SHEAR STRENGTH OF SOIL

By

Aamir Ali Solangi (12CE37)

Sadam Hussain Pitafi (12CE30)

Muhammad Rashid Phulpoto (12CE127)

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ABSTRACT

The shear strength is most important property of soil. It is resistance

provided by soil to sliding along any plane inside it. The nature of shear

strength is most difficult to grasp. Shear strength depends on interaction

between particles and shear failure occur when particles slides over each

other due to excessive shearing stresses. It is very much important to

understand behavior and analyze the property of shear strength to provide

soil stability regarding shear failures such as bearing capacity, slope stability

and lateral pressures on earth retaining structures. Shearing resistance of soil

is constituted basically of the structural resistance, the frictional resistance

and cohesion. The shear resistance in cohesion less soil is of friction alone

and in other soils is result of both friction and cohesion. The shear strength

of soil is determined in laboratory as well as in field.

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TABLE OF CONTENTS

ABSTRACT

Mohr-Coulomb Criteria 04

Direct Shear Test 07

Triaxial Shear Test 10

Van Shear Test 12

Conclusion 13

References 14

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Mohr-Coulomb Failure Criteria

This theory states that “material fails because of a critical combination of

normal and shear stresses alone”. Material fails essentially by shear. The

critical shear stress causing failure depends upon properties of material as

well as normal stresses on plane. The ultimate strength of the material is

determined by the stresses on the potential failure plane.

The shear failure was first defined by Coulomb (1776). The failure envelope

is a curved line which represents shear stress on failure plane as a linear

function of normal stress. It can be written as

Where is maximum shear stress that soil can carry without failure under

normal stress.

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This formula can be represented in the form of graph as shown in Figure 1.

Figure 1: Shear failure envelope [1].

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Figure 1 show that cohesive soils in which shear strength is result of both

friction angle and cohesion of soil particles and figure 2 represents that in

cohesion less soil shear stress is property of friction only.

Figure 2: For cohesion less soils value of c is ignored whose envelope starts

from oo coordinates of graph [2].

The Mohr-coulomb failure criteria simply states that “in saturated soil, the

total normal stress at a point is sum of effective stresses and pore water

pressure. It is written as

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We can say cohesion and frictions angles are the main properties of shear

strength of soil. These are parameters on which shear strength of soil

depends. Friction angles of sand of different type are given in table 1.

Table 1: Typical values of drained angles of friction for sand and silts [3].

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The shear failure will occur as shown below

Figure 3: Failure zone in envelope [4].

Figure 3 shows that line of failure envelope is touching circle, failure zone

will be shaded area and unshaded zone will not fail in shear because stresses

are not enough to fail that unshaded area.

Direct Shear Test

Direct shear test is also known as shear box test. This test is simplest and

oldest form of shear test arrangement. A sample of soil sample is placed in

box which is spited into two halves. The lower part is rigidly fixed and

upper part is allowed to move, a normal force is also applied on specimen.

Upper part of box is pushed by gear machine or by hand with constant

pushing force readings are noted on dialed gauge which is attached with

upper half of box. Shear box arrangement is shown in figure 4.

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Figure 4: Arrangement of shear box test [5].

Figure 5: Laboratory arrangement of shear box test [4].

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Direct shear test results

Figure 6: Plot of shear stress and shear displacement [4].

Figure 6 shows that for loose soils shearing stresses increase with increase in

shear displacement until failure shear stress after that shear resistance

become almost constant. For dense sand shear resistance increase with shear

displacement until failure occurs this is at peak shearing stress after that is

gradually decreases until critical point after that trend of dense soil is almost

of loose soil.

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Figure 7: Volume changes in shear box test [4].

Above figure 7 shows that in loose sand contraction of soil is observed at a

regular trend. Dense soils first contract then expands at constant trend.

Triaxial Shear Test

The triaxial shear test is one of the most reliable methods available for

determining shear strength parameters. It is widely used for research testing.

