Direct shear test

A direct shear test is a laboratory test to measure the shear strength properties of soil. The value internal friction angle and cohesion of the soil are required for design of many engineering problems such as foundations, retaining walls, bridges, sheet piling. Direct shear test can predict these parameters quickly.

The test is performed on three or four specimens from a relatively undisturbed soil sample.
A specimen is placed in a shear box which has two stacked rings to hold the sample; the contact between the two rings is at approximately the mid-height of the sample.

A confining stress is applied vertically to the specimen, and the upper ring is pulled laterally through a specified strain until the sample fails.

The load applied and the strain induced is recorded at frequent intervals to determine a stress–strain curve for each confining stress. As a vertical normal load is applied to the sample, shear stress is gradually applied horizontally, by causing the two halves of the box to move relative to each other. The shear load is measured together with the corresponding shear displacement.

Several specimens are tested at varying confining stresses to determine the shear strength parameters, the soil cohesion (c) and the angle of internal friction, commonly known as friction angle.

The results of the tests on each specimen are plotted on a graph on the y-axis and the confining stress on the x-axis. The y-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle.

Direct shear tests can be performed under several conditions. The sample is normally saturated before the test is run, but can be run at the in-situ moisture content.

The rate of strain can be varied to undrained or drained conditions, depending whether the strain is applied slowly enough for water in the sample to prevent pore-water pressure.

The test has several advantages:
•  It is easy to test sands and gravels.
•  Large samples can be tested in large shear boxes, as small samples can give misleading results due to imperfections such as fractures and fissures, or may not be truly representative.
•  Samples can be sheared along predetermined planes.

The disadvantages of the test include:
•  The failure plane is always horizontal in the test, and this may not be the weakest plane in the sample. Failure of the soil occurs progressively from the edges towards the centre of the sample.
•  There is no provision for measuring pore water pressure in the shear box and so it is not possible to determine effective stresses from undrained tests.
•  The shear box apparatus cannot give reliable undrained strengths because it is impossible to prevent localised drainage away from the shear plane.