Copyright 2013, Yiqing Wei

DEVELOPMENT OF EQUIVALENT SURCHARGE LOADS FOR THE DESIGN

OF SOIL NAILED SEGMENT OF MSE/SOIL NAIL HYBRID

RETAINING WALLS BASED ON RESULTS FROM

FULL-SCALE WALL INSTRUMENTATION

AND FINITE ELEMENT ANALYSIS

by

Yiqing Wei, BS

A Dissertation

In

Civil Engineering

Submitted to the Graduate Faculty

of Texas Tech University in

Partial Fulfillment of

the Requirements for

the Degree of

DOCTOR OF PHILOSOPHY

Approved

Priyantha W. Jayawickrama

Chair of Committee

William Lawson

Sanjaya Senadheera

Dominick Casadonte

Interim Dean of the Graduate School

May, 2013

ii

ACKNOWLEDGMENTS

I wish to acknowledge the financial support to this research by Texas Department

of Transportation

I am deeply grateful to my advisor, Dr. Priyantha Jayawickrama, for giving me

the opportunity to pursue this degree under his guidance and supervision.

My sincere appreciation goes to Dr. William Lawson and Sanjaya Senadheera for

accepting to serve in my committee.

I would like to thank the geotechnical lab team members: Rozbeh, Timothy,

Shannon, Douglas, Roderick, Asitha, Allen, John, for their help and encouragement.

I would also like to thank my wife Liming and my son Ivan for the constant

encouragement and support. You are always my power of study.

Finally, to my parents and sisters who have been the unceasing love and support.

Texas Tech University, Yiqing Wei, May 2013

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS............................................................................................ ii

ABSTRACT....................................................................................................................v

LIST OF TABLES............................................................................................................vi

LIST OF FIGURES........................................................................................................ vii

CHAPTER 1 OUTLINE.................................................................................................. 1

1.1 Background............................................................................................................. 1

1.2 Research Problem Statement ................................................................................. 5

1.3 Research Approach................................................................................................. 7

1.4 Organization of the Dissertation........................................................................... 10

CHAPTER 2 REVIEW OF CURRENT DESIGN METHODS FOR REINFORCED SOIL STRUCTURES..................................................................................................... 11

2.1 Mechanism of Reinforced Soil Structure.............................................................. 11

2.2 Design of Soil Nail Walls..................................................................................... 11

2.2.1 Nail Forces in Soil Nail Walls..................................................................... 13

2.2.2 Pullout Behavior of Soil Nail....................................................................... 16

2.3 Global Stability of Soil Nail Wall......................................................................... 22

2.3.1 Davis Design Method.................................................................................. 26

2.3.2 German Design Method............................................................................... 27

2.3.3 Kinematical Limit Analysis..........................................................................29

2.3.4 French Multicriteria Analysis.......................................................................35

2.3.5 FHWA 1996 Design Method........................................................................38

2.3.6 FHWA 2003 Design Method........................................................................40

2.3.6.1 GOLDNAIL...................................................................................... 41

2.3.6.2 SNAIL............................................................................................... 41

2.4 Introduction of MSE wall..................................................................................... 42

CHAPTER 3 INSTRUMENTATION AND MONITORING OF IH 410 MSE/SOIL NAIL HYBRID RETAINING WALL………………………………………………48


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3.1 Project Description................................................................................................ 48

3.2 Construction of the Hybrid Wall........................................................................... 52

3.3 Case Studies.......................................................................................................... 52

3.4 Instrumentation Plan............................................................................................. 54

3.5 Data Interpretation................................................................................................ 57

3.5.1 Inclinometer Data......................................................................................... 57

3.5.2 Grout Strain.................................................................................................. 57

3.5.3 Tensile Forces in Soil Nail............................................................................57

3.6 Discussion of the Results...................................................................................... 64

CHAPTER 4 2D FINITE ELEMENT ANALYSIS OF SOIL NAIL WALL............ 67

4.1 Introduction......................................................................................................... 67

4.2 Cases Study of the Finite Element Modeling for the Soil Nail Walls….............. 69

4.2.1 Polyclinic Wall in Seattle, Washington........................................................69

4.2.2 CLOUTERRE Wall......................................................................................72

4.2.3 Swift-Delta Soil Nail Wall............................................................................74

4.2.4 Simulation of Soil Nail Structures Using PLAXIS 2D.................................77

4.3 Reinforcement Pullout Behavior in Finite Element Program..............................79

4.4 Soil Nail Pullout Simulation by 2D PLAXIS........................................................86

4.5 Simulation of the MSE/Soil Nail Hybrid Retaining Wall.....................................92

CHAPTER 5 PARAMETRIC STUDY AND DEVELOPMENT OF EQUIVALENT SURCHARGE.... ... .. ... ... .. ... .. ... ... .. ... ... ... .. ... . .. .. ... ... . . ... ... .. ... .. ... ... .. ... . . 100

5.1 Parametric Study of MSE/Soil Nail Hybrid Wall............................................... 100

5.2 Equivalent Loads of the MSE Wall Portion…................................................... 101

5.3 Results and Discussion....................................................................................... 106

CHAPTER 6 SUMMARY AND CONCLUSIONS.................................................... 114

REFERENCE…………………………….………………………………………….118


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ABSTRACT

MSE/Soil Nail hybrid retaining walls have been used in cut/fill retaining systems

recently. In this type of wall a MSE wall is constructed above an existing soil nail wall.

Therefore, the soil nail wall portion of the hybrid wall system has much heavier

surcharge than the normal one. The dissertation demonstrates the results of

instrumentation and monitoring a MSE/Soil Nail hybrid retaining wall system. The

innovative 2D finite element models were used to simulate the behavior of the hybrid

retaining wall system, considering the soil nail ultimate pullout capacity and the effects of

the construction phase. In order to evaluate the global FOS of the soil nail wall portion,

the equivalent loads considering the vertical loads and horizontal loads of the MSE wall

portion are presented by the results of the finite element analysis. The vertical load factor

is 1.2 times of the self weight of the MSE wall. Meanwhile the horizontal load factors are

in function of the soil nail pullout capacities. The instrumentation data and numerical

analysis results are discussed below.


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LIST OF TABLES

2.1 Estimated bond strength of soil nails in soil and rock ...............................20

2.2 Results of the soil nail pullout displacement at the maximum

pullout forces (PDMPF) for a series of pullout tests in sands ...................21

3.1 The reinforcement of the MSE wall...........................................................50

3.2 Wall No.7 Construction Timeline ............................................................. 55

4.1 Materials properties for the normal numerical pullout test model

(a) Soil’s properties ....................................................................................83

(b) Properties of the facing and reinforcement ..........................................83

4.2 Interlayer’s properties according to different unit pullout capacity

of the nails ................................................................................................. 83

4.3 Material properties for the MSE/Soil Nail hybrid wall models

(a) Soil’s properties ....................................................................................99

(b) Properties of the reinforcements and facing ........................................ 99

5.1 Material properties for the MSE/Soil Nail hybrid wall models

(a) Soil’s properties ..................................................................................105

(b) Properties of the reinforcements and facing .......................................105

5.2 Value of μh

(a) For Soil nail Length=7 m, MSE/SN Height Ratio=1.38 ....................107

(b) For Soil nail Length=7.9 m, MSE/SN Height Ratio=1.38 .................107

(c) For Soil nail Length=8.8 m, MSE/SN Height Ratio=1.38 .................107

(d) For Soil nail Length=7 m, MSE/SN Height Ratio=0.88 ....................108

(e) For Soil nail Length=7.9 m, MSE/SN Height Ratio=0.88 .................108

(f) For Soil nail Length=8.8 m, MSE/SN Height Ratio=0.88. .................108

(g) For Soil nail Length=7 m, MSE/SN Height Ratio=0.55 ................... 109

(h) For Soil nail Length=7.9 m, MSE/SN Height Ratio=0.55 .................109

(i) For Soil nail Length=8.8 m, MSE/SN Height Ratio=0.55 ..................109

6.1 Global FOS of Wall 7 Section A and B analyzed by GOLDNAIL .........117

6.2 Nail forces of Wall 7 Section A and B analyzed by GOLDNAIL...........117


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LIST OF FIGURES

1.1 Use of fill type earth retaining wall in roadway expansion projects ............2

1.2 Use of cut type earth retaining wall in roadway expansion projects ...........3

1.3 Use of Cut/Fill Type Earth Retaining Wall in Side Hill Situations .............4

1.4 Schematic of a MSE/Soil Nail hybrid retaining wall...................................6

1.5 Types of global failure model of MSE/Soil Nail hybrid wall

(a) Type I......................................................................................................8

(b) Type II ....................................................................................................9

(c) Type III ...................................................................................................9

2.1 Typical construction sequences in soil nail walls ......................................12

2.2 Potential failure surfaces and soil nail tensile forces .................................14

2.3 Soil nail stress-transfer mechanism ...........................................................15

2.4 Mechanism of tension mobilization in soil nail wall .................................16

2.5 Typical load-displacement curve of in situ soil nail pullout testing ..........19

2.6 Typical load-displacement curve of laboratory soil nail pullout testing ....19

2.7 Principal modes of failure of soil nail wall systems ..................................24

2.8 Method for analysis global stability of soil nail wall ................................ 25

2.9 (a) Contours of factor of safety derived from finite element analysis .......26

(b) Limit equilibrium method for soil nail wall stability analysis .............26

2.10 German gravity wall method .....................................................................27

2.11 German method: Design chart for stability calculations............................28

2.12 Kinematical limit analysis approach:(a) mechanics of failure and

design assumption; (b) state of stress in inclusion; (c) theoretical

Solution for infinitely long bar Adopted for design purposes ...................32

2.13 Constant modulus of lateral subgrade reaction ..........................................33

2.14 Kinematical limit analysis design chart

(a) Design chart for perfectly flexible nails (N=0) ....................................34

(b) Design chart by kinematical method (N=0.33) ....................................34

2.15 Multicriteria slope stability analysis method


viii

(a) Schematic distribution of the lateral pressure along the nail ................36

(b) Representation of the various interaction mechanisms within

the normal force (Tn) and shear force (Tc) ..........................................37

2.16 Nail tension distribution diagram...............................................................38

2.17 Construction stability of soil nail wall .......................................................39

2.18 FHWA 1996 preliminary design chart for soil nail walls ..........................39

2.19 FHWA 2003 preliminary design chart for soil nail walls ..........................41

2.20 Stress transfer mechanisms for MSE wall reinforcement ..........................44

2.21 Typical Load-Displacement Curve for the metallic reinforcement

pullout test for the MSE wall .....................................................................45

2.22 Potential external failure mechanisms for a MSE wall ..............................45

2.23 Location of potential failure surface for internal stability design of

MSE walls ..................................................................................................46

2.24 Variation of stress ratio with depth in a MSE wall ....................................47

3.1 Profile view of Wall No.7 in San Antonio and wall panels selected

for instrumentation .....................................................................................49

3.2 Hybrid Wall Sections Selected for Instrumentation and Monitoring:

(a) Wall Section A; (b) Wall Section B .....................................................51

3.3 Cross section of nail tendon with strain gauge location at U.S.

Highway 26-89...........................................................................................53

3.4 Cross section of instrument section for Swift-Delta wall ..........................53

3.5 Spot Welded VWGs: (a) Schematic, (b) VWG covered with tape

for protection. .............................................................................................56

3.6 Model 4210 VWG: (a) Schematic, (b) VWG mounted on

centralizer ...................................................................................................56

3.7 Layout of Spot Welded and Embedment Type VWGs on a 7.9 m

long Soil Nail Tendon ................................................................................56

3.8 Horizontal displacement of Wall 7 Section A ...........................................59

3.9 Final strain of the grout of the soil nails ....................................................60

3.10 Distribution of nail forces during the construction of the hybrid walls

(a) Tensile forces measured in the top row nail of Wall 7 Section A ........60

(b) Tensile forces measured in the second row wail of Wall 7


ix

Section A ..............................................................................................61

(c) Tensile forces measured in the top row nail of Wall 7 Section B ........61

(d) Tensile forces measured in the second row nail of Wall 7

Section B ..............................................................................................62

(e) Tensile forces measured in the third row nail of Wall 7 Section B ......62

(f) Tensile forces measured in the fourth row nail in Wall 7 Section B ....63

(g) Tensile forces measured in the bottom row nail in Wall 7 Section B ..63

3.11 MSE/Soil Nail hybrid wall at U.S. Highway 26-89, Wyoming

(a) Typical cross section of MSE/Soil Nail hybrid wall Station

20+350 .................................................................................................. 65

(b) Slope inclinometer reading of the MSE/Soil Nail hybrid wall ............65

3.12 Loose fill soil nail slope under high surcharge in Hong Kong

(a) Soil nail slope with surcharge...............................................................66

(b) Soil nail slope horizontal displacement under high surcharge .............66

4.1 Comparison of Mohr-Coulomb model and typical triaxial test

results of soil ....................................................................................……..68

4.2 Mohr-Coulomb yield surface in principal stress when c=0………………68

4.3 Cross section and soil’s properties of Polyclinic wall in Seattle ...............71

4.4 Comparison between the results of the finite element analysis and

the measured data: (a) wall facing displacement; (b) maximum nail

forces ..........................................................................................................71

4.5 Schematic of CLOUTERRE Wall .............................................................72

4.6 Section view of the grout bars of CLOUTERRE wall ...............................73

4.7 Comparison between the results of the finite element analysis and the

measured data: (a) maximum nail forces; (b) wall facing displacement ...73

4.8 Cross section and construction sequence of Swift- Delta wall ..................74

4.9 Finite element model of Swift-Delta wall ..................................................75

4.10 Nail forces of Swift-Delta wall: (a) measured nail forces; (b) finite

element analysis results..............................................................................75

4.11 Horizontal displacement of Swift-Delta wall ............................................76

4.12 Soil nail wall models ..................................................................................77

4.13 Maximum nail forces with different cohesion ...........................................78


x

4.14 Horizontal displacements with different cohesion .....................................78

4.15 Representation of 3D and 2D models. .......................................................79

4.16 (a) Normal Numerical Pullout test model, (b) Facing opening,

reinforcement and force of the model ........................................................82

