soils strength

7/30/2019 Soils Strength

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CE 240

Soil Mechanics & FoundationsLecture 11.2

Shear Strength of Soil II

(Das, Ch. 11)


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Class Outlines

Direct shear test

Introduction

Critical void ratio

Triaxial shear test Introduction

Procedure & calculation Critical void ratio


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Mohr-Coulomb shear failure criterion

1

1

33n

f

1

3

2

f

= c + n

c

Failed Zone

(, )

ff


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Direct Shear Test Direct shear test is Quick and Inexpensive

Shortcoming is that it fails the soil on adesignated plane which may not be the

weakest one Used to determine the shear strength of

both cohesive as well as non-cohesive

soils

ASTM D 3080


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Direct Shear Test (cont.)

The test equipment consistsof a metal box in which thesoil specimen is placed

The box is split horizontallyinto two halves

Vertical force (normalstress) is applied through ametal platen

Shear force is applied bymoving one half of the boxrelative to the other to

cause failure in the soilspecimen

Soil

Normal stress n

Shear stress 3


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Direct Shear Test


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Calculations1. Determine the dry

unit weight, d

2. Calculate the void

ratio, e

3. Calculate the normal

stress & shearstress

1=

d

wGse

A

V

A

N== ;


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FiguresPeak Stress

s3

s2

s1

Shear

stress,s

N3 = 30 kg

N2 = 20 kg

N1 = 10 kg

Horizontal displacement, H


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Figures (cont)

Shear

Stress,s

(psf)

C

(1,s1)

(3,s3)(2,s2)

Normal Stress , psf


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Direct Shear Test Data

Shears

tress

Residual Strength

Peak Strength


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Direct Shear Test Data:

Volume change

H


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Triaxial Shear Test Developed by Casagrande in an attempt

to overcome some of the seriousdisadvantages of the direct shear test.

Advantages over DST More Versatile

Drainage can be well controlled

There is no rotation of the principal stresses

like the direct shear test

Also the failure plane can occur anywhere


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Triaxial Test: Increase the normal

stress in one direction

1

1 major principle stress

n

f3 3

Minor principle stress

Confining stress


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Principles of the Triaxial

Compression (TC) Test The triaxial compression test is used to

measure the shear strength of a soil undercontrolled drainage conditions

A cylindrical specimen of soil is encasedand subjected to a confining fluid/air

pressure and then loaded axially to failure.

The test is called "triaxial" because the

three principal stresses are assumed to be

known and are controlled.


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Principles of the TC Test

During shear, the major principalstress, 1 is equal to the appliedaxial stress (

= P/A) plus thechamber (confining) pressure, 3

The applied axial stress, 1 - 3is termed the "principal stressdifference" or sometimes the"deviatory stress

The intermediate principal stress,2 and the minor principal stress,3 are identical in the test, andare equal to the confining orchamber pressure

1 = + 3

3

1

3

2 = 3


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Triaxial Test Equipment

The Cell (Chamber)

Loading FrameControl Panel


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Soil Shear Strength under Drained

and Undrained Conditions .

Drained conditions occur when rate at

which loads are applied are slowcompared to rates at which soil material

can drain (k - dependent) Sands drain fast; therefore under most

loading conditions drained conditions

exist in sands

Exceptions: pile driving, earthquake

loading in fine sands


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Soil Shear Strength under Drained

and Undrained Conditions .

In clays, drainage does not occur quickly;

therefore excess pore water pressure

does not dissipate quickly

Therefore, in clays the short-term shearstrength may correspond to undrained

conditions Even in clays, long-term shear strength is

estimated assuming drained conditions


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Loose sand and norm.

consolidated clay

dense sand and over-

consolidated clay


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Types of Tests

There are three types of tests:1. Unconsolidated-undrained (UU or Q) Test

2. Consolidated-undrained (CU or R) Test3. Consolidated-drained (CD or S) Test


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Unconsolidated-undrained Test

This test is also called the quick test.

3 and are applied fast so the soil does

not have time to settle or consolidate. The test is performed with the drain valve

closed for all phases of the test.(Water is not allowed to drain)

UU test simulates short term shear

strength for cohesive soils. For this test, = = 0

s = cu

= Su = (1

-3

)/2 = (1

-3

)/2


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UU Test Results

1 3( )2

C lid d d i d T


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Consolidated-undrained Test

Apply 3 and wait until the soil consolidates Drainage valves open during consolidation phase but

closed during the shearing phase(Drainage and consolidation is allowed to take place during the application of

the confining pressure 3)

Loading does not commence until the sample ceases todrain (or consolidate).

This test can simulates long term as well as short termshear strength for cohesive soils if pore water pressureis measured during the shearing phase

For this Test, cT c and

T=

From this test we obtain;

c, and u (Effective stress)

cT, T (Total stress)

Skempton pore pressure parameter


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Skempton pore pressure parameter


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c.


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Part c

( ) 41.35 0.257( ) 160.85

d f

f

d f

uA

= = =

C lid t d d i d T t


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Consolidated-drained Test

Also called slow test. Drainage valves OPEN during consolidation as

well as shearing phases.

Complete sample drainage is achieved prior toapplication of the vertical load.

The load is applied at such a slow strain rate thatparticle readjustments in the specimen do notinduce any excess pore pressure. (can take up to2 weeks)

Since there is no excess pore pressure totalstresses will equal effective stresses.

This test simulates long term shear strength forcohesive soils.


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CD Test Results


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Triaxial Test on Sand

Conduct a CD test on sand.

Soil specimens will be loaded to failureunder 3 different confining pressures; 15,

30 and 45 psi

Failure will be defined as the peak or 3

maximum value of principal stress

difference reached.

ASTM D 2850

Test Results and


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Test Results and

Calculations

( )

c

c

A

P

A

A

ll

=

=

=

1

0

0


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Determining 1

= 2700 psf

32= 1000 psf

psf3700100027001

31

=+=

+=


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Mohrs Circles

1 = 3700 psf

3 = 1000 psf 3 = 1000 psf 1= 3700 psf

Triaxial Test on Sand -


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Triaxial Test on Sand -

Figures

45 psi

30 psi

15 psi

c

b

a


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Reading Assignment:

Das, Ch. 11

HW: Problem 11.11, 11.15

soils strength

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