Direct shear test for determining shear strength

Direct shear test for determining shear strength:

Direct shear test is performed in laboratory. It is one of the simpler forms of shear test. In this test, shear force is applied with a constant rate of strain until the shearing force failure is occurred. The soil samples under consideration are placed in a metal shear box of square or round shape spliced at the mid height in this test.

In direct shear test the system is designed so that the sample under consideration is made to fail on a pre-determined horizontal shear plane. By means of horizontal proving ring from which the peak shearing stress is determined, is the source of measuring shearing force. We can record horizontal and vertical deformations using displacement dial gauges used for this purpose. We can vary the normal force to the plane of shear failure and we can control the drainage of the sample by using solid or porous plates by placing them at the bottom and top.

The shear test can be stress controlled or strain controlled.

Stress controlled direct shear test:

In stress controlled direct shear test, the shear force is applied in equal increments until the specimen fails. The failure occurs along the plane of split of the shear box. After the application of each incremental load, the shear displacement of the top half of the box is measured by the horizontal dial gauge.  The change in height or volume change of the specimen can be obtained from the reading of the dial gauge that measures the vertical movement of the upper loading plate.

De merits of stress controlled direct shear test:

• In stress controlled test, only the peak shear resistance can be observed and plotted.
•  Peak shear resistance can only be approximated in stress controlled test. Because failure occurs at the stress level between the pre failure load increment and the failure load increment.

Strain controlled direct shear test:

In strain controlled test, a constant shear displacement is applied on one half of the box by a motor. The constant rate of shear displacement is measured by a horizontal dial gauge. The resisting shear force corresponding to any shear displacement can be measured by horizontal proving ring or load cell. The change in height or volume change of the specimen can be obtained from the reading of the dial gauge that measures the vertical movement of the upper loading plate.

Merits of strain controlled direct shear test :

• In case of dense sand, peak shear resistance that is at failure, as well as lesser shear resistance that is at a point after failure called ultimate strength, can be observed and plotted.