GOVERNMENT ENGINEERING COLLEGE - BHUJ

SUBJECT - SOIL MECHANICS

TOPIC - SHEAR STRENGTH OF SOIL

PREPARED BY - UMANG PARMAR

Contain..1. Introduction.2. Some definitions. 3. Mohr circle of stress.4. Mohr-coulomb’s strength theory.5. Tests for shear strength.6. Shear tests based on drainage conditions.

1. Introduction.. The shear strength of soil is the resistance to deformation

by continuous shear displacement of soil particles upon the action of a shear stress.

Shear strength is the principal engineering property which controls the stability of a soil mass under loads.

It governs the bearing capacity of soils. The stability of slopes, the earth pressure against retaining

wall, etc.

2. Some definitions..1) Cohesion :- The attraction between the molecules of the same

material is called cohesion. The cohesion of soil depends upon,i. Fineness of clay particlesii. Types of clay mineral iii. Amount of clayiv. Water content of soil

2) Internal friction:- The friction resistance between the individual soil particles at their contact points is known as internal friction.

3) Angle of internal friction:- It represents the frictional resistance between soil particles, which is directly proportional to the normal stress.

3. Mohr circle of stress At any point in a strained material, there

exist three mutually perpendicular planes on which only direct stresses are acting, i.e. no shearing stresses acting. These are known as principal planes.

The normal stresses that act on these planes are called the principal stresses.

The largest of these is called the major principle stress (σ1), the smallest, the minor principal stress (σ3), and the third one is called the intermediate principal stress (σ2).

4. Mohr – Coulomb’s strength theory Coulomb, in his investigations observed that

one component of the shearing strength, called the intrinsic cohesion is constant for a given soil and is independent of the applied stress.

The shear strength of a soil at a point on a particular plane was expressed by coulomb as a linear function of the normal stress on that plane, as

S = C + σ tanφ Where, S = shear strength of the soil

C = apparent cohesion σ = normal stress on the failure

plane φ = angle of internal friction

4. Mohr – Coulomb’s strength theory Types of soils based on total strength:1. Cohesionless soil (φ – soil ) :

• These are the soil which do not have cohesion . So C = 0.

• These soil derive the shear strength from the intergranular friction.

• These soils are also called frictional soils.• Equation for shear strength is,

S = σ tanφ• Examples : sands and gravels

4. Mohr – Coulomb’s strength theory2. Purely cohesive soil (C - soil) :-

• These are the soils which exhibit cohesion but, the angle of shearing resistance,φ = 0.

• These soil are called C – sails.• Equation for shear strength is S = C• Example : saturated clays

3. Cohesive frictional soils (C – φ soil ):-• These are composite soil having C and φ both.• These are also called C – φ soils.• The equation for shear strength is ,

S = C + σ tanφ• Examples : clayey sand, silty sand, sandy clay.

5. Tests for shear strength The following tests are used to measure the shear strength of a

soil.1. Direct shear test 2. Triaxial compression test3. Unconfined compression test4. Vane shear test

In shear test, there are to stages.a. Consolidation stage in which the normal stress is applied to

the specimen and it is allowed to consolidate.b. shear stage in which the shear stress is applied to the

specimen to shear it.

6. Shear Tests Based on Drainage Conditions Depending upon the drainage conditions, there are three type of tests.

1) Unconsolidated Un – drained test (UU test) : In this type of test, no drainage id permitted during the consolidation stage.

The drainage is also not permitted in the shear stage. As no time is allowed for consolidation or dissipation of excess pore water

pressure, the test can be conducted quickly in a few minutes. The test is also known as quick test (Q - test).

2) Consolidated Un – drained test (CU test) : In this type of test, the specimen is allowed to consolidate in the first stage.

The drainage is permitted until the consolidation is complete. In the second stage, when the specimen is sheared, no drainage is permitted. The test is also known as R test. Q used for quick test, S used for slow test and R falls between Q & S.

6. Shear Tests Based on Drainage Conditions

3) Consolidated Drainged test (CD test) : In this type of test, drainage is permitted in both the

stages. The sample is allowed to consolidate in the first stage.

When the consolidation is complete, it is sheared at a very slow rate to ensure that fully drained conditions exist and the excess pore water is zero.

The test is also known as drained test or slow test ( S test ).

Thank You…