4.17 Pullout forces versus pullout displacement for the normal

numerical Pullout test model with varied depth

(a) Unit pullout resistance versus pullout displacement for Rinter=0. .........84

(b) Unit pullout resistance versus pullout displacement for Rinter=1. ........84

4.18 Comparison of the unit pullout capacity of the normal numerical

pullout test model with different Rinter and the MSE wall design

value of highway IH410 located at San Antonio .......................................85

4.19 (a) Soil nail pullout test model ...................................................................88

(b) Interlayer and facing opening of the soil nail .......................................88

4.20 Pullout test results of the innovative soil nail pullout test model

with different unit pullout capacity, Quu

(a) Unit pullout force versus displacement for the interlayer of

Quu=24.5 kN/m/m ................................................................................89

(b) Unit pullout force versus displacement for the interlayer of

Quu=49 kN/m/m ...................................................................................89

(c) Unit pullout force versus displacement for the interlayer of

Quu=73.5 kN/m/m ................................................................................90

(d) Unit pullout force versus displacement for the interlayer of

Quu=98 kN/m/m ...................................................................................90

(e) Unit pullout force versus displacement for the interlayer of

Quu=122.5 kN/m/m ..............................................................................91

4.21 Finite element mesh: PLAXIS V8 finite element models for

MSE/Soil Nail hybrid retaining walls:

(a) Wall 7 Section A ..................................................................................93

(b) Wall 7 Section B……………………………………………………....94

4.22 Comparison between measured nail forces and finite element analysis

results

(a) Wall 7 Section A: Nail Forces on First Row Nail ................................94

(b) Wall 7 Section A: Nail Forces on Second Row Nail ...........................95


xi

(c) Wall 7 Section B: Nail Forces on First Row Nail ................................95

(d) Wall 7 Section B: Nail Forces on Second Row Nail............................96

(e) Wall 7 Section B: Nail Forces on Third Row Nail ...............................96

(f) Wall 7 Section B: Nail Forces on Fourth Row Nail .............................97

(g) Wall 7 Section B: Nail Forces on Bottom Row Nail ...........................97

4.23 Wall facing displacements of measured data and finite element analysis

results with varied Young’s modulus ........................................................98

5.1 Expected forces imposed by MSE wall on soil nail wall.........................102

5.2 PLAXIS finite element models for calibrating the equilibrium loads

(a) Total stresses of the hybrid wall .........................................................103

(b) Total stresses of the soil nail wall corresponding to the hybrid

wall under the equivalent loads..........................................................104

5.3 Comparison of the results between the hybrid wall model and soil

nail wall model under equivalent loads

(a) Comparison of the facing displacement .............................................110

(b) Comparison of the maximum nail forces ...........................................110

5.4 Relationship between the factor of the equivalent horizontal

distributed loads μh and MSE/SN Height Ratio .......................................111

5.5 Contour lines and potential failure surface of the finite element

models for the MSE/ Soil Nail hybrid walls

(a) MSE/SN Height Ratio equal to 1.35 ..................................................112

(b) MSE/SN Height Ratio equal to 0.88 ..................................................112

(c) MSE/SN Height Ratio equal to 0.55 ..................................................113

6.1 Soil nail walls’ models for the design by GOLDNAIL program

(a) Case 1 and case 2 ................................................................................116

(b) Case 3 .................................................................................................116


1

CHAPTER 1

OUTLINE

1.1 Background

Highway traffic congestion is a major source of frustration for many roadway

users especially those who commute in and around major metropolitan areas.

Customarily, this problem is addressed by widening the existing roadways and adding

extra travel lanes. However, in many metropolitan areas, it is becoming increasingly

difficult to undertake such roadway expansions due to the high cost of new right-of-way

acquisition and opposition from local groups. Therefore, it is important that

transportation agencies use innovative designs that allow improvement in highway

capacity by maximizing the use of available right-of-way. Such designs often involve the

use of multi-level structures that minimize the footprint of the improvement on the

surrounding landscape. Earth retaining structures allow grade separation to be achieved

within the existing right-of-way and therefore play an important role in the construction

of such multi-level structures.

A variety of earth retaining walls are used in modern transportation systems, with

different types optimally suited for given field situations. Some retaining wall types are

better suited for fill situations while other types are better used for supporting cuts.

Figure 1.1 illustrates two roadways that run parallel to each other. It represents a

situation where inadequate right-of-way requires the use of a retaining wall along the

embankment side slope to maintain grade separation. In this case, the wall is placed at

the bottom of the embankment along the edge of the lower roadway allowing widening of

the upper roadway. Clearly, the construction of the retaining wall is done in conjunction

with placement of a fill. Among the types walls used in fill situations such as this, MSE

walls with pre-cast concrete panels are, by far, the most widely used. They offer the

advantages of ease and speed of construction, cost efficiency, reliability in design and


2

aesthetic appeal. Modular block walls represent another type of fill wall that is

commonly used in applications where significant curvature in wall alignment exists and

when achieving exact line and grade is not very critical. CIP cantilever walls represent a

third type of fill walls. These walls are found to be cost effective only for small retaining

walls with wall areas less than about 1,000-sq.ft.

Figure 1.1 Use of fill type earth retaining wall in roadway expansion projects

(Jayawickrama, 2009)

A situation that is different the one described above arises if widening of the

lower roadway became necessary. In this case, the retaining wall would be placed along

the edge of the upper roadway as illustrated in Figure 1.2. This represents a cut situation

in which the embankment material must be removed to create additional space needed for

widening of the lower roadway. This type of wall is typically constructed from top down.

Soil nail walls, tied back walls and drilled shafts walls are the most commonly used cut

type walls. The wall height, soil and groundwater conditions usually govern the choice

of the optimum cut type wall. Often, other projects constraints such as the presence of

underground utility lines and/or other buried structures or lack of permanent easement

may also impact the wall selection. In these instances, a retaining wall system with small


3

footprint, such as the cantilevered drilled shaft retaining wall, may prove to be the

optimum design solution.

A third category of retaining walls, cut/fill type walls are encountered when

construction projects require retaining walls to be built into “side hill” situations. In this

case, the bottom portion of the wall is placed below existing ground, and the top portion

placed above. This is illustrated in Figure 1.3.

Figure 1.2 Use of cut type earth retaining wall in roadway expansion projects

(Jayawickrama, 2009)


4

Figure 1.3 Use of cut/fill type earth retaining wall in side hill situations (Jayawickrama,

2009)

Traditional design solutions used in side hill (or cut/fill) conditions have involved

full height MSE walls and drilled shaft walls. When full height MSE walls are used,

significant amounts of earth material must be removed near the bottom portion of the

wall to allow placement of the reinforced backfill. The excavated slopes must be

supported with temporary shoring while the MSE wall is being constructed. The use of

temporary shoring increases the overall cost of construction of the MSE wall significantly.

The other design alternative that has been used in side hill situations involves the use of

drilled shafts to support lateral soil loads. When this type of wall is used, the wall must

be constructed in two stages. In the first stage, a series of closely spaced drilled shafts

are installed to form the below-ground portion of the wall. Secondly, the drilled shafts

are extended above ground as columns using formwork and cast in place concrete to form

the above-ground portion of the wall. Finally the drilled shafts and columns are fitted

with facing panels and the above ground portion of the wall is backfilled with soil. The

drilled shaft walls resist lateral earth pressure by cantilever action. The depth of

embedment required generally varies from one to two times the wall height. Therefore

this type of wall is quite expensive to construct.


5

An innovative and more economical design alternative that has been used recently

in Texas for side hill walls involves the use of a soil nailed wall in the cut section, and an

MSE wall for the fill section (See Figure 1.4). Although such MSE/Soil Nail hybrid wall

systems have been found to be very cost effective, only a handful of walls of this type

have been built within the state until now.

1.2 Research Problem Statement

The limited use of MSE/Soil Nail hybrid walls in transportation projects is largely

due to lack of an established design procedure for these structures. In other words,

MSE/Soil Nail hybrid walls are still considered “experimental” and many questions

regarding their design and performance remain unanswered. Most importantly, the

FHWA Publication that outlines the design procedure for soil nail walls provides

minimum guidance regarding the design of soil nail walls that support MSE (or other

types of wall) constructed on top. However, it does state that the upper wall system may

be considered as an equivalent surcharge load with vertical, horizontal and moment

components when designing the soil nail wall (FHWA, 1996). This design manual does

not specify reasonable magnitudes of the surcharge loads to be used in the design.


6

Figure 1.4 Schematic of a MSE/Soil Nail hybrid retaining wall (Alhabshi, 2006)

The conventional methods for designing soil nail walls rely on the limit

equilibrium concepts. This approach does not consider the actual wall construction

sequence into account. Therefore, this method is not capable of modeling the

development of tensile loads on soil nails installed at different levels correctly. In a real

soil nail wall, the top row nails develop much larger tensile loads than bottom row nails

because of the specific sequence construction used. Despite this limitation, limit

equilibrium based analysis have been successfully applied for soil nail wall designs.

Nevertheless, it would be prudent to examine the applicability of the limit equilibrium

approach for MSE/Soil Nail hybrid walls before it is implemented as a routine design tool.

In particular it will be interesting to find out how the significant surcharge pressures

imposed by the MSE wall will impact the distribution of soil nail loads among soil nails

placed at various heights.


7

Another limitation in the limit equilibrium based design approach is that it does

not address wall deformation in an explicit manner. However, measured maximum wall

deformations of soil nail walls that have been designed using the limit equilibrium

approach have shown that the wall deformations are within acceptable limits. For

vertical soil nail walls of height, H and with typical nail length-to-wall height ratios and

negligible surcharge loadings, the peak wall deformations at the top of the wall tend to

vary from 0.1%H or less for weathered rocks and very competent and dense soils (such as

glacial tills), to 0.2%H for granular soils, and up to 0.4% for fine-grained clay type soils

(Recommendations Clouterre 1991-English Translation, 1993). Deformation of the

nailed soil mass in a hybrid wall is of special interest as it serves as the foundation for the

MSE wall. Excessive deformations of the soil nailed portion of the wall can have adverse

impact on the integrity of the MSE wall system. Therefore, it is important to ascertain

that any design procedure used for hybrid walls will result in nail lengths that are capable

of controlling wall deformations within acceptable limits.

The design of the MSE portion of the hybrid wall is not impacted to the same

extent as the soil nail portion of the hybrid wall because of the unusual wall configuration.

However, one wall failure mode that will be impacted is the global shear failure.

Therefore, it is important to gain insight into how the new wall configuration may

influence the location of the actual failure surface.

1.3 Research Approach

The general objective of this research study is to examine the widely used limit

equilibrium based soil nail wall design procedure with respect to its applicability to

MSE/Soil Nail hybrid walls. The research plan to accomplish this research objective

included several tasks. The first task involved instrumentation and monitoring of two

separate sections of a hybrid wall to observe its behaviors. The data collected from this

wall monitoring project included tensile loads on soils nails and wall deformations. The


8

research plan also included an independent effort to develop a series of numerical models

to simulate the MSE/Soil Nail hybrid walls by finite element method (FEM). FEM was

selected primarily because of its ability to provide valuable insight into the behaviors of

soil reinforced structures accommodating the actual construction sequence. Data from

the pullout tests of soil nails conducted at the wall construction site and laboratory were

used to develop an appropriate model for load transfer occurring at the soil-nail interface.

The FEM hybrid wall models are then validated with data collected from the

instrumented walls. A series of MSE/Soil Nail hybrid wall models with different MSE

wall to soil nail wall height ratio (MSE/SN Height Ratio) are built and use to come up

equivalent surcharge loading to represent the MSE walls. The failure model for the

hybrid walls will be tested in this dissertation. These walls are shown in Figure 1.5, and

are described as follows:

(a) Type I: The portion of the MSE wall is about 2 times height of the soil nail wall.

(b) Type II: the portions of the MSE and the soil nail wall are relatively even.

(c) Type III: The portion of the MSE is about 1/2 time height of the soil nail wall.

(a) Type I


9

(b) Type II

(c) Type III

Figure 1.5 Types of global failure model of MSE/Soil Nail hybrid Wall


10

1.4 Organization of the Dissertation

This dissertation describes the development of a LEM based design method for

MSE/Soil Nail hybrid earth retaining walls. The proposed design method relies on data

collected from instrumentation of a full-scale hybrid wall as well as results from finite

element analysis. Chapter 2 of the dissertation presents a comprehensive review of

existing methods for design of soil nail walls. This chapter also discusses the soil

reinforcement pullout behaviors and MSE walls design methods. Case studies involving

previous soil wall monitoring projects as well as instrumentation of the full-scale

MSE/Soil Nail hybrid retaining wall in this research project are described in Chapter 3.

The measured nail forces, and grout strains and the horizontal displacements of the wall

are also presented in this chapter. Chapter 4 presents the development of a finite element

model to simulate soil reinforcement pullout behavior. An innovative model to

accommodate the laboratory and in situ pullout behavior of soil nails is proposed. This

model is then incorporated in a more comprehensive finite element model that simulates

the entire MSE/Soil Nail hybrid wall system using finite element program PLAXIS V8.2.

The results from the finite element analyses and the instrumentation are compared and

discussed. Subsequently, Chapter 5 includes findings from a parametric study that was

performed using a series of MSE/Soil Nail hybrid wall models to identify the parameters

that have the most dominant influence on hybrid wall design. According to the

recommendation found in FHAW Design Manual (1996), factored horizontal and vertical

distributed surcharge loads are developed based on results from the finite element

analysis for use in limit equilibrium based design of MSE/Soil Nail hybrid earth retaining

walls. Chapter 6 presents the conclusions and recommendations from the research.


11

CHAPTER 2

REVIEW OF CURRENT DESIGN METHODS FOR REINFORCED

SOIL STRUCTURES

2.1 Mechanism of Reinforced Soil Structure

Soil nail wall and MSE wall are the typical reinforced soil structures which are

widely used in civil engineering area recently. The soil reinforcements are the passive

inclusions in the soil mass and then create a gravity structure which is similar with the

conventional gravity earth retaining wall. The basic design concept consists of

transferring the tensile forces in the reinforcements into the soil through the mobilized

friction at the interfaces. The factor of safety (FOS) of the global stability for the

reinforced soil structure highly depends on the pullout resistance or tensile strength of the

reinforcements. On the other hand, the tensile strength is usually higher than the pullout

resistance. Therefore, the reinforcement pullout resistances are the most important

parameters for the reinforced soil structure design. The reinforcement pullout resistances

mainly depend on the type of the structure, type of the reinforcements, type of the soils,

and construction methods.

