Simple Soil Structure Interaction in innovative foundation design.pdf
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Transcript of Simple Soil Structure Interaction in innovative foundation design.pdf
Simple Soil Structure Interaction Concepts in Innovative Foundation Design
Presented To: ASCE/SAME Engineering Conference
Presented By: Clyde N. Baker Jr., P.E., S.E.
STS Consultants Ltd.
Introduction
� Purpose of Talk – To share some experiences and insights gained over a 50
year career on a subject hopefully of interest to both geotechnical and structural engineers.
� Method – Use well documented case histories to illustrate points.
Key Points to Illustrate or Discuss
� Things are not always what they seem.
� Type of testing and amount can greatly influence predictions.
� Local geological knowledge and relevant case history experience more important than uncorrelated testing no matter how sophisticated.
� Simple tests and concepts correlated with relevant experience can result in predictions closer to reality than more elaborate testing and analysis not well correlated.
� We learn from experience but how close to the edge should we get? Take small steps. Make change gradually.
Four Simple Concepts
� Use dense soil layer over soft clay as a mat to spread out loads to permit footing design instead of a mat foundation.
� Use deep basement excavation stress relief effects in ways to maximize site building capacity.
� Use of piles as settlement reducers rather than as required structural elements for building support.
� Use of variable length piles under mat to minimize differential settlement.
Normal Consolidation vs. Preconsolidation
� Normal Consolidation Test - preconsolidation value often inaccurate.
� Thus settlement prediction based on test preconsolidation value is often inaccurate but usually conservative.
� Pressuremeter creep pressure quite similar to preconsolidation pressure in preconsolidated cohesive deposits.
Project Name:Dunbar Builders
Depth:
25 – 27.5
Water Content:28.8%
LL: 30.1
PL: 16.1
Soil Classification:
Silty Clay, Trace Sand and
Gravel, Gray (CL)
Project Name:Dunbar Builders
Depth: 25 – 27.5
Water Content:28.5%
LL: 33.4PL: 16.1
Soil Classification:Silty Clay, Trace Sand and Gravel,
Gray (CL)
Project Name:Dunbar Builders
Depth: 30 – 32.5
Water Content:21.0%
LL: 28.4PL: 15.1
Soil Classification:Silty Clay, Trace Sand and Gravel,
Gray (CL)
Modified Mat Foundation Design on Soft Clay
Concept – Use dense sand layer over soft clay as a mat to spread out loads to
permit footing design instead of mat foundation.
– Monitor pore pressure buildup and shear displacement in soft clay during 26-story construction to see that shear strength not exceeded.
Site Investigation and Lab Testing Program Soil Profile
– Possible foundation solutions – Bearing capacity and settlement analysis– Structural design concepts– Instrumentation Program – Settlement Measurements – Conclusions
Soil Profile and Properties
Settlement Analysis
Assumptions: Boussinesq stress spread through dense sand layer (conservative)
– Average net stress increase beneath mat = 2000 psf
– Clay Layer: Top 8’ Normally Consolidated, Lower Clay Preconsolidated
– Average t90 = 12-16 minutes
Calculated Settlement:
• 4” at center of mat• 2-1/2” at edge of mat• 90% occurring in 9 to 12 months
Summary of the Stresses Beneath the Center of the Building at a Depth of 5 Feet into the Soft Clay
Inclinometer Readings
Piezometer Readings
Settlement Data
Conclusions:� By utilizing the high bearing capacity of the dense sand layer overlying
the soft clay, a relatively thin and economical modified mat foundation design proved feasible at this site
� Construction of a building that developed theoretically induced shear stresses in the underlying soft clay soils, which exceeded the initial shear strength of the soft clay soil, was successfully accomplished by monitoring the performance of the structure and the critical soft clay. Construction loading occurred at a rate which permitted pore pressure dissipation and resultant shear strength buildup to occur fast enough so that excessive shear strains did not occur.
� Apparent pore pressure dissipation in the soft clay deposit at the site occurred more rapidly than was predicted by one-dimensional laboratory consolidation tests.
� Settlement of the structure has been less than anticipated, possibly due to insufficient recognition of overconsolidation effects.
Dearborn Center
� Concept
– Use deep basement excavation stress relief in combination with new basement mat over existing caisson foundations to maximize number of additional floors possible at site.
– Building load is distributed between new mat and existing caissons based on relative stiffness predicted by pressuremeter testing.
Site Investigation
� Soil
Borings and lab testing
In-situ pressuremeter testing
� Existing Foundations
Coring
Pressuremeter testing below caissons
Pressuremeter Set-Up
Pressuremeter Reduction
-100
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40
Pressure in TSF
Inje
cted
Vo
lum
e in
CC
-10
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40
Pressure in TSF
Cre
ep in
CC
VolumeCreep
Po PlPf
E+
Pressuremeter Data Reduction (BX)
Ed
STS Consultants, Ltd.
