1.Shear strength:
It is the strength of soil to give resistance to the deformation by continuous replacement of soil particle.
It is the strength of soil to give resistance to the deformation by continuous replacement of soil particle.
Shear strength of soil constituted by
1.
Structural resistant due to interlocking
2.
Frictional resistance
3.
Cohesion or adhesion
2.Coulomb’s Law:
According
to the coulomb’s law the strength of soil can be found by the following
equation.
S= Shear strength
C = Cohesion
= Angle of internal friction of shearing
resistance
3.Measurement of shear strength:
Measurement of shear strength is the
process of testing the soil for its failure in different set of conditions.
Shearing resistance can be determined in the laboratory by following methods.
i.
Direct shear test
ii.
Triaxial shear test
iii.Unconfined
compression test
iv. Vane
shear test
Again
depending upon its drainage conditions test developed as…
a.
Undrained test or Quick test
b.
Consolidated-undrained test
c.
Drained test
In the undrained test no discharge of
water is permitted. So there is no change of pore water pressure during the
test.
In
drained test Drainage is permitted so as there is no excess pore water pressure
and full consolidation occurs.
In Consolidated-undrained test the
drainage is permitted only for initially applied large stress. Full
consolidation occurs there after no drainage is permitted.
4.Direct Shear test:
This
is simple and commonly used test. It consists of shear box apparatus which has
two square or circular shear boxes. One is rigidly fixed at bottom and the top
box is movable with the help of proving ring. When the soil filled shearing box
is applied load, the soil fails at certain load is measured using proving ring.
This test can be performed under all the
conditions.
5.Tri-axial Compression test:
This is strength test more commonly used
in all research laboratory first introduced by A.Casangrande and Karl Terzaghi.
In this test the solid specimen cylindrical
shape is subjected to direct stresses acting three mutually perpendicular
directions. The major principal stress acts in the vertical direction and the
other two stresses acting in the horizontal direction.
From the test the following relation can be derived.
The failure plane or critical shear plane
6.Unconfined compression test:
This test is advanced to the triaxial
test. In this test the cell pressure or confining pressure is zero. The
horizontal stress is called cell pressure or confining pressure. The failure
load is determined by proving ring.
Generally this test is applicable for saturated clay for which angle of
internal friction is zero. While submitting angle of internal friction as zero
in the above equation.
6.Vane Shear test:
Vane
shear test is quick test used to determine undrained shear strength of cohesive
soil. The apparatus consists of four thin plates called vanes. During the test
the vanes are gently pushed in to the soil and torque is applied at the uniform
speed generally 10 per minute. The torque at failure is noted down.
Then
the failure toque is equal to shear resistance of soil.
d= Over all diameter Of vane
H= Height of vane= (2.d)
Examples:
1. A
dry sand sample with angle of internal friction 36o is subjected to tri-axial test. If the minor principal stress acting is 250 KN/m2 Find the value of Deviator stress.?
Ans:
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