2.2 Design of Soil Nail Walls

As a cut-type earth retaining wall, a soil nail wall is constructed top-to-bottom.

Soils in soil nail walls must possess enough true or apparent cohesion to stand long

enough by itself without any reinforcing system at the cut slope, to permit the increment

of excavation and the reinforcements to be installed. Soil nail is basically a rebar

encapsulated in a cement grout. The interaction between soil and soil nail in the walls is

mostly presented by the bond stress between the grout and soil. The bond stress

introduces tensile force in the soil nails. The mobilized nail forces are caused by: (1) the


12

stressed relief during the excavation; (2) the on-going ground movements of the

marginally stabilized ground; (3) the surcharge on the soil nail wall.

Construction sequences and procedures for a soil nail wall are shown in Figure

2.1.

Figure 2.1 Typical construction sequences in soil nail walls (Byrne et al., 1996)


13

2.2.1 Nail Forces in Soil Nail Walls

The soil-nail interaction that occurs in the soil nail wall is complex. Each stage of

the excavation has a critical failure surface, as shown in Figure 2.2. The critical failure

surfaces separate the soil nail wall in two parts: active zone and resistance zone. The

active zone is the reinforced mass tending to move outward and yielding the mobilized

lateral shear stresses outward in the reinforcement. The resistant zone is located at the

opposite side of the active zone. The reinforcements have the shear stresses inward and

against the pull out forces caused by the active zone. The tensile forces in the soil nail are

zero at the end of the nail. The forces increase to a maximum value, Tmax, with the effect

of shear stress surround the grout in the intermediate length, and decrease to a value To at

the facing (Figure 2.3). The maximum nail tensile forces in the nails are not certainly

presented at the cross point between the nails and the critical failure surface. In some

cases, upper nails could be totally located in the active zone and ineffective in improving

the global FOS. However, the upper nails should not be considered superfluous. The

functions of the upper nails are stabilizing the retaining wall during earlier stages of

excavation, helping reduce horizontal displacements, and ensuring the integrity of the

retaining wall. The instrumentation of the soil nail wall showed that the upper nails

receive the maximum tensile forces occurred at a distance of approximately 0.3 to 0.35 of

the total height of the wall at the crown, Figure 2.4 (Byrne et al, 1998). Peck et al. (1996)

suggested that the maximum tensile forces occurred at the distance on the order of 0.35 to

0.45 of the total height from the face of the wall at the crown.

Though the forces in soil nails are predominant in tension, there are also shear and

bending that aid in preventing the potential sliding soil mass. The evaluation of the shear

and bending was discussed by Elias and Juran (1991). However, the effect of the shear

and bending are mobilized only after relatively large displacement is taking place and

failure is approaching. The results of the research suggested that the shear and bending

resistance of the soil nails contributed less than 10% of the overall stability of the wall.


14

The recent design methods conservatively ignore the effects of the shear and bending

strength of soil nails.

Figure 2.2 Potential failure surfaces and soil nail tensile forces (Lazarte et al., 2003)


15

Figure 2.3 Soil nail stress-transfer mechanism (Lazarte et al., 2003)

Figure 2.4 Mechanism of tension mobilization in soil nail wall (Byrne et al., 1996)


16

2.2.2 Pullout Behavior of Soil Nail

The effect of the soil and soil nail interaction of the wall is mostly presented by

the bond stress between the grout and the soil. The bond stress introduces tensile force in

the soil nails.

The soil nail pullout capacity is affected by the perimeter of the grout, the length

of the nails and the ultimate bond strength. FHWA soil nail wall design manual

Geotechnical Engineering Circular No. 7, Soil Nail Walls (2003) suggested the soil nail

pullout capacity, Rp, expressed as:

(2.1)

With: (2.2)

where:

Qu = pullout capacity per unit length (also referred to as load transfer rate capacity);

Lp = Pullout length or nail length behind the failure surface

qu = ultimate bond strength.

DDH = average or effective diameter of the drill hole

Unlike MSE wall, in most of the situations, the pullout capacity in soil nail wall is

independent from overburden pressure of the soil (Cheng and Lawrence 1994; Byrne et al

1998; Lazaarte et al 2003; Li-Jun Su et al 2008; Yin and Zhou 2009). On the other hand,

the ultimate bond strength highly depends on the soil nail construction methods.

Elias and Juran (1991) showed that the bond stress under grout pressure of 350

kPa (50 psi) was one time higher than that of gravity-placed grout. Carlos A. Lazarte, et

al, (2003) mentioned that the soils’ properties and construction of the grout affect the

ultimate bond strength. Yin (2009) suggested that the overburden pressure could

influence the ultimate bond strength under high grout pressure (larger than 130 kPa).


17

FHWA manual (Byrne et al., 1998) provides the ultimate bond strength value for primary

design (Table 2.1). The values in this table correspond to gravity grouting method only.

Because of the difficulty in estimating bond strength, the field pullout tests are required

to verify the value considering the factor of safety. The Geotechnical manual (Texas

department of transportation, 2006) provides the design charts to estimate the bond

strength (allowable skin friction) based on Texas cone penetration test.

Recently, some researches concentrated on the behavior of soil nail pullout test.

The results of the pullout displacements at the maximum pullout forces (PDMPF) for a

series of soil nail pullout tests in sands are shown in Table 2.2 As it can be seen, the

PDMPF of laboratory pullout tests are ranged from 1.5 to 18 mm. The PDMPF of in-situ

pullout tests are about as much as two times of the PDMPF of laboratory tests. The data

of tensile test of bored pile (Krabbenhoft, et al, 2008) suggested that the PDMPF may

relate to the SPT value of the soils. Higher SPT value of the soils tends to have larger

PDMPF. Figure 2.5 and Figure 2.6 show the typical bond stress- displacement curves of

in-situ and laboratory pullout test. The bond stress in the Figures multiplied by the

perimeter of the grout and the length of the nails yields the maximum pullout force of the

soil nail. French National Project CLOUTERRE (1991) suggested that the ultimate

pullout force can be determined as either the maximum value or the point where increase

of force per 1mm displacement is less than 1%, or a point of the displacement equal to 30

mm.

Based on the above description, it can be concluded that:

1. The PDMPF for soil nail pullout tests in sands are independent from the ultimate

bond strength

2. The PDMPF for soil nail pullout tests in sands are ranged from 1.5 mm to 18 mm for

1 to 2 m long soil nails


18

3. The load-displacement curves for soil nail pullout tests in sands are close to linear

shape before achieving the maximum bond stresses

4. The influence of surcharge on soil nail pullout capacity and PDMPF is still unknown

and the current design method ignores the influence.


19

Figure 2.5 Typical load-displacement curve of in situ soil nail pullout testing (Zhang, et

al, 2009)

Figure 2.6 Typical load-displacement curve of laboratory soil nail pullout testing (Su, et

al, 2008)


20

Table 2.1 Estimated bond strength of soil nails in soil and rock (Elias and Juran, 1991)


21

Table 2.2 Results of the soil nail pullout displacement at the maximum pullout forces (PDMPF) for a series of pullout tests in sands

Paper or Report Authors and

the Time Nail Type

Grout

Diameter

Effective

Nail Length

(m)

Testing Method PDMPF

Rapid Pullout Test of Soil Nail Ooi Poh Hai

(2006)

Metallic hollow

circular pipes

25mm and

45 mm 0.75 and 1.65

Laboratory rapid

and quasi static

pullout test

Mostly ranged

at 1.5 to 3 mm

The Tensile Capacity of Bored

Piles in Friction Soils

Krabbenhof

t, et al, 2008

20mm bar, installed

vertically

140 mm and

250 mm 2 to 6 In-situ pullout test 8 to 39 mm

Influence of Overburden

Pressure on Soil-Nail Pullout

Resistance

Su, et al,

2008

25 mm bar, grouted

with high pressure 100 mm 1.2 Laboratory test

About 8 to

18mm

Uncertainties of Field Pullout

resistance of Soil Nails

Zhang, et al,

2008 N/A 57-100 mm 2 In-situ pullout test

About 2 to

15mm

Influence of Grouting Pressure

and Overburden Stress on the

Interface Resistance of a Soil

Nail

Yin and

Zhou, 2009 40 mm bar 100 mm 1.2 Laboratory test

About 2 to 8

mm


22

2.3 Global Stability of Soil Nail Wall

The limit conditions for analysis and design a soil nail wall are service limit

states and strength limit states. Service states failure models of soil nail walls involve

the problems of excessive wall displacement, differential settlement, cracking of

concrete facing, and fatigue caused by repetitive loading. The problems do not cause

collapse of the wall but impair the functions of the structures.

The strength limit states refer to the damage of the components and the failure

of the systems. For soil nail walls, the strength limit states are classified as external

failure model, internal failure model, and facing failure model. The failure models are

presented in Figure 2.7. Internal failure and facing failure are concern about the

failure of the components such as soil nail, nail head, and the facing. External failure

models consider the failure of the systems. Therefore, the consequences of the

external failure could be significant. The external failure models include global failure

model, sliding failure model and bearing failure model.

The analysis of sliding failure and bearing failure model for soil nail walls is

similar with the analysis of traditional gravity earth retaining walls. In this case, the

soil nail wall is treated as a rigid block. The failure of the systems is caused by the

excessive shear forces in the foundation soil.

The developments of soil nail wall design method are mostly about identifying

the global stability of the soil nail walls. Global stability refers to the overall stability

of the reinforced soil nail wall mass. This dissertation also concentrates in the

evaluation of the global stability of soil nail walls.

Current design methods of soil nail wall use 2D limit equilibrium methods

evaluate the global stability. Different limit equilibrium methods have different

assumptions about the active zone, slide surface, force equilibrium, and moment

equilibrium. The active zone is modeled as a rigid block or multiple vertical slices and

potentially sliding alone the slide surface, as shown in Figure 2.8. The equilibrium

equations are established based on the global force equilibrium in different directions

and (or) global moment equilibrium. The slide surface could be linear, bi-linear,

circular, parabolic, or log spiral. The force equilibrium could be total equations,

equations in horizontal and vertical direction, or equations perpendicular and parallel

to the slide surface. The moment equilibrium could be defined as:


23

(2.3)

or

(2.4)

where,

F is the FOS

Md is driving moment

Ms is the moment due to available shear strength of the soil

Mt is the moment due to reinforcement

Equation (2.3) can be transformed into:

(2.5)

Equation (2.4) can be transformed into:

(2.6)

It is easy to tell the differences between two approaches by equation (2.5) and

(2.6). The moment due to reinforcement is reduced by a FOS in the equation (2.3) and

(2.5). In the equation (2.4) and (2.6), the FOS applied only for soil strength.

For reinforced soil structures, the reinforcement forces are presented as

boundary forces of the slices. The values of the forces are determined by the tensile

force distribution diagram for the soil nails.

The following section presents a brief discussion of the different design

methods and their design concepts.


24

Figure 2.7 Principal modes of failure of soil nail wall systems (Byrne et al., 1996)


25

Figure 2.8 Method for analysis global stability of soil nail wall (Duncan & Right,

2005)


26

2.3.1Davis Design Method

Shen et al. develop an equilibrium method assuming a parabolic failure

surface, passing either entirely or partially within the inclusion. The assumption is

based on the contour of the factor of safety derived from finite element solutions. In

the analysis, the tensile and pullout resistance of nails crossing the failure surface are

considered the governing stabilizing forces. The slope stability analysis is presented

in Figure 2.9.

The nails are assumed to withstand only tension force and their failure can be

defined by either the breakage or the pullout resistance. The factors of safety are

defined by:

Fc = c/cm (2.7)

Fφ = tanφ/tanφm (2.8)

FL = Tp/T (2.9)

where cm, φm, and T are the soils’ mobilized cohesion, mobilized angle of friction

and the nail’s mobilized pullout resistance alone the potential slide surface,

respectively, and c, φ, and Tp are the soils’ cohesion, angle of friction, and the nail

pullout resistance, respectively. The global factor of safety FOS = Fc = Fφ = FL.

Figure 2.9 (a) Contours of factor of safety derived from finite element analysis (b)

limit equilibrium method for soil nail wall stability analysis (Shen et al., 1981)


27

2.3.2 German Design Method

This method was developed by Stocker et al. (1979) based on assuming a

bilinear slip surface with the consideration of tension forces in the nails. The global

equilibrium system of soil nail wall is shown in Figure 2.10. The forces acting on the

slide surfaces are shown in the force polygon.

The minimum global factor of safety can be plotted with dimensionless

variables and serve as design charts, Figure 2.11. In this case, the failure surface is

fixed by three angle δ1, δ2, and δ12. The minimum factor of safety can be found only

by varying δ1, and keeping δ12 =900, δ2 = 45

0 + φ/2.

Figure 2.10: German gravity wall method (Gassler and Gudehus, 1981)


28

Figure 2.11 German method: Design chart for stability calculations (Gassler and

Gudehus, 1981)


29

2.3.3 Kinematical Limit Analysis

Kinematical limit analysis design method was developed by Juran and Elias

(1990). This design approach is base on a limit-analysis solution associating a

kinematical admissible displacement/failure mode, as observed on model walls, with

a statically admissible limit equilibrium solution. The main design assumptions are

as follows and shown in Figure 2.12, (Juran et al., 1990):

(a) Failure occurs by quasi-rigid body rotation of the active zone that is limit by

a log-spiral failure surface;

(b) At failure, the locus of maximum tension and shear forces coincide with the

failure surface developed in the soil;

(c) The quasi-rigid active and resistant zones are separated by a thin layer of

soil at a limit state of rigid plastic flow;

(d) The shear resistance of the soil, as defined by Coulomb’s failure criterion, is

entirely mobilized alone the failure surface;

(e) The reinforced mass is divided into slices parallel to the nails.

(f) The horizontal components (Eh) of the inter-slice forces acting in the both

side of a slice compromising nail (Figure 2.12) are equal;

(g) The effect of a slope (or horizontal surcharge, Fh) at the upper surface of

the nailed soil mass on the forces in the conclusions linearly decreases alone

the failure surface.

