Civil and Building Engineering.

A method for testing soils that is similar to the sliding block test is the direct shear test. A cross-sectional view through a direct shear apparatus is shown in Figure 3.33. In this test, the soil is placed in a box that is split horizontally in a manner that allows the upper and lower halves to be displaced relative to one another while a vertical stress is applied to the upper surface of the test specimen. The shape of the shear box may be square or circular in plan.

The soil may be consolidated prior to shearing if the soil being tested is clay. During the test, the horizontal displacement of the shear box is increased and the shear force is measured until failure occurs or until maximum displacement is reached.

If a series of direct shear tests are performed on a dry sand using various vertical pressures, the shear stress at failure can be plotted versus the vertical normal stress in a diagram like that shown in Figure 3.34. By analogy with the block sliding on the plane, the slope of the line in this diagram is des- ignated by the Greek letter , and the angle is called the angle of internal friction.

The typical results for a direct shear test series on stiff clay are shown in Figure 3.35. In this diagram, the shear strength of the soil consists of two parts. The first part is indicated by the intercept of the vertical axis, labeled c, called the cohesion. The second part is indicated by the slope of the line and is called the internal friction. The shear strength of the soil is then given by the equation