8th Ishihara lecture: Holistic evaluation of liquefaction response

Abstract

Soil liquefaction during earthquakes is a highly dynamic process involving rapid development of excess pore water pressures (EPWP), reduction in soil stiffness and strength, and diffusion of EPWPs through water flow. In addition, the liquefaction response is characterized by strong cross-layer interactions that significantly influence the severity of liquefaction and overall system response of liquefying deposits. To emphasize the importance of dynamic interactions and provide an in-depth understanding of the liquefaction response, a framework for a holistic evaluation of the liquefaction response is presented. Three key aspects of the response are required to be concurrently considered in the holistic evaluation of the liquefaction response: element (soil) response, cross-layer interactions (system response) and intensity of the input motion. The proposed approach is applied to a comprehensive series of nonlinear dynamic analyses (NDAs) of well-documented case history sites from Christchurch, New Zealand, to identify key interaction mechanisms and quantify effects of dynamic interactions on the liquefaction response. As state-of-practice simplified liquefaction evaluation procedures ignore interactions within the deposit, results from simplified analyses substantially differ from those observed in NDA and show significant anomalies in the evolution of the liquefaction response throughout the depth of liquefiable deposits. A systematic approach for an NDA-based improvement of simplified analysis is proposed to incorporate interaction mechanisms and system response effects in simplified liquefaction evaluation procedures.