Testing diagram is shown in figure 8.In this test, a soil specimen 36mm in

diameter and 76mm long is generally is used. The specimen is encased by

thin rubber membrane and placed inside a plastic cylindrical chamber that is

usually filled with water or glycerin. The specimen is subjected to a

confining by compression of a fluid in chamber to cause shear failure in

specimen, one must apply axial stress through a vertical loading ram. The

following three types of triaxial tests are generally conducted.

1. Consolidated-drained test or drained test (CD test).

2. Consolidated-undrained test (CU test).

3. Unconsolidated-undrained test or undrained test (UU test).

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Figure 8: Diagram of triaxial test equipment [6].

In case of consolidated test, the test is then carried out by a first stage of

applying confining pressure in the pressure chamber and allowing drainage

of the sample. This stage corresponds to the consolidation of the sample.

The deviatoric load is then applied through the vertical axis. The deviatoric

stress is indeed the difference between the vertical stress and the confining

stress. During the deviatoric compression, the drainage valves can be open

(CD) or closed (CU&UU).

Triaxial test data, in general, include evolution of axial and volumetric

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strain, deviatoric and isotropic stress, and pore pressure evolution. From the

triaxial test results, it is possible to deduce the shear strength parameters,

namely friction angle, cohesion, dilatancy angle and the other dependent

parameters.

Vane Shear Test

Vane shear test is a quick test, used either in laboratory or in the field to

determine the untrained shear strength of cohesive soil. The van shear tester

consists of four thin steel plates connected orthogonally to a steel rod. A

torque measuring arrangement is set on the rod which is rotated by worm

gear. After putting van gently into soil, the torque rod is rotated at uniform

speed usually 1 degree per minute. The test can be conveniently used to

determine the sensitivity of the soil. Figure 9 shows the vane tester and

figure 10 shows van test being performed in field.

Figure 9: Vane tester [7]. Figure 10: van test being performed in the field [8].

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Conclusion

Shear strength of soil is most important property. It is bit complex to

understand, for stability of structure it has a prime importance. For design of

structures like dams, earth retaining structures, foundation shear strength of

soil is taken into consideration. The shear strength parameters of soils in its

every condition can be determined by tests mentioned above either in

laboratory or in field.

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References

[1] http://geosyntheticsmagazine.com/articles/0409_f1_cohesion.html

(sited on 17 November 2014)

[2] http://home.iitk.ac.in/~priyog/Shear%20Strength%20Lecture.pdf

(sited on 17 November 2014)

[3] Baraja M. Das, Principles of Geotechnical Engineering, 5th edition,

Bill Stenquist, USA

[4] http://www.geotechdata.info/geotest/triaxial-test.html (sited on 17

November 2014)

[5] http://www.engr.mun.ca/~spkenny/Courses/Undergraduate/ENGI672

3/Lecture_Notes/2008F_ENGI_6723_Guest_Lecture_McAfee.pdf

(sited on 17 November 2014)

[6] http://www.civil.uwaterloo.ca/maknight/courses/CIVE353/Labs/Triax

ial.pdf (sited on 17 November 2014)

[7] http://impact-test.co.uk/products/3152-Hand-Vane-Tester/ (sited on

17 November 2014)

[8] http://www.ehow.com/info_8775661_purpose-shear-vane-test.html

(sited on 17 November 2014)

[9] B. C Punmia, Ashok Kumar Jain, Arun Kumar Jain (2005), Soil

Mechanics and Foundation Engineering, Laxmi Publications (P) Ltd,

India.

[10] http://nptel.ac.in/courses/105104137/module1/lecture4.pdf (sited on

17 November 2014)

[11] http://www.engr.uconn.edu/~lanbo/CE240LectW111shearstrength1.p

df (sited on 17 November 2014)

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[12] http://www4.hcmut.edu.vn/~cnan/Principles%20of%20geotechnical

%20engineering%20(Fifth%20Edition,%20Das)/311-363.PDF (sited

on 17 November 2014)

[13] http://www.gtt.bme.hu/gtt/oktatas/feltoltesek/bmetkeogb01/4_shearstr

engthofsoils.pdf (sited on 17 November 2014)

[14] http://theconstructor.org/geotechnical/vane-shear-test-on-soil/3435/

(sited on 17 November 2014)

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