To determine the shear strength of soil using the direct shear apparatus.
In many engineering problems such as design of foundation, retaining walls, slab bridges, pipes, sheet piling, the value of the angle of internal friction and cohesion of the soil involved are required for the design. Direct shear test is used to predict these parameters quickly. The laboratory report covers the laboratory procedures for determining these values for cohesion-less soils.
The test is conducted as per IS: 2720- 13 (1986), method of tests for soils. One kg of air dry sample passing through 4.75mm IS sieve is required for this test.
Box shear tests can be used for the following tests.
1. Quick and consolidated quick tests on clay soil samples.
2. Slow test on any type of soil.
Only using box shear test apparatus may carry the drained or slow shear tests on sand. As undisturbed samples of sand is not practicable to obtain, the box is filled with the sand obtained from the field and compacted to the required density and water content to stimulate field conditions as far as possible. So far clay soil is concerned the undisturbed samples may be obtained from the field. The sample is cut to the required size and thickness of box shear test apparatus and introduced into the apparatus. The end surfaces are properly trimmed and leveled. If tests on remolded soils of clay samples are required; they are compacted in the mould to the required density and moisture content.
Coulombs equation is used for computing the shear parameters.
For clay soils
S=c+σtanØ
For sand
S= σtanØ
Where, S = shear strength of soil in kg/cm2
c=unit cohesion (kg/cm2 )
σ=normal load applied on the surface of the specimen (kg/cm2 )
Ø=angle of shearing resistance (degrees)
In a Direct Shear test, the sample is sheared along a horizontal plane. This indicates that the failure plane is horizontal. The normal stress (s) on this plane is the external vertical load divided by the area of the soil sample. The shear stress at failure is the external lateral load divided by the corrected area of soil sample. The main advantage of direct shear apparatus is its simplicity and smoothness of operation and the rapidity with which testing programmes can be carried out. But this test has the disadvantage that lateral pressure and stresses on planes other than the plane of shear are not known during the test.