Liquefaction Potential Mapping for San Francisco

Abstract

The potential of saturated deposits of cohesionless soil in downtown San Francisco to experience initial liquefaction due to seismically induced pore pressure is evaluated. Initial liquefaction, or the zero effective stress state, is used as the index of liquefaction potential because it provides the best available index for damage due to seismically induced pore pressures. Liquefaction potential is evaluated by comparing the conditional probability of liquefaction, or liquefaction susceptibility, to the expected intensity of seismic loading, or liquefaction opportunity. The probabilistic evaluation is made using a liquefaction hazard model developed by Chameau. Assuming the water table to be at the ground surface, results indicate that while no liquefaction is expected anywhere for an intensity with an annual probability of exceedance of 0.05, only the most resistant deposits will survive an event with an annual probability of exceedance of 0.02. For an event with an annual probability of exceedance of 0.01, initial liquefaction is expected to occur within all saturated, cohesionless soil deposits in the downtown San Francisco area. It must be emphasized that in dense soil deposits the consequences of initial liquefaction may be minimal because of their limited shear strain potential.