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 (FSL) and typically characterize earthquake loading using design parameters based on probabilistic or deterministic ground motion levels. Because FSL 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 FSL (TR,FS) obtained from conventional liquefaction hazard analysis (CLHA) using uniform-hazard ground motions. TR,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 TR,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, PGAL, for use in liquefaction models that result in consistent liquefaction design levels across all geographic locations. The mapped PGAL is shown to be somewhat sensitive to site-specific properties, and adjustment factors are developed and presented. The proposed PGAL mapping procedure produces FSL estimates that are consistent with those obtained from full PLHA at a target TR,FS, providing a promising roadmap to incorporating PLHA concepts into current liquefaction design methods.