Soil shear can be described as one layer or portion of soil moving over or with respect to another layer of soil. The interface between the two layers of soil can be abrupt and well defined, such as when a water saturated clay slides down a slick bedrock plane, or it can be gradational, which can occur in a soil mass where different zones move at different speeds. This may look more like a rolling motion in cross section.
Shear failure usually involves the soil shear affecting some man-made appurtenance that the soil provides support for, and occurs at some pre-determined level of soil shear, which generally involves limits of soil migration for a specific application. For example, if a portion of a building moves and another portion is stable, 1/4 inch cracks may develop in the building stemwall and we might say this was caused by soil shear failure.
