9.88E+05

Note: Shaded boxes not used in the average

APPENDIX E

INDIVIDUAL LOADING CYCLE PLOTS FOR TEST C2

30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

k = 0.145 X 106 lb/in.(0.0254 X 106 kN/m)

k = 0.531 X 106 lb/in.(0.0930 X 106 kN/m)

Fig. E.1. Test C2; Loop 1 of 5 - 5 Loading Cycles from 0 to 8 kips on Wetter Material (w ~ 6.1 %)

30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

k = 0.552 X 106 lb/in.(0.0967 X 106 kN/m)


30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

k = 0.111 X 106 lb/in.(0.0194 X 106 kN/m)

k = 0.469 X 106 lb/in.(0.821 X 106 kN/m)


30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

k = 0.625 X 106 lb/in.(0.109 X 106 kN/m)

k = 0.191 X 106 lb/in.(0.0334 X 106 kN/m)


30

25

20

15

10

5

086420

Force (kips)

0.6

0.4

0.2

0.0

403020100Force (kN)

k = 0.648 X 106 lb/in.(0.113 X 106 kN/m)

k = 0.160 X 106 lb/in.(0.0280 X 106 kN/m)


REFERENCES

American Association of State Highway and Transportation Officials

(AASHTO) (1986), T 222-81, “Nonrepetitive Static Plate Load Test of Soils and Flexible Pavement Components, for Use in Evaluation and Design of Airport and Highway Pavements”.

American Association of State Highway and Transportation Officials

(AASHTO) (1986), AASHTO Guide for Design of Pavement Structures, 1993.

Ahtchi-Ali, F. and J.C. Santamarina (1994); “Settlement of Footings on

Granular Materials: Low and Large Strain Parameters”; Vertical and Horizontal Deformations of Foundations and Embankments: Proceedings of Settlement ‘94 / Texas A&M University, College Station, TX, June 16-18, 1994; edited by Albert T. Yeung and Guy Y. Felio. ASCE Geotechnical Special Publication No. 40, pp 1287-1297.

Bay, J.A. (1997), Development of a Rolling Dynamic Deflectometer for

Continuous Deflection Testing of Pavements, Dissertation, The University of Texas at Austin, Dec, 1997.

Bueno, J.L., K.H. Stokoe III, J.J. Allen (1998), “Density and Stiffness Results

for Thick Lift Sections”, Proceedings of the International Center for Aggregages Research Conference, St Louis MO, April 19-21, 1998.

Burland, J.B. (1989), “Small is Beautiful - the stiffness of soils at small

strains,” Ninth Laurits Bjerrum Memorial Lecture, Canadian Geotechnical Journal, Volume 26, pp. 499-516.

211

Huang, Y.H., (1969), “Influence Charts for Two-Layer Elastic Foundations,” Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers (ASCE), Vol. 95, No. SM2, March, pp. 709-713.

Seed, H.B., Wong, R.T., Idriss, I.M., and Tokimatsu, K. (1986), “Moduli and

Damping Factors for Dynamic Analysis of Cohesionless Soils,” Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers (ASCE), Vol. 112, No. SM11, pp. 1016-1032.

212

213

Stokoe, K.H.; S.G.Wright; J.A. Bay; J.M. Roesset (1994), “Characterization of Geotechnical Sites by SASW Method,” Technical Report: Geophysical Characterization of Sites, International Society of Soil Mechanics and Foundation Engineering (ISSMFE) Technical Committee 10, edited by R.D. Woods, Oxford Publishers, January, 1994.

Terzaghi, K.T. and R.B. Peck (1948), Soil Mechanics in Engineering Practice,

John Wiley and Sons, Inc. New York, Copyright, 1948.

PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER PLATE ...

Documents

Transcript of PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER PLATE ...