Shear Strength of Soils (6)

8/16/2019 Shear Strength of Soils (6)

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Types of Triaxial Tests

Is the drainage valveopen?

yes no

Consolidated sample

Unconsolidate

d sample


yes no

Drained

loading

Undrained

loading

Under all-around cell pressureσc

σcσc

σc

σcStep 1

deviatoricstress (∆σ !"

Shearing(loading"

Step #

σc σc

σc$ !


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Types of Triaxial Tests


yes no

Consolidated sample

Unconsolidated sample

Under all-around cell pressureσc

Step 1

Is the drainage valve

open?yes no

Drainedloading

Undrained loading

Shearing(loading"

Step #

CDtest

CU

test

UUtest


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Unconsolidated- Undrained test (UU Test)

Data analysis

σC σ%

σC σ%&odrainage

Initial specimen

condition

σ% $ ∆σd

σ%

&odrainage

Specimencondition duringshearing

Initial volume of the sample ' )

*

+olume of the sample during shearing ' ) *

Since the test is conducted under undrained condition,

' ) * ' ) *

' )(* ∆*" ' ) *

' )(1 ∆*.*" ' z

A A

ε −=

1

0


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Step 1/ Immediately after sampling

$

Step #/ 'fter application of hydrostatic cell pressure

∆uc 0 ∆σ%

σC σ%

σC σ% ∆uc

σ% σ% - ∆uc

σ% σ% - ∆uc&odrainage

Increase of p2p due toincrease of cell pressure

Increase of cell pressure

S3emptons pore 2aterpressure parameter, 0

&ote/ If soil is fully saturated, then 0 1 (hence, ∆uc ∆σ%"


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Step %/ During application of axial load

σ% $ ∆σd

σ%

&odrainage

∆ud '0∆σd

∆uc 4 ∆ud

$

Increase of p2p due to

increase of deviator stress

Increase of deviator

stress

S3emptons pore 2aterpressure parameter, '

'

1 3 d c d u uσ σ σ = + ∆ −∆ ∆m

'

3 3 c d u uσ σ = −∆ ∆m


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Com5ining steps # and %,

∆uc 0 ∆σ% ∆ud '0∆σd

∆u ∆uc $ ∆ud

Total pore 2ater pressure increment at any stage, ∆u

∆u 0 6∆σ% $ '∆σd7S3emptons pore2ater pressuree!uation

∆u 0 6∆σ% $ '(∆σ1 ∆σ%7


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Ty(i*a "a+$s for (aram$%$r B

D$'r$$ ofsa%+ra%io&

f

!sa%+ra%io


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Ty(i*a "a+$s for (aram$%$r A


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stress criteria

Three identical saturated soil samples are sheared to failure in UU triaxialtests: ;ach sample is su5


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stress criteria

Three identical saturated soil samples are sheared to failure in UU triaxial

tests: ;ach sample is su5


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stress criteria

τ

σ

σ1σ%′σ1

′σ3

0ecause each sample is at failure, the fundamental e9ective stressfailure condition must also 5e satised: 's all the circles have the

same si@e there must 5e only one e9ective stress =ohr circle

τ σ φ= + ′cn

' tan '


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• The di9erent total stress =ohr circles 2ith a single e9ectivestress =ohr circle indicate that the pore pressure isdi9erent for each sample:

• 's discussed previously increasing the cell pressure 2ithout

allo2ing drainage has the e9ect of increasing the porepressure 5y the same amount (∆u ∆σc" 2ith no change in

e9ective stress:

• The change in pore pressure during shearing is a functionof the initial e9ective stress and the moisture content: 'sthese are identical for the three samples an identicalstrength is o5tained:

stress criteria


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• It is often found that a series of undrained

tests from a particular site give a value of φu that is not @ero (cu not constant": If this

happens either

– the samples are not saturated, or

– the samples have di9erent moisturecontents

• If the samples are not saturated analyses

5ased on undrained 5ehaviour 2ill not 5ecorrect

• The undrained strength cu is not a fundamental

soil property: If the moisture content changes

parameters


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σ%5 σ15


;9ect of degree of saturation on failure envelope

σ%a σ1aσ%c σ1c

τ

σ or

σ

S A 1B S 1B

om$ (ra* *a a(( *a o&s o


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τ τ in situ undrainedshear strength

Soft clay

1: ;m5an3ment constructed rapidly over a soft clay deposit

om$ (ra* *a a(( *a o&s oa&aysis for *ays



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#: arge earth dam constructed rapidly 2ithno change in 2ater content of soft clay

Core

τ Undrained shearstrength of clay core

τ



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%: Eooting placed rapidly on clay deposit

τ In situ undrained shear strength

&ote/ UU test simulates the short term condition inthe eld: Thus, cu can 5e used to analy@e the

short term 5ehavior of soils



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,&*o&-&$d Com(r$ssio& T$s% !,C T$s%#

