chapter 1(37-57)

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Advanced Soil Mechanics I

1.3Shear Strength of clays.

(1) Drained Strength.

Fundamental Shear Strength Parameters

- Hvoslev Parameters.

ce, e

- Work based on lab tests of saturated remolded clays.

Physico-chemical Forces (Intrinsic Forces)

Attractive Forces, A.

1) Electrostatic attraction.

2) Van der Waals force.

Repulsive Forces, R.

1) Electrostatic repulsion.

Geotechnical Engineering Lab.

37

d r a i n e d

U U b e h a v i o r

S u

N C c l a y

U U b e h a v i o r

0 . 8 P c '

S u

m a x . p a s t p r e s s u r e

O C c l a y

su

n

= 0 = 0

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- Factors affecting intrinsic forces .

1.2.

3.

( )

- Effective stress equations .

RAu +='

S shear strength cohesivegranular SS +=

ee

ccg

cgcg

cu

KRAKKu

KRAuKuSSS

+=

++=

++=+=

tan)(

)())((

)()(


38

Equivalent friction angle.material constant.

Equivalent cohesion.

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For N.C. Clay (drained test)

No longer 1 failure envelope, but a series of envelope.


39

3

Cause e (consolidation) increase ce

ce c

e

e

e

ce

e

e

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For O.C. Clay.

Ideal Soils No e during unloading.

In terms of Hvorslev parameters;

For any unloading (OCR > 1).

consta

consta

=

=

e

e

c

Practical decrease in strength due to increase in e during actual

unloading

So, Hvorslev ideal envelope (O.C. Clay) upper bound strength.


40

l o g P '

e

t y p i c a l d a t a

i d e a l i z e d d a t a

ce

e

Composite envelope(idealized)

ideal

practical

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Summary

An increase in effective stress has two effects on strength.

1.

2.


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(2) Undrained Strength

Sophistication for selecting su.

Triaxial or field tests.

su varies with depth and stress history.For truly N.C. clays,

const.''' ===c

u

vc

u

o

u

p

ss

p

s

For given OCR, const.' =o

u

p

s


42

depth

su

z

omogeneous clay,

Atterburg limit & wn

PLLLw

n

NC

For truly NC clay,w

n= LL

Consolidation data

Present effective stressPresent effective stress

NC

OC

Vertical consolidation stress

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To determine ''vc

u

o

u s

p

s

= (finding out Su).

(1) Using wn, LL or PL.

For example, if wn LL NC

const.' =o

u

p

sis valid.

Skempton, )(0037.011.0' PLps

o

u += .

(2) Run consolidation tests,

Ladd,8.0

' ))(04.023.0( OCRp

s

o

u =

Mesri, 22.0' =c

u

p

s

(3) Run a series of UU tests.

( a )

( b )

( c )

( c )

( b )

( a )

S u ( U U )

d e p t h

1 - 3

a

S u / P ' 0 = c o n s t a n t( i n c a s e w i t h

s a m e s t r e s sh i s t o r y )

(4) Run a series of CU (CIU or CK0U) tests get 'vc

us

.


43

31

a

c o n s t a n t'/0

=psu( in case withsame stress history )

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Considerations of lab Testing on Undrained Shearing Behavior of clays.

- Representing the in-situ field conditions before shearing and during

shearing.

1) In-situ field conditions before shearing.

a) Sample disturbance.

Perfect sample ( No change in water content and volume).

In filed in lab


44

P0+u

0

K0P

0+u

0

..'

SP

Perfect sample

= 0

= 0

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- Skempton.

)( 313 += DBuAssumption. Start with good sample,

No change in wn as a saturated sample (No change in

volume) undrained condition. )( 313 += Au (B=1)

)(

)(

'

00

'

00

'

00

0

'

000

'

00

'

00

pKpAupK

upKupAupKu

u

u

=

+++=

'

000

'

00

'

0

'

00

0

'

0

)}1({

)(

pKAK

pKpApK

uuu

uuu

u

u

psps

ps

+=

+=

==

+=

If soil is elastic,

11

0 =

=

K

'

0

' pps =

In real soils, typical values: 5.00 =K ,'

0

' 55.0 pps =

1.0=uA


45

=0.5 for undrained condition

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for NC clays,

Even in the condition of perfect sampling, volume change occurs during

consolidation.

To improve quality of results :


46

v o l u m e c h a n

C c1

f i e l d c o n d i t i o n

p e r f e c ts a m p l i n g

a s s u m i n gu n d r a i n e ds t a t e

0 . 5 5 P '0 P ' 0 P ' ( l o g s c a l e )

e

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Two ways to get high quality results.

1. Be careful (minimize disturbance).

2. Normalized Strength Concept (especially, N.C. clay)

For NC clay

(1) Consolidate samples to vc larger than pc (= p0).

" u n d i s t u r b e d b u t n o t

i n f i e l d s t r e s s c o n d i t i o

p 'c = p 'o

e

(2) Run shear tests to get Su. (3) Normalize the results by v .

