Probabilistic Evaluation of Liquifaction Potential for Downtown San Francisco

This report describes the evaluation of the potential of saturated deposits of cohesionless soil in downtown San Francisco for initial liquefaction due to seismically induced pore pressures. The condition of initial liquefaction, or the zero effective stress state, was evaluated because it provides the best available index for damage due to seismically induced pore pressures.

Liquefaction potential was evaluated in a probabilistic manner by comparing the conditional probability of liquefaction, or liquefaction susceptibility, to the expected intensity of seismic loading, or liquefaction opportunity. The probabilistic evaluation was made using a liquefaction hazard model developed by Chameau (1980).

Results indicate that while no liquefaction is expected anywhere for an intensity with an annual probability of exceedance of 5% (a return period of twenty years), only the densest deposits will survive an event with an annual probability of exceedence of two percent (fifty year return period). For an event with an annual probability of occurrence of 1%, or a hundred year return period, 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.