WES Theoretical Manual, Part X: Sliding Instability

This part covers sliding instability, in which the cell slides along its base or along a weak layer in the foundation. The manual uses a wedge method, in which trial failure surfaces are chosen and the forces on each wedge are balanced.

It gives several cautions that designers should remember. The earth pressures on the cell walls from the wedge method are not necessarily good approximations to the actual pressures, since they may correspond to a high factor of safety against sliding. If a weak stratum is present in the foundation, it should be included in the trial failure surfaces. And the wedge method gives an upper bound to the factor of safety, so there is no guarantee that the lowest value has been found; the result depends on the analyst's experience in predicting failure surfaces.

A more refined analysis that includes shear on all vertical boundaries may be considered when the simple check is not enough.