The effect of the bending stiffness of the inclusion on the actual nail

deformation and the generated resisting forces is analyzed considering the three

following case:

(a) Perfectly flexible nails that withstand only tension forces;

(b) Extremely rigid nails the withstand both tension and shear forces but do not

deform during construction;

(c) Nails with finite bending stiffness that governs their deformation and

thereby the generated shear forces.

In the third case, the actual nail deformation dβ is calculated from available

elastic solutions. As illustrated from Figure 2.12 (c), the tensile force (Tmax) and


30

shear force (Tc) are the maximum where the moment is 0. The maximum nail

deformation at the failure surface is given by:

(2.10)

where:

Tc is the maximum shear force;

Ks is the lateral soil retion modulus. It can be obtained by Soletanche charts (Figure

2.13, Pfisster et al. 1982);

D is the width of a flat strip reinforcement or diameter of a circular nail;

l0 is the transfer length that characterizes the relative stiffness of the inclution to the

nail, and is given by l0 = (4EI/KsD)1/4

.

The non-dimensional normalized maximum shear force (TS) and Tension force

(TN) are defined as:

(2.11)

(2.12)

hence:

(2.13)

where

is a non-dimensional bending stiffness

parameter, which depends on both the relative rigidity of the reinforcement to the

soil and the structural height. S is the length of the reinforcement in the active zone;

The design criteria with respect to pullout failure of each reinforcement expressed

as:

(2.14)

where

; la is the adherence length, ; L is the total

reinforcement length; fl is the limit interface lateral shear force; for circular


31

nails and 2 for flat strip reinforcement; Fp is the safety factor with respect to pullout;

S/H is the non-dimensional geometry of the failure surface

Failure by Breakage of Reinforcement

(2.15)

where, Fall and As are the allowable tension stress and the cross-sectional area of the

nail.

For the failure by coupled Tension and shear, the design should satisfy:

(2.16)

where

For failure by excessive bending, the design should satisfy:

(2.17)

where Mp is the plastic bending moment of the nail; FM is the factor of safety with

respect to the plastic moment of the nail; The TN, S/H, TS design chart are shown as

Figure 2.14.

The global safety factor can be defined using the design data per nail:

(2.18)

where n is the number of layers and I is the layer number.


32

Figure 2.12 Kinematical limit analysis approach: (a) mechanics of failure and design

assumption; (b) state of stress in inclusion; (c) theoretical Solution for infinitely long

bar Adopted for design purposes. (Juran et al., 1990)


33

Figure 2.13 Constant modulus of lateral subgrade reaction (Pfister et al., 1982)


34

(a) Design chart for perfectly flexible nails (N=0)

(b) Design chart by kinematical method (N=0.33)

Figure 2.14 Kinematical limit analysis design chart


35

2.3.4 French Multicriteria Analysis

French national project “CLOUTERRE” presented a Multicriteria analysis

procedure for soil nail wall design. The overall stability is evaluated assuming a

circular slip surface utilizing the simplified Bishop’s method. The method considers

the shear and bending contribution of the nails. It also mentions that the nail shear and

bending capacity increase the global FOS less than 10%. The Multicriteria analysis is

conducted to evaluate the factors of safety with respect to the following (Figure 2.15).

(a) Soil strength.

The soil is characterized by the Mohr-Coulomb criteria. The shear resistance τ

follows the relationship presented as:

τ φ (2.19)

where c is the soil cohesion and φ is internal friction angle of the soil and σ is the

normal vertical stress

(b) Nail resistances.

The nail resistances include resistances of tension, shear, and moment. The

maximum resistance of the nails is depending on the soil-nail interaction criteria.

The maximum shear force, Tc mobilized at the point of intersection of the

failure surface, is given by:

; p < pmax (2.20)

The maximum bending moment mobilized at the distance (π/4) l0 from point

O is given by:

(2.21)

where:

p = Passive maximum shear force, Tc mobilized at the point of intersection between

the failure surface as shown earth pressure on the nail

pmax = Maximum passive resistance that can be mobilized in the soil

l0 = Transfer length given by equation (2.4)

Mp = Limit bending capacity of the nail


36

The nails must withstand both tension (T) and shear force (V). Assuming the

nail element follows Tresca’s failure criterion (Elias and Juran, 1991):

(2.22)

where:

Fy = tensile strength of the nail

Rc = shear strength of the nail (Rc = Fy/2)

(c) Soil-nail interaction.

The soil nail interaction is examined by the limit skin friction.Assuming the

skin friction constant along the embedment length, the nail tensile strength, Tn is

evaluated using the following relationship (CLOUTERRE, 1991).

π (2.23)

where D is the diameter of the soil nail and La is the embedment nail length in the

resistant zone.

The Global FOS=Fc=Fφ=FT=FS, where Fc, Fφ, FT, FS arethe FOS of cohesion,

angle of friction, soil nail resistance, and skin friction, respectively.

(a) Schematic distribution of the lateral pressure along the nail


37

(b) Representation of the various interaction mechanisms within the normal force

(Tn) and shear force (Tc)

Figure 2.15 Multicriteria slope stability analysis method (Schlosser, 1982)


38

2.3.5 FHWA 1996 Design Method

Manual for Design and Construction of Soil Nail Walls was published by U.S.

Federal Highway Administration (FHWA) in 1996. The limit equilibrium method in

the manual considers only tension in the nails. The proposed tension distribution of

tensile forces in the nail is given by the manual, as shown as Figure 2.16. The tension

distribution diagram is based on the nail head strength and the bond strength between

grout and soil. The design approach implements both the Service Load Design (SLD)

and the Load and Resistance Factor Design (LRFD). Checking the FOS for each stage

of construction, following excavation of each lift and prior to the installation of the

associated row of nails, is required. The procedure is illustrated in Figure 2.17. The

equilibrium equations are vertical and horizontal forces in equilibrium and solved by

spreadsheet iteratively for the global FOS, which is identified by the soil’s strength

along the sliding surface.

A set of simplified preliminary charts were developed for the soil nail walls

preliminary, and are shown in Figure 2.18.

Figure 2.16 Nail tension distribution diagram (Byrne, et al., 1996)


39

Figure 2.17 Construction stability of soil nail wall (Byrne, et al., 1996)

Figure 2.18 FHWA 1996 preliminary design chart for soil nail walls (Byrne, et al.,

1996)


40

2.3.6 FHWA 2003 Design Method

In 2003, The FHWA published a new edition design manual, “Geotechnical

Engineering Circular No.7”, for soil nail wall design. The soil nail wall design

approach is based on Allowable Stress Design (ASD) method. A series of preliminary

design charts are also presented by this manual (Figure 2.19). The charts have been

developed using computer program SNAIL for a factor of safety of 1.35. The key

parameter for the design charts are face batter and back slope of the soil nail walls,

and effective friction angle of the soils. The limit equilibrium computer program

SNAIL or GOLDNAIL are used to calculate FOS.

Figure 2.19 FHWA 2003 preliminary design chart for soil nail walls (Lazarte, et al.,

2003)


41

2.3.6.1 GOLDNAIL

GOLDNAIL version 3.11 is a versatile Windows-based proprietary program

developed in 1993-1998 by Golder Associates. The soil strength criterion is a linear

Mohr-Coulomb envelope with the option of using a bi-linear strength envelope. The

program can model up to 13 soil layers, complex slopes and subsurface geometries,

horizontal and vertical surcharge distributions, groundwater, and pseudo-static

horizontal coefficients. The slip surfaces are circular and pass at or above the toe.

This program satisfies moment and force equilibrium. Similar to conventional

slope stability methods, GOLDNAIL divides the potential sliding mass into vertical

slices. The program modifies iteratively the normal stresses distribution at the base of

the slices until force and moment equilibrium is obtained.

2.3.6.2 SNAIL

SNAIL is a computer program developed by the California Department of

Transportation (CALTRANS) in 1991. The program is based on two-dimensional

limit equilibrium that considers force equilibrium only. The failure surface is bi-linear

(with the failure surface originating at the toe) or tri-linear (with the failure surface

originating at the bottom of the excavation at a point away from the toe). For the case

of a tri-linear failure surface, the resisting forces in the lower wedge beneath the wall

are calculated assuming passive earth pressure conditions, with the inclination of the

passive force fixed at the mobilized friction angle.

SNAIL allows up to two uniform (vertical) surcharge distributions and an

internal or external force (horizontal or oblique).Up to seven soil layers can be

modeled. The maximum of two slope segments can be modeled at the toe. The soil

strength criterion used in SNAIL is the conventional linear Mohr-Coulomb envelope.

Bond strength input is associated with the soil input, not with the nail input. Hence, if

different bond strengths need to be modeled in an otherwise homogeneous soil profile,

a new soil layer must be defined.


42

2.4 Introduction of MSE wall

Mechanically Stabilized Earth Wall (MSE) is fill-type reinforced soil structure

and constructed from the bottom up. Multiple layers of reinforcement are placed in

the retaining wall equal in the vertical distance. Granular materials are filled and

compacted between the reinforcements. The reinforcements of MSE could be metallic

and nonmetallic reinforcements. The stresses between soil and reinforcements are

depending on the material type and the geometry of the reinforcements, shown as

Figure 2.20. Figure 2.21, shows the typical load- displacement curve of the metallic

reinforcement pullout test for MSE walls. The pullout resistance of the MSE wall

reinforcements is defined as the maximum force before the pullout displacement

equal to 12.5 mm (0.5 inch) or 20 mm (0.75 inches) (Senanayake, 2011). It is

important to mention that the load-displacement curve presented in the Figure

mentioned above (Figure 2.21) has a non-linear behavior and the steps as shown in

the Figure are due to rupture of the reinforcements.

The pullout resistance of MSE wall, Pr, follows the equation:

(2.24)

where:

Le = the embedment or adherence length in the resisting zone behind the failure

surface

C = the reinforcement effective unit perimeter; e.g., C = 2 for strips, grids, and

sheets

F* = the pullout resistance (or friction-bearing-interaction) factor

α = a scale effect correction factor to account for a non linear stress reduction over

the embedded length of highly extensible reinforcements, based on laboratory data

(generally 1.0 for metallic reinforcements and 0.6 to 1.0 for geosynthetic

reinforcements).

= the effective vertical stress at the soil-reinforcement interfaces.


43

The design method of MSE wall is based on limit equilibrium method. The

types of stability include external, internal, and in some cases, combined

external/internal stability. Shown in Figure 2.22, the external stability of the wall

consists of sliding, overturning, bearing capacity, and deep seated stability. The MSE

wall is treated as a rigid body for the external stability analysis. Meanwhile, because

of the flexibility and satisfactory field performance of MSE walls, the adopted values

for the FOS for external failure are in some cases lower than those used for traditional

gravity earth retaining walls.

The internal failure model could be: i) failure by elongation or breakage of the

reinforcements; ii) failure by pullout. The critical slip surface for internal failure

model is assumed to match the maximum tensile force line of the reinforcements,

illustrated in Figure 2.23. The location of the maximum tensile forces is related to the

type of the reinforcements. The relationships between the stress ratio and overburden

stress with different type of reinforcements are shown in Figure 2.24.

FHWA suggests the computer program MSEW as the official MSE wall

design program.


44

Figure 2.20 Stress transfer mechanisms for MSE wall reinforcement (Elias et al., 2001)


45

Figure 2.21 Typical Load-Displacement Curve for the metallic reinforcement pullout

test for the MSE wall (Senanayake, 2011)

Figure 2.22 Potential external failure mechanisms for a MSE wall


46

Figure 2.23 Location of potential failure surface for internal stability design of MSE

walls.


47

Figure 2.24 Variation of stress ratio with depth in a MSE wall


48

CHAPTER 3

INSTRUMENTATION AND MONITORING OF IH 410 MSE/SOIL

NAIL HYBRID RETAINING WALL

3.1 Project Description

The MSE/Soil Nail hybrid wall project documented in this dissertation was

approximately 2200 ft in length and was located near the IH-410 overpass over

Ingram Road in San Antonio, Texas (Figure 3.1). As shown in Figure 3.2, the heights

of soil nail wall and MSE wall portions varied along the length of the wall. Two

separate sections of the wall were selected for the purpose of instrumentation and

monitoring. The first, Wall 7 Section A, is located at Station 703+80. The height of

the soil nail wall at this location is 4.0 m and the height of the MSE wall is 5.4 m. The

MSE/Soil Nail hybrid wall has a MSE/SN Height Ratio of 1.35. The second wall

section, Wall 7 Section B, is located at Station 705+40. It has a 5.0 m soil nail wall

and a 4.5 m MSE wall yielding a MSE/SN Height Ratio of 0.88. Cross sectional

views of the two wall sections are shown in Figures 3.2 (a) and (b).

The reinforcement of MSE wall consisted of 6.7 m geogrid mats anchored on

the width of 2.3 m precast panels. There were 2 goegrid mats per panel for each layer.

The types of geogrid mats are show in Table 3.1. Soil nails were installed at 1.0 m and

1.05 m horizontally and vertically, respectively. Grouting holes were 150 mm in

diameter and rebars were 25 mm in diameter. The length of the soil nails in Wall 7

Section A were 8.5 m for the first row and 7.9 m for the remaining rows. Wall 7

Section B had a length of soil nails equal to 7 m. The inclination of nails was 15

degrees below horizontal. The soil for MSE wall was free drain granular material with

an angle of friction of 34 degree and a unit weight of 19.6 kN/m3. The soil for soil nail

walls consisted of gravelly silty sand with a design angle of friction of 35 degree and

a unit weight of 19.6 kN/m3.


