Seismic Bearing Capacity and Settlements of Foundations

Field and laboratory observations of seismic settlements of shallow foundations on granular soils that are not attributable to changes in density or liquefaction are explained in terms of seismic degradation of bearing capacity. Limit analysis using a Coulomb-type mechanism including inertial forces in the soil and on the footing gives expressions for seismic bearing capacity factors that are directly related to their static counterparts. Comparison of the two depicts clearly the rapid deterioration of the overall foundation capacity with increasing acceleration. Such periodic inertial fluidization causes finite settlements that are possible even in moderate earthquakes. Reduction in foundation capacity is due to both the seismic degradation of soil strength and the lateral inertial forces transmitted by shear to the foundation through the structure and any surcharge. A straightforward sliding-block procedure with examples is also presented for computing these settlements due to loss of bearing capacity for short time periods. This approach also leads to a design procedure for footings based on limiting seismic settlements to a prescribed value.