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???