PSEUDO-ELASTIC ZONE
PLASTIC ZONE
Ed = Deformation Modulus
Eo = Rebound Modulus
E+ = Recompression Modulus
Pf = Creep Limit
Pl = Limit Pressure
α = Ed/E+
Rules with Pressuremeter
Dead Load Pressure
+ Long-term Real Live Load+ Effective Overburden Pressure
Must be less than Creep Pressure
Rules with Pressuremeter (cont.)
Allowable Bearing Pressure
Where K varies from 0.8 to 1.8 depending on depth to diameter ratio and material type
Pl = Limit PressurePo = Beginning of Psuedo Elastic RangeF.S. = Factor of Safety
( )..SF
PPK ol −≤
Dearborn Center Pressuremeter Profile
Dearborn Soil Profile
Geotechnical Analysis
� Settlement Prediction
– Existing Caissons
– New Mat
Dearborn Center Deflection and Spring Calculations
Dearborn Center Deflection and Spring Calculations (cont.)
Dearborn Center Deflection and Spring Calculations (cont.)
Dearborn Center Deflection and Spring Calculations (cont.)
Dearborn Center Deflection and Spring Calculations (cont.)
Dearborn Center Foundation Plan
Structural
� Local distribution through shear walls � 3-dimensional SAP model used to determine
overall behavior using geotechnical developed springs.
Observed Settlement
� Full structure dead load in place
� Live load not in yet
� Estimated 70% of total design load. So predicted settlement would be about 0.7 inches or about to ¾ inches
� Measured settlement varied from:� 0” on the North wall reported to be on rock caissons to
� ½” on the West wall� ” on the South wall and interior mat
� Possible likely settlement based allowing for surveyaccuracy is ” to ¾”
85
81
85
South Side Office Building
� Use of straight shaft piers as settlement reducers in combined footing design over Chicago soft clay.
� Predict settlement of 12-story building on mat or strip footings on medium dense sand layer over soft clay with and without supplementary pile or pier settlement reducers
� Monitor load distribution between footing and piers and settlement during and after construction
Concept:
Strain Gage Readings and Measured Settlement of Column B-6
-1000
-800
-600
-400
-200
0
200
3-Oct-94 3-Apr-95 3-Oct-95 2-Apr-96 2-Oct-96 2-Apr-97 2-Oct-97 2-Apr-98 2-Oct-98
Date
- = C
ompr
essi
on M
icro
stra
ins
+ =
Ten
sion
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
Sett
lem
ent (
Inch
es)
Black - 1
Gray-1
Average
Settlement
Strain Gage Readings and Measured Settlement of Caisson B-6
-500
-400
-300
-200
-100
0
100
200
3-Oct-
94
3-Apr-9
5
3-Oct-
95
2-Apr-9
6
2-Oct-
96
2-Apr-9
7
2-Oct-
97
2-Apr
-98
2-Oct-
98
Date
- =
Com
pres
sion
Mic
rost
rain
s +
= T
ensi
on
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Set
tlem
ent (
Inch
es)
Gage 8745Gage 8746AverageSettlement
Strain Gage Readings and Measured Settlement of Column C-2
-1000
-800
-600
-400
-200
0
200
3-Oct-94 3-Apr-95 3-Oct-95 2-Apr-96 2-Oct-96 2-Apr-97 2-Oct-97 2-Apr-98 2-Oct-98
Date
- = C
ompr
essi
on M
icro
stra
ins
+ =
Ten
sion
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
Sett
lem
ent (
Inch
es)
Black - 2
Gray-2
Average
Settlement
Strain Gage Readings and Measured Settlement of Caisson C-2
-200
-150
-100
-50
0
50
100
150
200
3-Oct-94 3-Apr-95 3-Oct-95 2-Apr-96 2-Oct-96 2-Apr-97 2-Oct-97 2-Apr-98 2-Oct-98
Date
- = C
ompr
essi
on M
icro
stra
ins
+ =
Ten
sion
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
Sett
lem
ent (
Inch
es)
Gage 8747
Gage 8748
Average
Settlement
0.9481377
(59%)
2327C-2
1.25761496
(75%)
1978B-6
Settlement (inches)
Shaft Base Pressure
(ksf)
Shaft Load (kips)
Column Load (kips)
Column Number
Measured Column/Caisson Load Distributions as of April 14, 1998
Conclusion:
Innovative cost effective solutions to foundation design problems are sometimes possible using combinations or mixtures of foundation elements provided that ground deformation and response to structure loading can be reasonably predicted within allowable tolerances.
Petronas Towers
� Worlds tallest building (1482’)
� Worlds deepest high rise foundations up to 430’
� Worlds deepest ground improvement up to 530’
Tower Foundation Profile
Concept:
� Predict settlement based on modulus values based on bored pile load testing and extensive in-situ pressuremeter testing
� Use simple equivalent footing approach as well as more complete finite element computer programs to predict settlement
� Monitor settlement and load distribution in piles and on mat during and after construction
Use of variable length piles under mat to minimize critical differential settlement of worlds tallest building – Petronas Towers.
Standard Penetration Resistance Profile
535MPa190MPa223MPa226MPa176MPa391.8MPa186.9MPaAvg.