σ1 σ+C $ ∆σ

σ%

Conning pressure is @ero in the UC

test


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,&*o&-&$d Com(r$ssio& T$s% !,C T$s%#

σ1 σ+C $

∆σf

σ%

S h e a r

s t r e s s ,

τ

&ormal stress,σ

!u

&ote/ Theoritically !u cu , *o2ever in the actual

case !u A cu due to premature failure of the

sample


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O%h$r a.ora%ory sh$ar %$s%s

Direct simple sheartest

Torsional ring shear test

Flane strain triaxial test


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Dir$*% sim($ sh$ar %$s%

Direct shear test = 80 mm

Soil specimenPorous

stones

Spiral wire

in rubber

membrane

Direct simple shear test


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Torsio&a ri&' sh$ar %$s%

PeakResidual

τ

Shear displacement

τf

σ’

’max

’res


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σ

Preparation of ring shaped

undisturbed samples is very

difficult. Therefore, remoulded

samples are used in most cases


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/a&$ s%rai& %ria0ia %$s%

σ

’1, 1

σ’2,

2

σ

’3, 3

Plane strain test

σ’2 ≠ σ’3

2 = 0

σ’!

σ’"

σ’#

σ’"

$i%id platens

Specimen


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Drai&$d a&d +&drai&$d *o&di%io&s

• D1AINED condition occurs 2hen there is no change in pore

2ater pressure due to external loading

• In a drained condition, the pore 2ater can drain out of thesoil easily, causing volumetric strains in the soil

• ,ND1AINED condition occurs 2hen the pore 2ater isuna5le to drain out of the soil

• In undrained condition the rate of loading is much !uic3erthan the rate at 2hich the pore 2ater is a5le to drain out ofthe soil

• 's a result, most of the external loading is ta3en 5y the pore2ater, resulting in an increase in the pore 2ater pressure:

• The tendency of soil to change in volume is suppressed

during undrained loading:


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• The existence of either a drai&$d or an +&drai&$d

condition in a soil depends on/ ((i"soil typesG ne-grained orcoarse grained, (ii" geological formation and (iii" rate ofloading"

• Eor a rate of loading associated 2ith a normal constructionactivity, saturated coarse grained soils (e:g: sands andgravel" experience drained conditions and saturated ne-grained soils (e:g: silts and clays" experience undrainedconditions

• If the rate of loading is fast enough (e:g: during an

earth!ua3e", even coarse-grained soils can experienceundrained loading, often resulting in li!uefaction:


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!Li2+$fa*%io

>hen loading is rapidly applied and large enough such that it does notHo2 out in time 5efore the next cycle of load is applied, the 2aterpressure may 5uild to an extent 2here they exceed the contactstresses 5et2een the grains of soil that 3eep them in contact 2ith eachother: These contact 5et2een grains are the means 5y 2hich the2eight of the 5uildings and overlying soil layers are transferred from

the ground surface to layers of soil or roc3 at greater depth: This loss ofsoil structure causes it to lose all of its stren th and it ma 5e o5served


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!Li2+$fa*%io

Uplift of se2erageduring &iigata

earth!ua3e #

Collapse of Hat houseduring the 1JK

&iigata earth!ua3e, Lapan:


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• The shear strength of a ne-grained soil under undrained

condition is called the undrained shear strength and denotedas su:

• su is the radius of the =ohrs circle of total stress/M

N,N

( )1 f σ ( )3 f σ

u s

( ) ( )1 3

2

f f

u s

σ σ −=

• The undrained shearstrength depends only on

the initial void ratio or theinitial 2ater content of thesoil

Total stress circle


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• The undrained shear strength is not a fundamental soil

parameter:

• Its value depends on the values of the initial conningstresses:

M

N,N

• 'n increase in initial

conning stressescauses a decrease invoid ratio and anincrease in undrainedshear strength

1u s2u

s

( )1 f σ ( )3 f σ ( )1 f σ ( )3 f σ


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i d d d i d h h


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Drai&$d a&d ,&drai&$d sh$ar s%r$&'%h

Co&di%io& Drai&$d ,&drai&$d

E0*$ss (or$a%$r(r$ss+r$

&ot @eroG could 5e positive or

negative

o+m$ *ha&'$ Compression Fositive excess pore2aterpressure

;xpansion &egative excess pore2aterpressure

Co&soida%io& Oes, ne grained soil &o

Com(r$ssio& Oes Oes, 5ut lateral expansionmust occur so that thevolume change is @ero

A&aysis ;9ective stress (;E'" Total stress (TS'"

D$si'& (aram$%$rs

0/

uS ' '

(or )cs p

φ φ

*ome2or3/ Do reading from page #% #P (Section Q:R" Soilmechanics and foundations

Shear Strength of Soils (6)

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