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Evaluation ofUndrained Shear Strength

from In-Situ Tests

Paul W. Mayne

Georgia Institute of Technology

Mix & Match ofUndrained ShearStrengthsfrom differenttests

Pentre, UK Site(Lambson, et al.1996, Large ScalePile Tests in Clay,Thomas Telford)

Undrained StrengthAnisotropy and Effectsof Strain Rate, BoundaryConditions, and InitialStress State

UndrainedShearStrength(c = cu = su)

Classical Methods for su in Clays & Silts

Undrained Shear Strength

Classical interpretation from CPT in clays:

undrained shear strength = cu = su

kt

votu

N

qs

=

Which su ?

HC CIUC PSC CK0UC DSC DSS DS PSE CK0UC CIUE UU UC

Nk = 15

Undrained Shear Strength of Clays

Theoretical formulation from critical-state soil

mechanics for intact clays (Cambridge Univ.):

where

1 - Cs/Cc Experimental work by MIT with simple shear

device:

Low OCRs (assume = 28o): suDSS 0.2 p

su/voDSS = sin'OCR

su/voDSS = 0.23 OCR0 8

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NC Normalized DSS Undrained Strength

0.0

0.1

0.2

0.3

0.4

0.3 0.4 0.5 0.6 0.7 0.8

sin'

su

/vo

'NC

(DSS)

Bothkennar Mexico City

Amherst AGS Plastic

Hackensack Onsoy

James Bay Rissa

Porto Tolle Portsmouth

Boston Blue San Francisco

Silty Holocene Cowden

Wroth (1984)

su/vo'NC(DSS) = sin'

Undrained Shear Strength from Stress History

Intact Clays

Independentlyobtained by

experiments

(SHANSEP)

And theory

(CSSM)

Shear-Induced Porewater Pressures OC Normalized DSS Undrained Strength

0.1

1

10

1 10 100

Overconsolidation Ratio, OCR

DSSU

ndrained

Strength,su

/vo

' DrammenPortsmouth

Portland

Maine

Boston Blue

Silty Holocene

Haga

Upper Chek Lok

Lower Chek Lok

Bangkok

Atchafalaya

Conn Valley

Paria

Hackensack

McManus

Cowden

Brent Cross

20

30

40

Fissured 20

' = 40o

20o

30o

su/vo' = sin'OCR

Note: = 0.8

Fissured

Intact

Strength Anisotropy Strength Anisotropy

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Undrained Strength Anisotropy

0

0.1

0.2

0.3

0.4

0.5

0.6

NCUndrainedShea

rStrengthRatio

15 20 25 30 35 40 45

Effective Stress Friction Angle (deg)

CIUC

KoPSC

CKoUC

SS

DSS

KoPSE

CKoUE

Wroth Prevost Constitutive Model

Undrained Strength Anisotropy

(Jamiolkowski, et al. 1985; Ladd, 1991)

Undrained Strength by SHANSEP

(Ladd &

Foott, 1974)

(Ladd, 1991)

Undrained Strength by SHANSEP

(Koutsoftas

and Ladd,

1985)

Troll Investigation, North Sea

0

5

10

15

20

25

30

0 500 1000 1500

Piezocone Readings (kPa)

Depth

(meters)

qt (tip stress)

u2 (shoulder)

uo (hydrostatic)

Troll, North Sea

Amundsen et al (1985)

Troll Investigation, North Sea

0

5

10

15

20

25

30

0 5 10 15

Nm= Q (a' = 0)

Depth

(meters)

0

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8 1.0

Bq

0

5

10

15

20

25

30

20 25 30 35 40 45

' (deg)

Lab TX

CPTu

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Troll Investigation, North Sea

Troll, North Sea

0

5

10

15

20

25

30

0 1 2 3 4 5

Overconsolidation Ratio, OCR

Depth

(meters)

CPTu2Oed.1985Oed.1989

Amundsen et al. (1985)

' = 29o

= 0.85

Troll Site, North Sea (NGI, 1989)

0

5

10

15

20

25

0 100 200 300 400 500

Preconsolidation Stress,p' (kPa)

Depth

(meters)

qtnetOed.1985

Oed.1989svo'

0

5

10

15

20

25

0 100 200 300 400 500

Preconsolidation Stress, p' (kPa)

Delta u2Oed.1985Oed.1989svo'

Troll Site, North Sea (NGI, 1989)

0

5

10

15

20

25

0 100 200 300 400 500

Preconsolidation Stress,p' (kPa)

Depth

(meters)

SCE-CSSMOed.1985Oed.1989svo'

Triaxial CKoUC -Troll, North Sea

Troll, North Sea

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80

Undrained Shear Strength, su(kPa)

Depth

(meters)

CPTu2

TC 1984

TC 1987TC 1988

TC 1989

NGI (1989)

DSS Strengths - Troll, North Sea

Troll, North Sea

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80Undrained Shear Strength, su(kPa)

