Nonlinear shear stress reduction factor (rd) for assessment of liquefaction potential in Christchurch Central Business District

Abstract

Simplified procedures for evaluating liquefaction triggering potential use the nonlinear shear stress reduction factor, rd, to estimate the peak earthquake-induced cyclic shear stress within the soil strata. Previous studies have derived rd by considering the response of representative ground profiles subjected to input ground motions with a range of ground motion characteristics. In this study, site–specific rd for serviceability limit state (SLS) and ultimate limit state (ULS) design ground motions are developed using site response models of the Christchurch Central Business District (CBD). The site response models are generated for typical geologic conditions of Christchurch CBD with shear wave velocity, Vs, profiles developed from the results of multichannel analysis of surface waves (MASW) surveys conducted across Christchurch CBD. A total of 528 simulations were conducted using 1D nonlinear time domain site response analyses using a suite of input ground motions that are representative of controlling ground motion scenarios for seismic hazard of Christchurch. The results of the ground response analyses are used to determine Christchurch CBD-specific rd relationships for liquefaction triggering assessments. The proposed relationships provide a better estimate of the cyclic stress ratios induced below Christchurch CBD when subjected to design SLS and ULS ground motions as compared to typical practice using generic liquefaction assessment methodologies.