49

Figure 3.1 Profile view of Wall No.7 in San Antonio and wall panels selected for instrumentation


50

Table 3.1The reinforcement of the MSE wall

The row of the MSE wall

(from the top down)

Mat Length

(m)

Wire Size Spacing

(m*m)

Mat Width

(m)

Trans Wire

(qty/mat)

Long Wire

(qty/mat)

Row1-4 6.7 W9.5xW11 0.23x0.30 0.46 13 3

Row5-6 6.7 W11.5xW11 0.23x0.30 0.46 13 3

Row7-14 6.7 W14.5xW11 0.23x0.30 0.69 13 4


51

(a) (b)

Figure 3.2 Hybrid Wall Sections Selected for Instrumentation and Monitoring: (a) Wall Section A; (b) Wall Section B

a

a

a

a

b

b

b

b

c

c

c

c

d

d

d

d

Instrumented Soil Nails

Inclinometer Casing 0.75 m from Wall

Face Inclinometer Casing 7.0ft from Wall Face

MSE Wall 5.4 m

Soil Nail

Wall 4 m

4.5 m

Embedment

a

b

c

d

a

a

a

b

b

b

c

c

c

d

d

d

a

b

c

d

MSE Wall 4.5 m

Soil Nail

Wall 5 m

4.5 m

Embedment

Inclinometer Casing 7.0ft from Wall Face

Instrumented Soil Nails


52

3.2 Construction of the Hybrid Wall

The construction started from Feb 19th

2008 and the phases are shown in Table

3.2. The increments of the first excavation for the soil nail wall portions was 1.5 m

and the rest varied from 1.05 m to 1.35 m. The construction of the MSE wall started

after all the soil nail wall sections were finished. By May 30th

2008, the construction

of the MSE wall was terminated and the pavement above the MSE wall was

completed by August 14th

2008.

3.3 Case Studies

In order to study the behaviors of the soil nail walls, the instrumentations and

monitoring of these walls have been performed since 1980s. Two aspects are mainly

concerned by the instrumentation: the maximum tensile forces of the nails, and the

horizontal displacement of the wall. The maximum tensile forces of the nails illustrate

the interaction between the soil and the nails. Furthermore, the locations of the

maximum tensile forces at the nails may present the critical failure surface of the soil

nail wall. The horizontal displacements reveal the failure pattern of the soil nail walls.

The serviceability for these walls also requires the horizontal displacements less than

the tolerable deformation and must not affect the other structures behind the walls.

Strain gages and inclinometer are used to measure the strain of nails and the

horizontal displacement of soil nail walls respectively. The nail forces then can be

calculated by converting the strains to stresses. The installation of the strain gages

could be different. The soil nail wall located at U.S. Highway 26-89 (Turner and

Jensen, 2005) and the soil nail wall located at Seattle (Thompson and Miller, 1991)

had the strain gages located at 3 o’clock position on the rebars to minimize the

potential bending interference as shown in Figure 3.3. The CLOUTERRE soil nail

wall (Plumelle et al., 1991) and Swift- Delta soil nail wall (Barrows, 1994) had the

strain gauges installed on the top and bottom of the rebars, illustrated in figure 3.4.

Then, the non-uniform strains resulting from bending of the bar can be detected. The

average of the top and bottom measurements was used as the axial strain.

Thompson et al. (1991) and Banerjee et al. (1998) suggested that the grout

could provide the tensile resistance before creeping or cracking. Therefore, the

stresses in the rebars may not be able to present the total stresses in the nails. The

grout also provides the nail the ability of bending resistance. The bending moments


53

and shear forces in the nails could be activated under large deformation of the soil nail

walls.

Figure 3.3 Cross section of nail tendon with strain gauge location at U.S. Highway

26-89 (Turner and Jensen, 2005)

Figure 3.4 Cross section of instrument section for Swift-Delta wall (Barrows, 1994)


54

3.4 Instrumentation Plan

Two types of vibrating wire strain gages (VWG) were installed. The first type

of VWG was spot welded to the soil nails in order to measure rebar strains. The

instrumentation scheme is illustrated in Figure 3.5 (a) and (b). The spot welded

gauges were place at the top and bottom of the rebars at the gage location.

The second type of VWGs were Model 4210 VWG, illustrated in Figure 3.6 (a)

and (b), embedded in the grout to measure the its strains. The Model 4210 VWG have

plates at either end of the gage that lock into place in the grout. The gage then behaves

the same way as the spot welded gage. Figure 3.6 (b) shows a Model 4210 VWG that

had been mounted on the plastic centralizer attached to the steel bar. Figure 3.7 shows

the gage layout on the bar for a 7.9 m long nail that carried both types of strain gages.

The inclinometer casing was installed 0.75 m behind the facing at Wall 7 Section A

to determine the deformation in the soil mass behind the wall. The casing was placed

by drilling, down to approximately 4.6 m below the final bottom ground level.


55

Table 3.2 Wall No.7 Construction Timeline

Date (2008)

Wall Section A Wall Section B

Feb 19

--- Depth of excavation = 1.5 m; Top

row nails installed and grouted

Feb 20 Depth of excavation = 1.5 m

Top row nails installed and

grouted

Shotcreted top nail

Feb 21 Shotcreted top nails ---

Feb 22 --- Depth of excavation = 2.6 m

Feb 23 Depth of excavation = 2.6 m

Feb 25 --- The second row nails installed and

grouted

Feb 26 The second row nails installed,

grouted and shotcreted

The second row nails shotcreted

Feb 27 --- Depth of excavation = 3.7 m; The

third row nails installed and

grouted

Feb 28 --- The third row nails shotcreted;

Depth of excavation = 5.0 m

Mar 4 Depth of excavation = 4.0 m ---

Mar 5 The third row and fourth row

nails installed, grouted and

shotcreted

---

Mar 7 --- The fourth and fifth row nails

installed and grouted

Mar 10 --- The fourth and fifth row nails

shotcreted

May 2 MSE wall height = 0.3 m MSE wall height = 0.3 m

May 9 MSE wall height = 2.4 m MSE wall height = 1.5 m

May 30 MSE completed, height = 5.4 m MSE completed, height = 4.4 m

Jun 16 Construction of traffic barriers Construction of traffic barriers

Aug 14 Pavement Construction Complete Pavement Construction Complete


56

(a) (b)

Figure 3.5 Spot Welded VWGs; (a) Schematic, (b) VWG covered with tape for

protection

(a) (b)

Figure 3.6 Model 4210 VWG ; (a) Schematic, (b) VWG mounted on centralizer

Figure 3.7 Layout of Spot Welded and Embedment Type VWGs on a 7.9 m long Soil

Nail Tendon

Welded VWG Model 4210 VWG


57

3.5 Data Interpretation

3.5.1 Inclinometer Data

The horizontal displacement of Wall 7 Section A is shown in Figure 3.8.

Under high surcharge, the maximum displacements were recorded as 38 mm and 47

mm by the end of MSE wall construction phase, and the pavement completion,

respectively. The ratio of horizontal displacement versus height of the soil nail

wall, δ

, wall was 1.18%.

3.5.2 Grout Strain

The grout used in the soil walls had a theoretical maximum tensile strain of

202 με and maximum compressive strain of 901 με. Grout strains in the soil nail walls

were mostly over the limit for tensile and undergoing large cracking after the

completion of the pavement construction (Figure 3.9). Therefore, the grout sustained

almost nothing of tensile force when the soil nail walls had significant surcharge. The

tensile forces in the rebars were representing the most part of the tensile forces of the

soil nails. Likewise, there was no significant bending resistance in the soil nails.

3.5.3 Tensile Forces in Soil Nail

The strain gage data provided behavior of soil nails under significant

surcharge. The development of soil nail forces was found to be associated with the

stages of construction. The fourth and fifth row nails in Wall 7 Section B were

installed after the last excavation of the soil nail wall. Therefore, the nail forces of

fourth and fifth row for Wall 7 Section B were 0 kN at the completion of soil nail wall.

All of the nail forces kept increasing with different increments during the following

construction of the MSE wall and the pavement. Figure 3.10 shows the distribution

and increments of nail loads along the soil nails under different stages.

The highest nail forces for wall 7 sections A and B, happened at the first row

nail. The maximum tensile force of the first row of Wall 7 Section A occurred at the

nail head during the beginning of the several construction stages. The maximum

tensile force moved to the right-hand side of the soil nail in the last construction stage.

This phenomenon suggests that the potential failure surface of the retaining wall

system moved to the right-hand side of the wall. The active zone of the soil nail wall

may pass through the top nail since the maximum tensile force was found to be 3/4 of


58

the length of the soil nail which was caused as a result of high surcharge. This

situation was not presented in Wall 7 Section B, although the soil nail portion in wall

Section B is greater than the one in Section A. The maximum nail force of first row in

Wall 7 Section A was twice as high as the one of Wall 7 Section B. Since Wall 7

Section A had higher MSE wall and shorter soil nails compared to Wall 7 Section B,

the nail length and value of surcharge could be the key parameters to control the

maximum nail forces of the soil nail walls.

A salient phenomenon was found in Wall 7 Section B. The nail force of the

bottom row nail had a maximum nail force equal to 42 kN, which was close to the one

at first row nail. Meanwhile, the normal soil nail walls with vertical-cut presented

relative small nail forces for the bottom row nails (Thompson and Miller, 1991;

Plumelle et al, 1991; Briaud et al., 1994). The phenomenon could be caused by self

weight and construction loads of the MSE wall and the pavement.


59

Figure 3.8 Horizontal displacement of Wall 7 Section A

0

1

2

3

4

5

6

7

8

9

-20 -10 0 10 20 30 40 50 60

Hei

ght

fro

m t

he

Bo

tto

m o

f th

e In

clin

om

eter

(m

)

Horizontal Displacement (mm)

MSE Wall Construction Completed

Pavement Construction Completed


60

Figure 3.9 Final strain of the grout of the soil nails

(a) Tensile forces measured in the top row nail of Wall 7 Section A

-1000

-500

0

500

1000

1500

2000

0 1 2 3 4 5 6 7 8

Mic

rost

rain

(με)

Distance From The Facing (m)

Wall 7 Section A Row 1 Wall 7 Section B Row 1

Wall 7 Section B Row 3 Wall 7 Section B Row 4

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)

Distance from Facing (m)

Soil Nail Wall Completed MSW Wall Height = 2.4 m(8 ft)

MSW Wall Height = 5.4 m(17.8 ft) Pavement Construction Completed


61

(b) Tensile forces measured in the second row wail of Wall 7 Section A

(c) Tensile forces measured in the top row nail of Wall 7 Section B

0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7 8

Tesi

le F

orc

e (

KN

)




0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7 8

Tesi

le F

orc

e (k

N)

Diatance from Facing (m)




62

(d) Tensile forces measured in the second row nail of Wall 7 Section B

(e) Tensile forces measured in the third row nail of Wall 7 Section B

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)

Distance from Facing (m) Soil Nail Wall Completed MSW Wall Height = 1.5 m(5 ft)


0

5

10

15

20

25

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)

Distance from Facing (m) Soil Nail Wall Completed MSW Wall Height = 1.5 m(5 ft)



63

(f) Tensile forces measured in the fourth row nail in Wall 7 Section B

(g) Tensile forces measured in the bottom row nail in Wall 7 Section B

Figure 3.10 Distribution of nail forces during the construction of the hybrid walls

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)


MSW Wall Height = 1.5 m(5 ft) MSW Wall Height = 4.4 m(14.5 ft)


-5

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(KN

)


MSW Wall Height = 1.5 m(5 ft) MSW Wall Height = 4.4 m(14.5 ft)



64

3.6 Discussion of Results

The instrumentation of soil nail strengthened slope under high surcharge have

been performed by Turner et al. (2005) and Li et al. (2008), as shown in Figure 3.11

(a) and 3.12 (a). The soil nail strengthened slope in Wyoming was 1.8 to 3 m in height

and had the MSE wall of 4 to 7.6 m height above it. Another soil nail strengthen slope

in Hong Kong was 4.75 m in height and 9 m in width. Five layers of concrete blocks

were placed at the crest of the slope as surcharge. The dimensions of each concrete

block were 1.0x1.0x0.6 m.

The horizontal displacements of the soil nail strengthen slopes are show in

Figure 3.11 (b) and 3.12 (b). The δ

values of the horizontal displacements were

much higher than the ones of a normal soil nail wall which usually has δ

value less

than 1/333 (FHAW 2003). The soil nail strengthen slope in Wyoming had the δ

values equal to 1.15% and the soil nail strengthen slope in Hong Kong had the δ

values equal to 0.8%. Also, the soil nail wall portion of the MSE/Soil Nail hybrid wall

in IH 410 had δ

equal to 1.18%.

The conclusions made for the instrumentation of the MSE /Soil Nail hybrid

wall are shown below:

1. High surcharge could cause large horizontal displacement of the soil nail wall in

its facing direction. The measured data showed that the ratio of horizontal

displacement versus height,

, of the soil nail wall may be greater than 1%. The

similar phenomenon was observed by Turner et al (2005) and Li et al (2008).

2. The grout of the soil nail was undergoing large cracking with high surcharge and

the tensile forces were mostly carried by the rebars, and the soil nails presented

very limited bending resistance.

3. The soil nail forces were significantly influenced by the surcharge. The bottom

row nails might have the maximum nail forces close to the first row nails.


65

(a) Typical cross section of MSE/Soil Nail hybrid wall Station 20+350

(b) Slope inclinometer reading of the MSE/Soil Nail hybrid wall

Figure 3.11 MSE/Soil Nail hybrid wall at U.S. Highway 26-89, Wyoming (Turner

and Jensen, 2005)


66

(a) Soil nail slope with surcharge

(b) Soil nail slope horizontal displacement under high surcharge

Figure 3.12 Loose fill soil nail slope under high surcharge in Hong Kong (Li et al.,

2008)


67

CHAPTER 4

2D FINITE ELEMENT ANALYSIS OF SOIL NAIL WALL

4.1 Introduction

Even though limit equilibrium methods (LEM) are capable of analyzing a soil

nail wall’s overall stability and the rupture and pullout capacities of the

reinforcements, they do not have the capability to model the wall’s deformation

behavior. The nail forces calculated by limit equilibrium methods do not present the

working forces since the theories are based on the extreme failure conditions.

On the other hand, finite element methods (FEM) can provide insight into the

deformation behavior and assess the overall performance of soil nail walls under

various conditions (geometry, soils’ properties, and reinforcements’ properties).

Soil materials are the heterogeneous their behavior is strongly influenced by

factors, such as: grain size, mineralogy, structure, pore water pressure, and initial

stress state etc. Finite element models require additional soil parameters, such as

Young’s modulus and Poisson’s ratio that are not considered in LEM, for describing

the stress-strain relationship of soils.

