Several hundred people die each year in trench collapses. Unfortunately, production and safety goals seem to be at odds when dealing with trench work. The soil that snuffs out life in a trench collapse is not only suffocating, it is crushing, weighing easily 120 pounds per cubic foot. To put that in perspective, keep in mind that a cubic yard is as heavy as an automobile.
When a trench is cut, lateral stresses that once pushed against the face of the trench wall (by the column of soil that was removed) no longer exist. The soil in the trench wall immediately begins to move into the trench. At the same time, the surface of the ground next to the trench subsides. Tension cracks appear due to soil's weakness in tension. The cracks occur at a distance equal to about one-third to two-thirds of the trench's depth. When these cracks develop, the weight of the soil in the trench wall is no longer carried by the soil back from the face of the wall (through shear). Next, the lower part of the trench wall fails under great stress from the weight of the soil above it. The second failure, from the upper part of the trench, will occur very soon after the first failure at the bottom.