A

B

C

(4) Back calculate su for any p0 by uv

u sps

= '0' .


47

VC (log)

VAVBVC

1 - 3

a

V O'

31

0'

2

V

us

a

Free from sample disturbancenot in field stress condition

VC(A)VC(B)

VC(C)

VC(A)

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b) K0 consolidation condition.

In field K0 state.In lab Generally isotropic state (Triaxial test) to simplify the field

conditions.

< For sendimented soils>

Field :

( anisotropy )

Lab : During isotropic consolidation for undisturbed anisotropic samples,

soil structure may be altered to have isotropic characteristics. And

total confining stress 3/)(''

3

'

2

'

1 ++=p is different from (larger

than) that in filed.

The effect of isotropic consolidation on undrained behavior for N.C. or

lightly O.C. clays. ( K0 < 1 piso > pKo )

(1) Compression shearing.

(2) Extension shearing.

(3) Isotropic consolidation has no effect on .


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1 3

1 3u e

I s o t r o p i c

I s o t r o p i c

I s o t r o p i c

I s o t r o p i c

K oK o

K o

K o

u e

( - )

( + )

( - )

T X E R T X E

The Effect of Isotropic Consolidation on Shearing Behavior.

Triaxial

Compression

Trialxial

Extension

Structure Change

su( Stiffness )

( ue)

su( Stiffness )

( ue)Increasing p su su

Anisotropy of Su .

KoCu

Eu

iCu

Eu

s

s

s

s

>

)(

)(

)(

)(


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a

a

TXC

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Mayne (1985), for 42 aoil types,For comp. , ( ) ( ) isovcuKovcu ss '/87.0'/

For Ext. , ( ) ( ) isovcuKovcu ss '/60.0'/

Sivakugan and et al. using K0 (=1-sin ) and pore pressure parameter at failure, Af for isotropic

and K0 consolidation.

( )00,,00

,00)1(

)1(2

)1(2

'

'KKA

AKK

AKK

s

s

if

Kof

if

CIUCvc

u

CKoUCvc

u

+

+

+=

Wroth

( )=

213

'sin23

'

'a

s

s

CIUCvc

u

CKoUCvc

u

wherec

r

C

Ca =

= 1

)'sin23(2

'sin3,

For heavily OC clays

1,4))(sin1( 0sin

0 == KOCRForOCRK

So, the higher OCR (

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2) Shearing Conditions.

a) Anisotropy

2 types 1) Material Inherent anisotropy.2) K0 1 Stress system anisotropy .

will affect

, c ext > com (?).su.deformation parameters ( - response).

Pore pressure response.

As the direction of major principal stress changes,

0 4 5 9 0

1 . 0 O C c l a y

N C c l a y

T X C D S S T X E

S u ( 0 )

S u ( )


51

1f 1f1f

1f

Inter-related

)0(

)(

u

u

s

s

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b) Plane strain tests vs. Triaxial tests.

Plane strain tests comparing to Triaxial tests.

3

c) Aging effect.

Aging

p ' ( l o g

e t = 1 y e a r

t = 1 d a

In lab


52

p(quasi)

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d) Rate of shearing.

S u

R a t e o fS h e a r i n

0 . 5 % / h r 5 % / h r

C o n v e n t i o n a l t e


53

(su)conventional test = 1.3 su(0.5%/hr)

su

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(3) SHANSEP ( Charles C. Ladd (MIT) )

Stress history and Normalized Soil Engineering Properties.

Soil has same normalized strength with same stress history.

1 Sample disturbance + Stress History

NC soils consolidate samples to 1.5 , 2.0 and 4.0 times larger than pc(maximum past pressure) with K0 state.

OC soils same procedure as NC soils and then unload to a given valueof OCR.

p 'c

4 . 0 p 'c

2 . 0 p 'c1 . 5 p 'cO C R = 4 O C R = 2

e

l o g ' v c

2 Strain rate + anisotropy

Shear samples with 0.5%/hr strain rate for CK0U TXC and RTXE, and

DSS conditions.


54

log vc

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3 We get normalized results from step .

+ Normalized Pore

Pressure Response.

4 Combine the results.

1 . 0 4 . 0O C R ( l o g s c a l e )

2 . 0

T X E

D S S

T X C


55

C''3

OCR=1.0

OCR=1.0

4.0

2.0

2.0

4.0

a

VC'

'' 31

VC

us

'TXC

DSS

TXE

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Problems with SHANSEP.

1.

2.

3.

4.

5.


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Notes.

1. Mohr failure envelopes over a range of stress spanning thepreconsolidation stress, p.

z

EET

T

e f f e c t i v e s t r e s s

t o t a l s t r e s s

Typical point z 2p (Hirschfeld)

2. su vs. ff (shear stress on failure plane at failure.)

)(max uff s=

chapter 1(37-57)

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