In order to characterize the stress-strain behavior of soil materials before and

after failure, a series of constitutive models have been developed. The most common

constitutive model used in soil mechanics is the Mohr-Coulomb model, also named

perfectly elastic-plastic behavior. Figure 4.1 shows the basic idea of a perfectly

elastic-plastic model for the finite element programs. The plasticity is associated with

the development of irreversible strain.

The full Mohr-Coulomb yield condition consists of six yield functions:

(4.1)

(4.2)

(4.3)

(4.4)


68

(4.5)

(4.6)

where:

are the principal effective stresses and arranged in algebraic

order:

; φ is the angle of friction; c is the cohesion.

The six yield functions together present a hexagonal cone in principal stress

space, shown in Figure 4.2.

Figure 4.1 Comparison of Mohr-Coulomb model and typical triaxial test results of

soil (Popa and Batali, 2010)

Figure 4.2 Mohr-Coulomb yield surface in principal stress when c=0 (PLAXIS

Material Manual)


69

The advantages of using Mohr-Coulomb model are:

(a) The model requires fewer parameters than other constitutive models.

(b) The parameters required by the model are easier to be evaluated or

experimentally determined

On the other hand, Mohr-Coulomb model does not match the typical

experimental results of triaxial tests on soil, shown in Figure 4.1. Relatively larger

error may occur when the soil is under frequent loading-unloading cycles.

Other constitutive models for soil, such as hardening soil model, are more

sophisticated but reuqire more parameters as input. Some of the parameters may

require unconventional soil laboratory tests. The selection of the soil’s constitutive

model should depend on the available information.

4.2 Cases Studies of Finite Element Modeling of the Soil Nail Walls

Previous research studies that performed finite element modeling of soil nail

walls are presented and discussed in this section.

4.2.1 Polyclinic Wall in Seattle, Washington

A 16.7 m (55 ft) soil nail wall was designed and constructed by DBM

Contactors, and instrumented by Golder Associates in Seattle area (Thompson and

Miller, 1990). The soil conditions consisted of fill to a depth of 2.4 m (8 ft), underlain

by very dense glacial out wash and gravel and very dense lacustrine fine sand and silt.

The cross section, soil’s properties and nail length are shown in Figure 4.3. The nails

were in installed on 1.8 m (6 ft) centers horizontally and vertically with the inclination

on 15 degrees below the horizontal. The grout was 203 mm (8 inches) in diameter.

The wall was modeled by FES2D, a non-linear finite element program.

Anisotropic soil modulus (i.e. different values for soil moduli in horizontal and

vertical directions) was used to simulate the soil’s behavior. The soil moduli were

treated as the primary variables that were adjusted until reasonable agreement was

reached between the measured data and finite element results, as shown in Figure 4.4.

The horizontal deformation and the overall maximum nail forces, both were compared.

The top row nail yield an error about 50% of maximum nail force and the nail at 7th

row had an error of maximum nail force about 100%. The authors mentioned that,

when isotropic soil modulus was used, the FEM results overestimated vertical heave


70

response of the excavation and yielded a deformation pattern that did not match with

the measured deformations.


71

Figure 4.3 Cross section and soil’s properties of Polyclinic wall in Seattle

(Thompson and Miller, 1990)

Figure 4.4 Comparison between the results of the finite element analysis and the

measured data: (a) wall facing displacement; (b) maximum nail forces. (Thompson

and Miller, 1990)


72

4.2.2 CLOUTERRE Wall

In order to study the failure modes of a soil nail wall, a full-scale test was

conducted as a part of French National Research Project, CLOUTERRE (Plumelle et

al., 1990). This soil nail wall was 7 m height and constructed on Fontainebleau sand

foundation. Figure 4.5 shows the schematic of the soil nail wall. The nails used in this

wall were aluminum tubes that were embedded in a grout column with 62 mm in

diameter (Figure 4.6). The spacing of the nails was 1.0m in the horizontal direction

and 1.15m in the vertical direction.

The finite element model used perfectly elastic-plastic soil model. The

maximum nail forces showed a good agreement with experimental values. Figure 4.7

shows the experimental and calculated results of the nail forces and the wall facing

horizontal displacement. The 2nd

row nail had an error of about 50% for the maximum

nail force and an error of about 30% for the 3rd

row nail. The horizontal displacement

was under predicted. The reason was likely caused by the inability of elastic-plastic

model to present the complex soil behavior under cyclic loads, and the difficulty in

correctly determining the elastic modulus.

Figure 4.5 Schematic of CLOUTERRE Wall ((Plumelle et al., 1990)


73

Figure 4.6 Section view of the grout bars of CLOUTERRE wall (Plumelle et al.,

1990)

(a)

(b)

Figure 4.7 Comparison between the results of the finite element analysis and the

measured data: (a) maximum nail forces; (b) wall facing displacement. (Plumelle et

al., 1990)


74

4.2.3 Swift-Delta Soil Nail Wall

Swift-Delta soil nail wall was constructed below the Oregon Slough Bridge

near Portland. The cross section of the soil nail wall and the construction sequence are

shown in Figure 4.8. The properties of the soil at the Swift Delta Wall construction

site were as follows: internal angle of friction = 32.4o, cohesion = 4.7 kPa, and unit

weight =18 kN/m3

Briaud and Lim (1994) used 3D ABAQUS to create a numerical model for

swift-delta wall (Figure 4.9). The soil model used was a modified Duncan-Chang

Hyperbolic model. The numerical maximum nail forces matched the measured data

very well (Figure 4.10). Meanwhile, the results of the finite element analysis failed to

match displacement pattern of the measured data (Figure 4.11).

Figure 4.8 Cross section and construction sequence of Swift- Delta wall (Briaud and

Lim, 1994)


75

Figure 4.9 Finite element model of Swift-Delta wall (Briaud and Lim, 1994)

Figure 4.10 Nail forces of Swift-Delta wall: (a) measured nail forces; (b) finite

element analysis results. (Briaud and Lim, 1994)


76

Figure 4.11 Horizontal displacement of Swift-Delta wall (Briaud and Lim, 1994)


77

4.2.4 Simulation of Soil Nail Structures Using PLAXIS 2D (Sivakumar Babu, G. L.,

Singh, V. P., 2009, 2010)

Babu and Singh (2009, 2010) presented findings from a study in which used

“plate” or “geogrid” elements for simulating soil nails in the finite element program

PLAXIS. Plate element has the capacity to resist bending moment, tensile forces, and

shear forces while geogrid element can resist only tensile forces. Two soil nail walls

of 10 m and 18 m vertical height were used in the above study (Figure 4.12). The

authors concluded that the plate element and geogrid element provide similar results

of global FOS and horizontal displacement of the walls. The maximum tensile forces

developed in the nails simulated using geogrid elements were 15% more in

comparison to that of the plate elements. The impact of the cohesion of the soil also

was presented by the authors with a 6.6 m height soil nail wall model. The maximum

nail forces had a slight effect due to change of cohesion (Figure 4.13). Meanwhile, the

horizontal displacement had increments of 100% when the cohesion was increased

from 10 kPa to 20 kPa (Figure 4.14).

Figure 4.12 Soil nail wall models (Babu and Singh, 2009)


78

Figure 4.13 Maximum nail forces with different cohesion (Babu and Singh, 2010)

Figure 4.14 Horizontal displacements with different cohesion (Babu and Singh, 2010)


79

4.3 Reinforcement Pullout Behavior in Finite Element Program

Although the finite element simulation studies described in Section 4.1

provided important information on the overall behavior of soil nail walls, the

interaction at the soil-nail interface was not adequately considered. Wei and Chen

(2010) suggested that the technique may be more suitable for fill type reinforced soil

structures.

First of all, an approximation must be made in order to model 3D earth

reinforcements such as soil nails using 2D finite elements, as shown in Figure 4.15.

The grout is ignored in the numerical models since it would undergo cracking at large

strains and therefore would not be capable of carrying significant tensile forces at the

final stage. This phenomenon was observed in IH 410 MSE/soil nail wall

instrumentation project. Accordingly, the equivalent elastic modulus of soil nails,

EAeq, in 2D models, is defined as follows:

(4.7)

where EA is the elastic modulus of soil reinforcements and Sh is the horizontal

spacing of the soil reinforcements.

One important parameter in the 2D numerical model is the Unit pullout

capacity Quu,

(4.8)

where Rp is reinforcement pullout capacity, Qu is pullout capacity per unit length, Lp

is Pullout length.

Figure 4.15 Representation of 3D and 2D models (Zevgolis and Bourfeau, 2007)


80

The finite element program PLAXIS was used to investigate the reinforcement

pullout behaviors in the finite element models. The soil constitutive model used in this

analysis was the Mohr-Coulomb model. French National Project CLOUTERRE (1991)

concluded that the mobilized bending moment and shear force to be mobilized in the

nails occurred only if a shear zone developed in the soil nailed mass. The shear and

bending of the soil nails are also disregarded in FHWA soil nail wall design

guidelines (Lazarte et al, 2003). Therefore, the soil nails were simulated as geogrid

elements in the finite element model developed in this study. Geogrid elements in the

PLAXIS program can only resist tensile forces.

PLAXIS program allows the users to introduce an interface parameter, Rinter,

between the soil nails and soil. PLAXIS uses Rinter to calculate the strength parameters

corresponding to the interface elements based on the following the rules:

(4.9)

(4.10)

where ci and φi are the interface material properties, csoil and φsoil are the soil’s

properties. Rinter is less or equal to 1.0 in the program.

The PLAXIS is also capable calculating a global FOS for the models. The

global FOS provided by PLAXIS, , is defined as below:

φ

φ

(4.11)

where the soil strength parameters with the subscript “input” refer to the input

properties entered in the material sets and the parameters with the subscript “reduced”

refer to the reduced values used in the analysis.

The finite element models of the soil structure reach fully failure under the

reduced parameters. Unlike in the limit equilibrium models, the failure of the finite

element models may not relate to a failure surface.

A normal numerical pullout test model was created as shown in Figure 4.16.

The wall facing was supported by anchors and struts in order to minimize the effect of

wall facing displacement. There were the openings of 0.03 m (0.1 ft) on the facing for

the reinforcements. A series of reinforcements were 0.9 m (3 ft) in length and had a

vertical spacing of 1.2 m (4 ft). The soil and reinforcement parameters are shown in


81

Table 4.1. The numerical model simulated the construction stages are as indicated

below:

(a) The excavation of the vertical slope and installation of the facing, anchor,

and strut

(b) The installation of the reinforcements

(c) The pullout test of the reinforcements one by one

The displacements of the reinforcements under different forces and interfaces

were recorded. The pullout behaviors of the reinforcements are shown in Figure 4.17.

The unit pullout force, Puu, is defined as:

(4.12)

where, Pn is the force acted on the reinforcements of the numerical model, Lp is the

length of the reinforcement.

For the normal numerical pullout test model, the test results show that:

1. The reinforcement unit pullout capacity Quu and pullout displacement at

maximum pullout force, PDMPF are dependent on the soil’s properties and

the magnitude of the soil interface parameter, Rinter.

2. The reinforcement unit pullout capacity Quu and PDMPF linearly increase

with depth.

3. The reinforcement unit pullout capacity Quu and PDMPF do not behave the

same as the typical soil nail pullout test performance.

4. The curves of the pullout displacement match typical MSE wall metallic

reinforcement pullout test results

5. By varying the magnitude of the interface parameter, Rinter, it is possible to

achieve close agreement between the reinforcement unit pullout capacity Quu

and the MSE wall design value, as shown in Figure 4.18.


82

(a) (b)

Figure 4.16 (a) Normal Numerical Pullout test model, (b) Facing opening,

reinforcement and force of the model


83

Table 4.1 Materials properties for the normal numerical pullout test model

Material Unit Weight

(kN/m3)

Young’s

Modulus (MPa)

Poisson’s

Ratio

Angle of

friction (o)

Cohesion

(kPa)

Soil 19 20 0.3 30 2.5

(a) Soil’s properties

Facing EA

(kN/m)

EI (kN/m2/m) Poisson’s

Ratio

Weight

(kN/m2)

MSE Wall 120000 3140000 0.15 1.6

Reinforcement 73000

(b) Properties of the facing and reinforcement

Table 4.2 Interlayer’s properties according to different unit pullout capacity of the nails

Unit Pullout Capacity of the Soil Nail, Quu Interlayer’s Properties，Angle of friction φ= 10

Quu=24.5 kN/m/m (0.5 kip/ft/ft) c=34.3 kPa, E = 9.8 MPa, Rinter=0.3, ν=0.3

Quu=49 kN/m/m (1.0 kip/ft/ft) c=73.5 kPa, E = 24.5 MPa, Rinter=0.3, ν=0.3

Quu=73.5 kN/m/m (1.5 kip/ft/ft) c=102.9 kPa, E = 49 MPa, Rinter=0.3, ν=0.3

Quu=98 kN/m/m (2.0 kip/ft/ft) c=161.7 kPa, E = 73.5 MPa, Rinter=0.3, ν=0.3

Quu=122.5 kN/m/m (2.5 kip/ft/ft) c=191.1 kPa, E = 98 MPa, Rinter=0.3, ν=0.3


84

(a) Unit pullout resistance versus pullout displacement for Rinter=0.4

(b) Unit pullout resistance versus pullout displacement for Rinter=1.0

Figure 4.17 Pullout forces versus pullout displacement for the normal numerical

Pullout test model at various depths

0

20

40

60

80

100

0 5 10 15 20 25 30

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=4.9 m (16 ft)

Depth=3 m (10 ft)

Depth=1.2 m (4 ft)

0

20

40

60

80

100

120

0 5 10 15 20

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=4.9 m (16 ft)

Depth=3 m (10 ft)

Depth=1.2 m (4 ft)


85

Figure 4.18 Comparison of the unit pullout capacity of the normal numerical pullout

test model with various Rinter values and the MSE wall design value of highway IH410

located at San Antonio

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Dep

th (

m)

Ultimate Unit Pullout Forces (KN/m/m)

Rinter=0.4

Rinter=0.6

Rinter=1.0

the Design Value of Highway IH410 Located at San Antonio


86

4.4 Soil Nail Pullout Simulation by 2D PLAXIS

In this research, an innovative numerical model was introduced in order to

simulate the soil nail pullout behavior. The soil nail pullout test model is shown in

Figure 4.19 (a). The model was similar to the normal numerical pullout test model

except it had a 1:2 back slope. Also, an interlayer with thickness of 0.15 m was

introduced around every soil nail, as shown in Figure 4.19 (b). The interlayers were

defined as the soils with an angle of friction of 1o, but with different cohesions and

different Young’s moduli. The values of the cohesion and Young’s modulus were

depended on the desired unit pullout capacity Quu and PDMPF. Table 4.2 shows the

interlayer’s properties according to varied unit pullout capacity of the nails. Figure

4.20 shows the test results of the soil nail pullout test model. In most situations, the

interlayers were able to simulate the pullout performance of the soil nails. For some

cases, such as those shown in Figure 17 (c), (d), and (e), the Quu value were lower

than the expected value when the overburden was relatively small. However, even in

these instances, Puu/displacement ratios were the same when compared to others. Also

this problem was not critical for the hybrid wall since the soil nail wall portion had

high surcharge.