Depth

(meters)

CPTu2

DSS 1984

DSS 1987

DSS 1988

DSS 1989

NGI (1989)

Triaxial CKoUE -Troll, North Sea

Troll, North Sea

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80Undrained Shear Strength, su(kPa)

Depth

(meters)

CPTu2

TE 1984

TE 1987

TE 1988

TE 1989

NGI (1989)

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Troll Investigation, North Sea

(Jamiolkowski, et al. 1985, ICSMFE)

(Ladd, 1991, Terzaghi Lecture, JGE)

Anchorage, Alaska

Summary POA Undrained Shear StrengthsCIUC Triaxial and DSS Tests

Triaxial: su/vo' = 0.33 OCR0.77

Simple Shear: su/vo' = 0.23 OCR0.70

0.1

1

10

1 10 100

Overconsolidation Ratio, OCR

StrengthRatio,su

/vo

'

POA Summary: Triaxial Effective Stress Paths

POA Triaxial Summary

0

100

200

300

400

0 100 200 300 400 500 600

Effective Stress, p' = (1'+3')/2 kPa

ShearStress,q=

(1-

3)/2

kPa

OC Envelope

TB 9 (138 ft)

TP11 (155 ft)

TB15 (109 ft)

TB15 (124 ft)

TB15 (149 ft)

TB25 (69 ft)

TB25 (90 ft)

TB25 (190 ft)

TB28 (95 ft)

TB31 (126 ft)

TB31 (126 ft)

TB31 (126 ft)

TB39 (115 ft)

TB39 (160 ft)

TB47 (120 ft)

TB49 (100 ft)

TB49 (115 ft)

TB56 (115 ft)

Overconsolidated

Effective Strength Envelope

' = 27o

c' = 20 kPa

POA Summary: Triaxial Effective Stress Paths

POA Triaxial Summary

0

100

200

300

400

0 100 200 300 400 500 600

Effective Stress, p' = (1'+3')/2 kPa

ShearStress,q=

(1-

3)/2

kPa

OC Envelope

TB 9 (138 ft)

TP11 (155 ft)

TB15 (109 ft)

TB15 (124 ft)

TB15 (149 ft)

TB25 (69 ft)

TB25 (90 ft)

TB25 (190 ft)

TB28 (95 ft)

TB31 (126 ft)

TB31 (126 ft)

TB31 (126 ft)

TB39 (115 ft)

TB39 (160 ft)

TB47 (120 ft)

TB49 (100 ft)

TB49 (115 ft)

TB56 (115 ft)

Normally-Consolidated

Effective Strength Envelope

' = 27o

c' = 0

Summary POA Undrained Shear StrengthsCIUC Triaxial and DSS Tests

0.1

1

10

1 10 100

Overconsolidation Ratio, OCR

StrengthRatio,su

/vo

'

DSS Data

CIUC Data

MCC Pred CIUC

MCC Pred DSS

Critical State Soil Mechanics

(Modified Cam Clay)

' = 27o

= 0.75

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Undrained Shear Strength Anisotropy

Stability Analyses on Clays

Deterministic Solution

Using Limit Plasticity

Trial & Error Solution

Using Limit Equilibrium

TC

DSS

TE PSC DSSPSE

Undrained Strength Anisotropy

Wroth-Prevost Constitutive Soil Model

0

0.1

0.2

0.3

0.4

0.5

0.6

20 25 30 35 40 45

Effective Stress Friction Angle, ' (deg)

NCStrengthRati

o,su/vo'

CIUC

KoPSC

CKoUC

SS

DSS

KoPSE

CKoUE

POA Bootlegger

Cove Clay

Undrained Strength Anisotropy

(Jamiolkowski, et al., ICSMFE, 1985)

POA In-Situ and Lab Undrained Strengths

Mudline

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160 180 200

Undrained Shear Strength, su(kPa)

Elevation

(meters)

Lab DSS Data

Lab Triaxial Data

Vane Shear

TC from CPTu

DSS from CPTu

Upper

Bootlegger

Cove Clay

Lower

BootleggerCove Clay

Soft

Silt

Piezocone Sounding C-28 (TB-33A)

Port of Anchorage SCPT

10

15

20

25

30

35

40

45

0 1 2 3 4

Tip Stress, qt(MPa)

Depth

(meters)

0 50 100 150

Friction, fs(kPa)

0 1 2 3

Porewater, ub(MPa)

1 10 100

Dissipation,t50(min)

0 100 200 300 400 500

Shear Wave, Vs(m/s)

Mudline

Undrained Shear Strength of Clays

Theoretical formulation from critical-state soil

mechanics for intact clays (Cambridge Univ.):

where

1 - Cs/Cc Experimental work by MIT with simple shear

device:

Low OCRs (assume = 28o): suDSS 0.2 p

su/voDSS = sin'OCR

su/voDSS = 0.23 OCR0 8