Effects of partial saturation on the liquefaction response of two Christchurch deposits

Abstract

This paper focuses on the evaluation of effects of partial saturation on the liquefaction response of two free-field level-ground deposits from Christchurch (New Zealand). The first deposit is composed of vertically continuous liquefiable soils with a low-resistance critical zone at shallow depth and is typical of sites that manifested moderate-to-severe liquefaction at the ground surface in several events during the 2010-2011 Canterbury earthquake sequence. The second deposit consists of liquefiable soils of low liquefaction resistance interbedded with non-liquefiable layers, and is typical of sites that did not manifest liquefaction in any of the 2010-2011 seismic events. High-resolution measurements of compression wave velocity (V_p) have indicated the existence of a partially saturated zone at shallow depth below the groundwater table in both deposits, though with somewhat different characteristics. We assess the performance of the two deposits for the 22 February 2011 Christchurch earthquake using simplified liquefaction triggering analysis as well as advanced nonlinear dynamic analysis. Partial saturation effects are considered in both types of analyses by “correcting” the liquefaction resistance of the partially saturated soils on the basis of an empirical V_p-based relationship. The analyses indicate that partial saturation contributes to the formation of a specific sequence of system-response mechanisms that collectively act to mitigate liquefaction manifestation in the case of the interbedded deposit. In the case of the vertically continuous deposit, however, the mitigating effect of partial saturation is counteracted by system-response mechanisms that intensify the effects of liquefaction. The results highlight the importance of considering the effects of partial saturation in the context of the overall system response of liquefying deposits and consequent liquefaction manifestation.