The use of interlayer partially changed the soil’s properties of the soil nail. For

the soil nail wall portion of the hybrid wall, the interlayer was only 15% of the soil.

After using the inter layer, the overall shear strength of the soil for the soil nail wall

portion was about 90% to 110% of the original one. Considering the soil nail wall

portion was only about 50% of the hybrid wall, the influence of this on the hybrid

wall was even smaller.

The advantages for introducing the interlayer for soil nail are:

(a) The soil nail unit pullout capacity Quu can be simulated and has little impact

on the soil normal stresses.

(b) Under high surcharge, all of the soil nails will yield the desired soil nail unit

pullout capacity Quu.

(c) The PDMPF of soil nails can be described by the model and has relatively

uniform values.

(d) The soil nail pullout performance is independent of the soil’s properties of


87

the retaining wall.

(e) The curves of the pullout displacement match the in situ test results from soil

nail pullout test.


88

(a)

(b)

Figure 4.19 (a) Soil nail pullout test model, (b) Interlayer and facing opening of the

soil nail.

Interlayer

0.15 m


89

(a) Unit pullout force versus displacement for the interlayer of Quu=24.5 kN/m/m

(b) Unit pullout force versus displacement for the interlayer of Quu=49 kN/m/m

0

5

10

15

20

25

30

0 2 4 6 8 10

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=0.45 m

Depth=3.65 m

Depth=7.9 m

0

10

20

30

40

50

60

0 2 4 6 8 10

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=0.45 m

Depth=3.65 m

Depth=7.9 m


90

(c) Unit pullout force versus displacement for the interlayer of Quu=73.5 kN/m/m

(d) Unit pullout force versus displacement for the interlayer of Quu=98kN/m/m

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=0.45 m

Depth=1.5 m

Depth=3.65 m

Depth=7.9 m

0

10

20

30

40

50

60

70

80

90

100

110

0 2 4 6 8 10

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=0.45 m

Depth=1.5 m

Depth=3.65 m

Depth=7.9 m


91

(e) Unit pullout force versus displacement for the interlayer of Quu=122.5kN/m/m

Figure 4.20 Pullout test results of the innovative soil nail pullout test model with

different unit pullout capacity, Quu

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

0 2 4 6 8 10

Uin

t P

ullo

ut

Forc

es

Pu

u (

KN

/m/m

)

Displacement (mm)

Depth=0.45 m

Depth=1.5 m

Depth=2.6 m

Depth=3.65 m

Depth=7.9 m


92

4.5 Simulation of the MSE/Soil Nail Hybrid Retaining Wall

According to results from the two numerical pullout test models described in

Section 4.2 and 4.3 above, an FEM models of the MSE/Soil Nail hybrid wall were

built for Wall 7 Section A and Section B, as shown in Figure 4.21. The models of the

construction stages for soil nail wall and MSE wall portions were simulated according

to Table 3.2. Each lift of the construction for the MSE wall was 0.3 m (1 foot) and had

24.5 kPa (500 psf) compaction load. The models did not simulate the construction of

the pavement since the pavement structures and construction loads were different

from the ones of the MSE wall. The interlayer was introduced in order to simulate the

soil nail pullout design value. The soil nail wall portion had an ultimate bond strength

of qu=49.2 kPa with the factor of safety equal to 2.0. Therefore, unit pullout capacity

is expressed as follows:

π

Since the Puu has the factor of safety equal to 2.0, the numerical models used:

Quu =23.2*2=46.4 ≈ 49 kN/m/m.

The materials’ properties of the MSE/ Soil Nail hybrid wall models are shown

in Table 4.3.

The comparison of the nail forces between measured data and the finite

element analysis is shown in Figure 4.22. As it is noted, the finite element analysis

provided results that agreed well with the measured data except for the third and

fourth row nails of Wall 7 Section B.

The soil nail wall facing horizontal displacements under different soil’s

Young’s modulus also were tested by the finite element models. The soil’s Young’s

modulus of the hybrid wall numerical models were 9.8 MPa (200 ksf), 14.7 MPa (300

ksf), 19.6 MPa (400 ksf), and 29.4 MPa (600 ksf). The soil nail wall facing horizontal

displacements of the measured data and finite element analysis results are shown in

Figure 4.23. The finite element results suggested that the wall facing displacement

was highly dependent of the value of the soil’s Young’s modulus. Though, the finite

element model did not match the pattern of the measured horizontal displacement, it

was able to provide the close value of the maximum horizontal displacement.


93

According to the finite element models, the maximum horizontal displacements of

Wall 7 Section B were 2/3 of the maximum horizontal displacements of Wall 7

Section A with the same soil properties. The finite element analysis results also

showed that the Young’s modulus of the soil had little impact on the maximum nail

forces. The differences of the maximum nail forces were less than 10% for the first

row nails and 2% for the remaining rows with different soils modulus value.

The differences between the measured data and finite element results may due

to uncertainties in the properties of the soil and soil-nail interaction. The construction

of the hybrid wall occurred during a 6 months period during which rainfall caused

construction delays. The performance of the soil nails could be significantly affected

by the rainfall infiltration (Cheng and Hansen, 1994; Zhou et al, 2009). It may also

caused by the inability of isotropic Mohr-Coulomb model to accurately model the

complex soil behavior under cyclic shear stresses.

(a) Wall 7 Section A


94

(b) Wall 7 Section B

Figure 4.21 Finite element mesh: PLAXIS V8 finite element models for MSE/Soil

Nail hybrid retaining walls

(a) Wall 7 Section A: Nail Forces on First Row Nail

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)


Measured,MSW Wall Height = 2.4 m(8 ft) Analysis, MSE Wall Height=2.4 m (8 ft)

Measured,MSW Wall Height = 5.4 m(17.8 ft) Analysis, MSE Wall Height=5.4 m (17.8 ft)


95

(b) Wall 7 Section A: Nail Forces on Second Row Nail

(c) Wall 7 Section B: Nail Forces on First Row Nail

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8

Tesi

le F

orc

e (

KN

)


Measured,MSW Wall Height = 2.4 m(8 ft) Analysis, MSE Wall Height=2.4 m (8 ft)

Measured,MSW Wall Height = 5.4 m(17.8 ft) Analysis, MSE Wall Height=5.4 m (17.8 ft)

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8

Tesi

le F

orc

e (

kN)

Diatance from Facing (m)

Measured, MSW Wall Height = 1.5 m(5 ft) Analysis, MSE Wall Height=1.5 m (5 ft)

Measured, MSW Wall Height = 4.4 m(14.5 ft) Analysis, MSE Wall Height=4.4 m (14.5 ft)


96

(d) Wall 7 Section B: Nail Forces on Second Row Nail

(e) Wall 7 Section B: Nail Forces on Third Row Nail

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)




0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)





97

(f) Wall 7 Section B: Nail Forces on Fourth Row Nail

(g) Wall 7 Section B: Nail Forces on Bottom Row Nail

Figure 4.22 Comparison between measured nail forces and finite element analysis

results

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

Te

nsi

le F

orc

e (

KN

)




-5

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8

Ten

sile

Fo

rce

(K

N)





98

Figure 4.23 Wall facing displacements of measured data and finite element analysis

results with varied Young’s modulus

0

1.5

3

4.5

0 10 20 30 40 50 60

Elev

atio

n o

f So

il N

ail W

all F

acin

g (m

)

Horizontal Displacement (mm)

Measured E=9.8 MPa E=14.7 Mpa

E=19.6 MPa E=29.4 MPa


99

Table 4.3 Material properties for the MSE/Soil Nail hybrid wall models

Soil Type Unit

Weight

(kN/m3)

Young’s

Modulus

(MPa)

Poisson’

s Ratio

Angle of

friction (o)

Cohesio

n (kPa)

Interfac

e, Rinter

EA

(kN/m)

Soil for MSE

wall

19 20 0.3 30 2.5 0.4 -

Soil for Soil

Nail Wall

19.5 20 0.3 35 25 - -

Interlayer for

Quu=49 kN/m/m

19.5 24.5 0.3 1 73.5 0.3 -


Reinforcement and Facing EA (kN/m) EI (kN/m2/m) Poisson’s Ratio Weight

(kN/m2)

Soil Nail 38000 - - -

Reinforcement of the MSE Wall 73000 - - -

Soil Nail Wall facing 290000 630000 0.15 2

MSE Wall facing 120000 3140000 0.15 1.6

(b) Properties of the reinforcements and facing


100

CHAPTER 5

PARAMETRIC STUDY AND DEVELOPMENT OF EQUIVALENT

SURCHARGE

5.1 Parametric Study of MSE/Soil Nail Hybrid Wall

The previous chapter described the development of a PLAXIS based Finite

Element Model to simulate the behavior of the MSE/Soil Nail hybrid retaining wall

constructed in San Antonio. As explained in that chapter, this model had the

following unique features:

(a) The FE model simulates each stage of construction as it actually takes place

in the field

(b) The interaction between soil reinforcements and surrounding soil material is

modeled using interface elements so that FEM predicted pullout behavior

would match those observed during pullout testing of soil nails and MSE

reinforcements.

It was further demonstrated that the predictions made by the PLAXIS FE

model for tensile forces developed in soil nails were in good agreement with those

measured in the field. Accordingly, this FE model can now be considered as a reliable

predictor of how an actual MSE/Soil Nail hybrid wall would perform with respect to

tensile forces that develop in soil nails. Therefore, it can be used as the basis for

development of equivalent surcharge loads representing MSE walls constructed on

top of soil nail walls. The first step in the development of equivalent surcharge loads

involved a parametric study to determine which MSE wall parameters has the greatest

influence on the performance of the soil nail wall that supports it.

Such a parametric study was conducted by Alhabshi (2006) using PLAXIS. In

this study, the performance of the soil nail wall portion was assessed based on three

aspects of the soil nail wall performance: maximum horizontal displacement of the

soil nail wall facing δmax, global FOS calculated by PLAXIS, and the maximum

forces of the nails Fmax. Alhabshi suggested that the length of the reinforcements of

the MSE wall has very limited effect on the soil nail wall when it length is larger than

the height of the MSE wall.


101

Using the hybrid wall model of Wall 7 Section A, the same parametric study

was performed by the authors. The results obtained from this parametric study are as

follows:

1. When the angle of the friction of the soil in the MSE wall was varied from

30o to 40

o, δmax, FOS, and Fmax varied 13.4% , 10.2%, and 10.4%

respectively.

2. When the pullout capacity of the reinforcement for the MSE wall was

changed by varying the Rinter parameter from 0.4 to 1.0, as shown in Figure

4.18, then the corresponding changes in δmax , FOS, and Fmax were observed

to be 2.9% , 3.2%, and 4.9% respectively.

3. When the stiffness of the reinforcement for the MSE wall was changed by

varying its EA from 120,000 kN/m to 360,000 kN/m, then the corresponding

changes in δmax , FOS, and Fmax were less than 2%.

The results of the parametric study shows that the above-mentioned material

properties have little effect on the soil nail wall behavior and, therefore, can be

ignored for further research purposes of the MSE/Soil Nail hybrid wall.

5.2 Equivalent Loads of the MSE Wall Portion

FHWA Manual for Design & Construction Monitoring of Soil Nail Walls

(1996) suggested that the MSE wall portion of the hybrid retaining wall should be

treated as surcharge when designing the soil nail portion of the composite wall system.

Therefore, in the next step, parallel analyses were performed using two FE models,

one that simulates a hybrid wall system and another that simulates a soil nail wall

with vertical and horizontal surcharge. The objective of this analysis was to find the

magnitudes of the equivalent loads that would produce the same response in soil nail

wall performance as the actual MSE wall.

The estimation of the equivalent loads is based on the following assumption:

the MSE wall is treated as a rigid body and resists the lateral earth pressure behind the

MSE wall. Therefore, the equivalent loads should consist of two parts: vertical

distributed load and horizontal distributed load, as shown in Figure 5.1. The vertical

distributed load is caused primarily by the self-weight of the MSE system, the soils

behind the MSE wall, and the construction load. These vertical distributed loads may

be described by the following form of the equation:


102

(5.1)

where:

qv = the equivalent vertical distributed load

μv = the vertical distributed load coefficient, to be determined

M = the density of the soil of MSE Wall

hM =the height of the MSE wall

The horizontal distributed load is primarily caused by the lateral earth pressure

behind the MSE wall and the load transfer of that load to the soil nail portion at the

base of the MSE wall. Therefore, the length of the distributed load is equal to the

length of the reinforcement of the MSE wall. The horizontal distributed load may be

expressed as follows:

(5.2)

where:

qh = the equivalent horizontal distributed load

μh = the horizontal distributed load coefficient, to be determined

LM = the length of the reinforcement of the MSE wall

Figure 5.1 Expected forces imposed by MSE wall on soil nail wall (Alhabshi, 2006)

A series of PLAXIS finite element models of the MSE/ Soil Nail hybrid walls

and the corresponding soil nail walls under equivalent loads (Figure 5.2) were built in


103

order to identify the the equivalent distributed load coefficients, μv and μh, . The

following parameter combinations were considered in this analysis:

(a) MSE/SN Height Ratio equal to 1.35, 0.88, and 0.55

(b) The length of soil nail equal to 7.0 , 7.9 , and 8.8 m

(c) The angle of friction for the soil in the soil nail wall equal to 30o, 35

o, and

40o

(d) The Unit pullout capacity Quu equal to 24.5, 49, 73.5, 98.0, 122.5 kN/m/m

All of the hybrid wall models had the same height of 9.4 m (31 feet). The

length of the reinforcement in the MSE wall is 6.7 m (22 feet). A conservative

estimate of 30o was used as the angle of friction of the MSE wall backfill. Since the

soils in the hybrid walls were sandy soil, the relative small value of cohesion was used

for the soils. The cohesion of the soil in MSE wall was 2.5 kPa and 10 kPa for the soil

in the soil nail wall. The models had the same construction phases and construction

load as Wall 7 Section A and B, as shown in Table 3.2.

