**Branch :**Civil Engineering

**Subject :**Soil Mechanics

## OTHER LABORATORY DEVICES TO MEASURE SHEAR STRENGTH

There are several other types of apparatuses that are used to determine the shear strength of soils in the laboratory. These apparatuses are, in general, more sophisticated than the shear box and the triaxial apparatus.

**Simple Shear Apparatuses**:

The purpose of a simple shear test is to determine shear strength parameters and the stress–strain behavior of soils under loading conditions that closely simulate plane strain and allow for the principal axes of stresses and strains to rotate. Principal stress rotations also occur in the direct shear test, but are indeterminate. The stress states in soils for many geotechnical structures are akin to simple shear. There are two types of commercially available simple shear devices. One deforms an initial cuboidal sample under plane strain conditions into a parallelepiped (Figure 10.28a). The sample is contained in a

box made by stacking square hollow plates between two platens.

The top platen can be maintained at a fi xed height for constant-volume tests or allowed to move vertically to permit volume change to occur (constant load test). By displacing the bottom of the box relative to the top, the soil is transformed from a cube to a parallelepiped. A load cell mounted on the top platen measures the excess porewater pressures. The lateral stresses are deduced from one of the hollow plates outfi tted with strain gages. The stresses and strains deduced from measurements in the cuboidal simple shear apparatus are shown in Figure 10.28b. If the excess porewater pressures are measured in undrained (constant-volume) tests, then the effective stresses can be determined.

The other apparatus tests a cylindrical sample whose vertical side is enclosed by a wire-reinforced rubber membrane (Figure 10.28c). Rigid, rough metal plates are placed at the top and bottom of the sample. Displacing the top of the sample relative to the bottom deforms the sample. The vertical and horizontal loads (usually on the top boundary) as well as displacements on the boundaries are measured, and thus the average normal and shear stresses and boundary strains can be deduced. In the cylindrical apparatus, the stresses measured are sz and tzx, and the test is referred to as direct simple shear.

Cuboidal simple shear apparatus: (a) simple shear box, (b) stresses imposed

on samples, and (c) direct simple shear.

Simple shear apparatuses do not subject the sample as a whole to uniform stresses and strains. However, the stresses and strains in the central region of the sample are uniform. In simple shear, the strains are εx 5 εy 5 0, εz 5 Dz/Ho, and gzx 5 Dx/Ho. A plot of shear stress tzx versus gzx is used to determine G. The shear displacement Dx must be applied in small increments to comply with the above defi nition.

The principal strains from Equations are

The dilation angle is determined from Equation (10.10). Taylor failure criterion is suitable for interpreting the results of simple shear tests. You can also use Mohr–Coulomb failure criterion with the effective stresses to determine f9cs and f9p, and Tresca failure criterion with the total stresses to determine su.