The document discusses the shear strength of soil, highlighting its crucial role in soil stability and addressing shear failures. It explains the Mohr-Coulomb failure criteria, various testing methods such as direct shear tests, triaxial shear tests, and vane shear tests to determine shear strength parameters. The complexity of shear strength characterization is emphasized, noting its importance in the design of structures like dams and foundations.

1. SHEAR STRENGTH OF SOIL
By
Aamir Ali Solangi (12CE37)
Sadam Hussain Pitafi (12CE30)
Muhammad Rashid Phulpoto (12CE127)
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2. 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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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. Figure 4: Arrangement of shear box test [5].
Figure 5: Laboratory arrangement of shear box test [4].
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10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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16. [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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