Under the equivalent loads, the soil nail portion in the hybrid wall and the soil

nail wall under the equivalent loads should have:

(a) Similar maximum nail forces

(b) Similar wall facing displacements

(c) Similar global FOS calculated by the PLAXIS program

(a) The total stresses of the hybrid wall


104

(b) The total stresses of the soil nail wall corresponding to the hybrid wall under the

equivalent loads

Figure 5.2 The PLAXIS finite element models for calibrating the equivalent loads


105

Table 5.1 Material properties for the MSE/Soil Nail hybrid wall models

Soil Type Unit Weight

(kN/m3)

Young’s

Modulus

(MPa)

Poisson’s

Ratio

Angle of

friction (o)

Cohesion

(kPa)

Interface,

Rinter

EA (kN/m)

Soil for MSE wall 19 20 0.3 30 2.5 0.4 -

Soil for Soil Nail

Wall

19.5 20 0.3 35 25 - -

Interlayer for

Quu=49 kN/m/m

19.5 24.5 0.3 1 73.5 0.3 -


Reinforcement and Facing EA (kN/m) EI (kN/m2/m) Poisson’s Ratio Weight (kN/m2)

Soil Nail 38000 - - -

Reinforcement of the MSE Wall 73000 - - -

Soil Nail Wall facing 290000 630000 0.15 2

MSE Wall facing 120000 3140000 0.15 1.6

(b) The properties of the reinforcements and facing


106

5.3 Results and Discussion

Comparison of soil nail performance as predicted by the two FE models

described above revealed that a vertical distributed load coefficient, μv of 1.2 provided

the best overall agreement. The horizontal distributed load coefficient, μh, however,

varied depending on two main parameters, MSE/SN Height Ratio and Ultimate Soil

Nail Pullout Capacity. These effects are shown in Table 5.2. As it is noted, Quu equal

to 98 kN/m/m and 122.5 kN/m/m presented same μh values. The finite element hybrid

wall models failed when the angle of friction φ and unit pullout capacity Quu were low

and therefore μh values were not available for these situations.

Figure 5.3 shows the typical comparison of the facing displacements and the

maximum nail forces between the soil nail wall portion of the hybrid wall and the soil

nail wall, under equivalent load situation. The data show that the facing displacement

and global FOS will have a 7% to 15 % difference between the previous mentioned

cases. Meanwhile, the maximum nail forces in the bottom rows of soil nails wall

under equivalent loads are about 20% to 60% less than that of the of the hybrid wall.

The phenomenon suggests that the numerical models under equivalent surcharge

loads may underestimate the mobilized tensile forces of the bottom rows of nails.


107

Table 5.2 Value of μh

Quu=24.5 kN/m/m Quu=49 kN/m/m Quu=73.5 kN/m/m Quu=98 kN/m/m Quu=123.5 kN/m/m

φ=30o N/A 0.23 0.23 0.23 0.23

φ=35o 0.28 0.26 0.24 0.24 0.24

φ=40o 0.31 0.26 0.26 0.24 0.24

(a) For Soil nail Length=7 m, MSE/SN Height Ratio = 1.38


φ=30o N/A 0.24 0.24 0.24 0.24

φ=35o 0.26 0.22 0.22 0.22 0.22

φ=40o 0.26 0.22 0.22 0.22 0.22

(b) For Soil nail Length=7.9 m, MSE/SN Height Ratio =1.38


φ=30o N/A 0.24 0.24 0.24 0.24

φ=35o 0.24 0.22 0.22 0.22 0.22

φ=40o 0.24 0.22 0.22 0.22 0.22

(c) For Soil nail Length=8.8 m, MSE/SN Height Ratio =1.38


108


φ=30o N/A N/A 0.22 0.24 0.24

φ=35o N/A 0.24 0.24 0.24 0.24

φ=40o 0.35 0.28 0.24 0.24 0.24

(d) For Soil nail Length=7 m, MSE/SN Height Ratio = 0.88


φ=30o N/A 0.23 0.23 0.23 0.23

φ=35o N/A 0.26 0.26 0.26 0.26

φ=40o 0.38 0.28 0.26 0.26 0.26

(e) For Soil nail Length=7.9 m, MSE/SN Height Ratio = 0.88


φ=30o N/A 0.24 0.24 0.24 0.24

φ=35o 0.36 0.28 0.28 0.28 0.28

φ=40o 0.4 0.28 0.28 0.28 0.28

(f) For Soil nail Length=8.8 m, MSE/SN Height Ratio = 0.88


109


φ=30o N/A 0.32 0.26 0.26 0.26

φ=35o N/A 0.32 0.26 0.26 0.26

φ=40o 0.52 0.32 0.26 0.26 0.26

(g) For Soil nail Length=7 m, MSE/SN Height Ratio =0.55


φ=30o N/A 0.32 0.26 0.26 0.26

φ=35o N/A 0.34 0.28 0.28 0.28

φ=40o 0.52 0.4 0.32 0.32 0.32

(h) For Soil nail Length=7.9 m, MSE/SN Height Ratio = 0.55


φ=30o N/A 0.32 0.26 0.26 0.26

φ=35o 0.52 0.36 0.3 0.3 0.3

φ=40o 0.56 0.42 0.36 0.36 0.36

(i) For Soil nail Length=8.8 m, MSE/SN Height Ratio = 0.55


110

(a) Comparison of the facing displacement

(b) Comparison of the maximum nail forces

Figure 5.3 Comparison of the results between the hybrid wall model and soil nail wall

model under equivalent loads.

Figure 5.4 shows the relationship between the equivalent horizontal equivalent

distributed load coefficient and MSE/SN Height Ratio for Quu equal to 24.5, 49.0, and

122.5 kN/m/m. The μh values showed the regression according to the Quu. The

regression shad close relationship the values presented by the situation which the soil

nail wall portions had the angle of friction equal to 35o and the nail length equal to 7.9

0

2

4

6

8

10

12

14

0.5 0.7 0.9 1.1 1.3 1.5

Ele

vati

on

(ft

)

Displacement (in)

For Phi=35, P=1.5 kip/ft/ft

Hybrid Wall Displacement

Equivalent SN Wall

0

5

10

15

0 5 10 15

Ele

vati

on

(ft

)

Nail Forces (kips)

For Phi=35, P=1.5 kip/ft/ft

Hybrid Wall Nail Forces

Equivalent SN Wall


111

m. Therefore, the authors tested the equivalent loads for the soil nail wall that had

cohesion equal to 25 kPa, angle of friction equal to 35o, and soil nail length equal to

7.9 m. By changing the cohesion from 10 kPa to 25 kPa, the μv still is equal to1.2,

while μh increased about 8%.

Figure 5.4 Relationship between the equivalent horizontal distributed load coefficient,

μh and MSE/SN Height Ratio

The contour lines of the total displacement obtained for three different

MSE/SN Height Ratios are shown in Figure 5.5. The potential failure surfaces of the

models can be identified by the density of the contour lines. The highest density of the

contour lines represents the potential failure surface. This failure surface consists of

two separate portions as seen in the figures. The first portion of the failure surface,

which is in the soil nail wall, passes through the soil nails. The second portion of the

failure surface, which is in the MSE wall, passes behind the reinforcements. When the

MES/SN Height Ratio is 0.55, the model had two potential failure surfaces. One of

the potential surfaces was similar to the other hybrid wall and the second one was

similar to the potential surface of the traditional soil nail wall.

y = 0.3375x-0.754 R² = 0.9495

y = 0.2614x-0.421 R² = 0.757

y = 0.2463x-0.224 R² = 0.5308

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

μh

MSE /SN Height Ratio

Quu=24.5 KN/m/m Quu=49 KN/m/m

Quu=122.5 KN/m/m Power (Quu=24.5 KN/m/m)

Power (Quu=49 KN/m/m) Power (Quu=122.5 KN/m/m)


112

(a) MSE/SN Height Ratio equal to 1.35

(b) MSE/SN Height Ratio equal to 0.88

Potential failure surface



113

(c) MSE/SN Height Ratio equal to 0.55

Figure 5.5 Contour lines and potential failure surface of the finite element models for

the MSE/ Soil Nail hybrid walls



114

CHAPTER 6

SUMMARY AND CONCLUSIONS

This research was undertaken with the primary objective of developing a

simplified design method that can be used in the routine design of hybrid retaining

walls constructed in side hill situations. This type of hybrid wall uses a soil nail wall

in the cut section and an MSE wall in the fill section. Therefore, the loads imposed

by the upper MSE wall must be appropriately taken into account when designing the

lower soil nail wall. The FHWA design manual for soil nail walls (FHWA, 1996)

suggests that the upper wall may be considered as an equivalent surcharge load with

vertical and horizontal components when evaluating the soil nail wall for global factor

of safety. However, the design manual does not provide specific guidelines regarding

the magnitudes of these surcharge loads. The primary goal of this research study was

to determine load coefficients associated with these vertical and horizontal surcharge

loads.

The methodology used in this research relied on data collected from two

separate sections of an MSE/Soil Nail hybrid wall constructed at highway IH 410 and

Ingram Road, in San Antonio, Texas. The soil nails installed on these wall sections

were provided with strain gages so that tensile forces that develop on these soil nails

could be monitored during wall construction. Strain gages were mounted on the steel

tendon as well as within the grout column. The strain gage readings in the grout

showed that much of grout had suffered cracking under tensile loads. This observation

implied that the tensile forces on the soil nail were resisted only by the steel tendon

The strain gages mounted on diametrically opposite sides confirmed that

reinforcements did not develop significant bending resistance. Furthermore, the

strain gage results indicated that the nail forces increased significantly due to the

construction of the MSE walls. Unlike the normal soil nail walls, in the MSE/Soil

Nail hybrid walls even the soil nails in the bottom row carried significant tensile

forces. The surcharge also caused large horizontal displacement in the soil nail wall.

Similar to strengthened soil nail slopes under high surcharge, the soil nail wall portion

of the hybrid wall experienced horizontal displacements larger than 1% of the soil nail

wall height.


115

To develop guidelines for equivalent surcharge load coefficients, a series of

innovative 2D finite element models of the hybrid wall were built. These 2D finite

element models were capable of simulating the pullout behavior of the reinforcements

for soil nail wall and the MSE wall. The equivalent distributed loads are evaluated

based on three soil nail wall performance criteria: maximum horizontal displacement

of the soil nail wall facing δmax, global FOS calculated by PLAXIS , and the

maximum soil nail forces Fmax.

Based on the findings from finite element analysis, two equations (Equation

5.1, and 5.2) were developed to calculate the equivalent vertical and horizontal

distributed loads representing the MSE wall portion.

The vertical distributed load coefficient, μv recommeded is 1.2, whereas the

horizontal distributed load coefficient μh varied within the range 0.23 to 0.4 according

to the relationships presented in Figure 5.4.

Limit equilibrium program GOLDNAIL was used to evaluate the global FOS

of IH 410 Wall 7 Section A and B for three different cases (Figure 6.1):

(a) Case 1: Based on the equivalent loads analysis in the chapter 5, the soil nail

walls had the equivalent horizontal and vertical distributed loads calculated using

Equations 5.1and 5.2.

(b) Case 2: The equivalent loads considered only the self weight of the MSE wall

and the active lateral pressure behind the MSE. Therefore, the value of the

vertical and distributed loads can be described as:

(6.1)

(6.2)

where, Ka is the coefficient of active lateral pressure.

(c) Case 3: The hybrid walls were treated as full height soil nail walls.

The nail forces and global FOS are shown in Table 6.1 and 6.2.

The results show that the global FOS in Case 1 is smaller than other cases and

the nail forces are greater than other cases. It means the global FOS is over-estimated

and the nail forces are under- estimated in Case 2 and 3.


116

The finite element models show that the potential failure surfaces of the

MSE/Soil Nail hybrid walls would normally consist of two separate portions, as

shown in Figure 5.5. The first portion of the failure surface is in the soil nail wall and

passes through the soil nails. The second portion is in the MSE wall and behind the

reinforcements. When the MSE/SN Height Ratio is 0.55, the model has two potential

failure surfaces. One of the potential surfaces is similar to the other hybrid wall and

the other one is similar to the potential surface of the traditional soil nail wall. These

phenomena suggest that the traditional limit equilibrium design methods that use a

circular or bilinear failure surface may not be suitable for the design of MSE/Soil Nail

hybrid wall when it is analyzed as one unit.

(a) Case 1 and case 2

(b) Case 3

Figure 6.1 Soil nail walls’ models for the design by GOLDNAIL program


117

Table 6.1 Global FOS of Wall 7 Section A and B analyzed by GOLDNAIL

Wall 7 Section A Wall 7 Section B

Case 1 1.52 1.67

Case 2 1.69 1.85

Case 3 2.49 2.91

Table 6.2 Nail forces of Wall 7 Section A and B analyzed by GOLDNAIL

Row

Wall7 Section A (kN) Wall7 Section B (kN)

case 1 case 2 case 3 case 1 case 2 case 3

1 77.9 73.2 19.3 73.92 68.2 21.2

2 77.9 73.2 19.7 73.92 68.2 21.3

3 72.3 69.3 20.5 73.5 68.0 21.5

4 63.8 60.3 21.5 67.7 62.8 21.6

5 -- -- 23.2 59.4 55.0 21.6

6 -- -- 24.5 -- -- 21.6

7 -- -- 24.4 -- -- 21.6

8 -- -- 23.6 -- -- 20.7

9 -- -- 20.4 -- -- 18.0


118

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