Framework for Mapping Liquefaction Hazard–Targeted Design Ground Motions

Liquefaction-induced ground failure poses substantial challenges to geotechnical earthquake engineering design. Current approaches for designing against liquefaction hazards, as specified in most seismic provisions, focus on estimating a liquefaction factor of safety ($F{S}_L$) and typically characterize earthquake loading using design parameters based on probabilistic or deterministic ground motion levels. Because $F{S}_L$ is estimated deterministically, this basis of design neglects considerable uncertainties for estimating liquefaction triggering and its consequences and results in a lack of liquefaction-specific design criteria, particularly as structural design has advanced toward risk-targeted performance objectives. This study presents a framework for developing liquefaction-targeted design criteria based on a minimum acceptable return period of liquefaction, informed by probabilistic liquefaction hazard analysis (PLHA). PLHA quantifies annualized rates of liquefaction by considering contributions from (1) the full ground-motion probability space, and (2) uncertainties in liquefaction triggering using probabilistic models. PLHA is used in this study to characterize the current, effective return periods of $F{S}_L$ ($T_{R,FS}$) obtained from conventional liquefaction hazard analysis (CLHA) using uniform-hazard ground motions. $T_{R,FS}$ is evaluated in a parametric study of nearly 100 sites throughout the conterminous United States. The results indicate large geographic variations in acceptable liquefaction hazard levels, with implied $T_{R,FS}$ ranging between approximately 1,000 to 3,000 years. To address these inconsistencies without the computational demands of full PLHA, a framework is proposed for developing a liquefaction-targeted design peak ground acceleration, $PG{A}_L$, for use in liquefaction models that result in consistent liquefaction design levels across all geographic locations. The mapped $PG{A}_L$ is shown to be somewhat sensitive to site-specific properties, and adjustment factors are developed and presented. The proposed $PG{A}_L$ mapping procedure produces $F{S}_L$ estimates that are consistent with those obtained from full PLHA at a target $T_{R,FS}$, providing a promising roadmap to incorporating PLHA concepts into current liquefaction design methods.