27312525271517# of Tests
383.3495590.3496851931479Max.
68.347.857.7325522.327.5ER Min.
149MPa64.1MPa101.8MPa109.8MPa67.9MPa133.9MPa37.6MPaAvg.
27312626271518# of Tests
470157199.422268330999Max.
11.7MPa18.3MPa38.5MPa17.8MPa32MPa10MP19.3MPaEd Min.
T2-54T2-26T1-54T1-24T1-10B23B14Boring
Pressuremeter Test Results
Overall Weighted Ed Avg. = 94.3
ER Avg. = 267
Settlement Analysis Using Equivalent Footing Method
dEE =1
dEE =2
dEE ×= 205,4,3
MPaE
E
EEE
E
B
B
B
1359420
19485.0
1941
2.3
185.0
112.3
5,4,321
=×
+×
+=
+×
+=
kPaq 130,1=
kPaq 610=
Av 60m piles
Pressuremeter Data
MpaE
MpaE
Av
Avd
267
3.94
=
=+
35.0== +E
Edα , Use 0.4
Settlement Calculation – Menard Empirical Method
1
3
020 5.43
33.1
E
Rq
R
RqR
Es
BMenard
λαλα
+���
����
�
×=
1, 32 =λλ for a circle
cmR 300 =
945.42
500,761.04.0
30
500,730610.0
1353
33.14.0
×
××+�
�
���
�×××
=Menards
mmcmcmsMenard 1.2716.255.0 =+=
Settlement Calculation – Elastic Theory
mms
E
qBs
Elastic
Elastic
59000,250
000,75100,692.035.0
10
=×××=
=µµ
Elastic Compression of Shaft Down to Equivalent Footing Level
mm
kPaEm
kNkN
E
L
conc
conc
4.14000,000,27
000,40727,9
000,000,27
727,98.22.182
000,680,22
=×=∆
≅
=××
=
=∆
�
�
σ
σ
Total Predicted Settlement By Menard Empirical Method
mmmmmmS
sS Menard
5.414.141.27 =+=∆+= �
By Elastic Theory
mmmmmmS
sS Elastic
4.734.1459 =+=∆+= �
Settlement Analysis Using Equivalent Footing Method
Settlement Maps and Rock Contour Plan – Tower 1
Settlement Maps and Rock Contour Plan – Tower 2
Layout Plan of Instrumented Barrettes
0.0m0.5m (Lev. A)
7.5m (Lev. B)
15.0m (Lev. C)
22.0m (Lev. D)
30.0m (Lev. E)
37.5m (Lev. F)
41.25m (Lev. G)
Load (kN)
Thousands
Dep
th b
elo
w c
ut-
off
leve
l
Met
ers
LOAD DISTRIBUTION CURVECOMPUTED FROM SGs MEASUREMENT
-5 5 15 25 35
0
10
20
30
40
50
0.0m
0.5m (Lev. A)
9.5m (Lev. B)
12.5m (Lev. C)
27.5m (Lev. D)
36.5m (Lev. E)
45.5m (Lev. F)
54.25m (Lev. G)
Load (kN) - Thousands
Dep
th b
elo
w c
ut-
off
leve
l
Met
ers
LOAD DISTRIBUTION CURVECOMPUTED FROM SGs MEASUREMENT
-5 5 15 25 35 450
20
40
60
Pressure in PSI(1 PSI = 6.9 kPa)
To
wer
Bu
ildin
g L
oad
in M
N
RELATION OF AVERAGE MAT PRESSURE CELL READINGS AND BUILDING LOAD
Settlement of Tower 1 ColumnsS
ettle
men
t –m
.m.
Petronas Towers: Conclusions and Lessons Learned
� In-situ testing with empirical correlations works well enough for engineering purposes .
� Menard Empirical procedures yield better settlement predictions compared to elastic theory using test pressuremeter modulus values as the Young’s modulus for the soil and geologic conditions reported herein.
� Simple hand calculations for settlement and bearing capacity can be as reliable as sophisticated computer solutions.
� Innovative cost effective foundation solutions are often possible with close interaction of geotechnical and structural engineer and cooperation of experienced contractor.
References � C.N. Baker, Jr. and T.P. Wiesinger, “Modified Mat
Foundation Design Over Soft Clay”, ASTM Special Technical Publication 584
� C.N.Baker, Jr., T.D. Bushell, Rob Diebold, “Dearborn Center: A Unique Soil Structure Interaction Design”, Fifth International Conference on Case Histories, N.Y., N.Y., April, 2004.
� C.N. Baker, Jr., T.A. Kiefer, Kolbjorn Saether, “Use of Straight Shaft Piers as Settlement Reducers in Combined Footing Design Over Soft Clay”, Fifth International Conference on Case Histories, N.Y., N.Y., April, 2004.
� C.N. Baker, et. Al., Foundation Design and Performance of the World’s Tallest Building, Petronas Towers,” Fourth International Conference on Case Histories, St. Louis, March 1998.
Questions???