Foundation design for gravity retaining walls under earthquake

Economical design of gravity retaining wall systems subject to earthquake can be achieved by recognising that, although modest earthquake acceleration may bring the wall system to limiting equilibrium, the criterion for satisfactory performance is the residual displacement of the wall after the earthquake. The bases of such walls are long in relation to their width so are considered as strip foundations. This means that the shallow-foundation bearing strength surface provides insight into the capacity of the foundation under static and earthquake actions. It could be argued that another consideration is the demand on the shear strength of the material beneath the foundation from earthquake-induced stresses. However, this is unlikely to be a serious limitation. Pseudo-static analysis of simple gravity wall system geometries reveals that, usually, the foundation capacity is controlled by moment-induced bearing failure as it will not be economical to design walls able to enforce sliding. Thus the critical deformation mode is rotation of the foundation rather than horizontal displacement (unless the foundation is constructed on a rock-like layer with large bearing strength). This finding calls for a re-evaluation of methods of considering the earthquake response of gravity retaining systems.