Frame shear device

A frame shear device is a geotechnical instrument for testing soil samples .

application

Effective shear parameters of a drained (drained) soil are determined experimentally in geotechnical engineering using shear tests in direct shear devices . Direct shear devices are very easy to use and more cost-effective compared to other test facilities ( three-axis device ). Another advantage is the clarity of the test method. The sample shape can be circular or rectangular. The latter configuration is referred to as a box or frame shear device.

Technical principle

In the case of direct shearing devices, two frames (or rings) lying one above the other are arranged in which the soil sample to be examined is installed. The size of the shear box depends on the largest grain of the sample. The samples built into the direct shear device are loaded by a previously defined normal force through a filter stone and punch until the subsidence subsides , as in the oedometer . Usually the normal force is applied by means of weights via a lever system. With the so-called “shearing”, either the lower part is moved parallel to the upper fixed box or, conversely, the upper box is moved towards the fixed lower part. Due to this movement, a shear surface is forced into the sample.

construction

In modern shearing devices, the shear box and the punch for the normal force transmission are guided by means of ball or roller bearings that prevent the components from tilting or tilting. With these devices, due to their design, it is possible to set a very small gap between the two shear boxes (or rings). This ensures that as little as possible of the sample material gets into the gap during shearing. During the shearing process, the built-in specimen is usually loaded by a constant, perpendicular normal force acting on it. When the soil sample is sheared, the horizontal displacement, the shear force with the associated normal force and, if necessary, the change in height of the top plate are measured. If the shear force for evaluation is related to the initial sample surface , as suggested by DIN 18137-3 , a shear stress is obtained. The change in area due to the shear path is not taken into account.