Direct Shear Apparatus

The shear strength of soil is the maximum resistance that it can offer to any stress on it. Every building or structure which is founded in or the earth imposes loads on the soil that support the foundation.The stresses set up in the soil causes deformations of the soil. One stress failure is caused by slipping of soil particles one on another, which may lead to the sliding of one body of soil relative to the surrounding mass.The process is known as shear failure and occurs when shear stresses set up in the soil mass exceeds the maximum shear resistance, which the soil can offer i.e. its shear strength.

Direct Shear Apparatus:- The soil specimen is placed in a circular box divide into two halves. The specimen is subjected to a constant load while an increasing horizontal force is applied to one of the sections of the shear box. This force causes the specimen to shear along the juncture between the box sections. The shear force and the normal load, both are measured directly. By testing a number of specimens under normal loads a relationship between the normal stress and maximum shear stress can be developed, which is a straight line in most of the cases. The angle which the resulting straight line makes with the horizontal axis and the intersect which the straight line makes with the vertical axis are reported as the angle of shear resistance (angle of internal friction) and cohesion intercept respectively.

The information about these shear characteristics provided by the direct shear test is of importance in the design of sub structure, in determining bearing capacity of soils, in stability calculations of earth slopes, design of retaining walls etc.