Simplified CPT-based liquefaction ejecta severity model using Christchurch data

K.M. Azul, R.P. Orense & L.M. Wotherspoon
University of Auckland, Auckland, New Zealand.

ABSTRACT
Christchurch, New Zealand, and its surrounding areas experienced significant liquefaction-induced damage during the 2010-2011 Canterbury Earthquake Sequence (CES) events. Ejecta is one of the liquefaction manifestations commonly observed and can cover significant parts of a site. This study focuses on developing a model that classifies sites based on whether liquefaction ejecta manifests or not, and the severity of the ejecta manifestation. Utilizing a database with 5000+ CPT data investigations in the region, the input parameters used in the model are simplified representations of fundamental geotechnical properties closely linked to liquefaction and the surface manifestations. Several models were developed using various combinations of these input parameters to segregate the sites into varying levels of ejecta severity, and their performances were compared. The final model presented can estimate the severity of ejecta at a site, specifically the lowest and highest levels (i.e. no ejecta and most ejecta classifications). Further analysis of various predictive capacity measures showed how much the model under- and over-predicted the observations. Combining severity levels into a single level was also studied to see its effects on the accuracy and performance of the model. Overall, given the relative simplicity of some of the inputs, the model shows promise as part of a large-scale liquefaction severity prediction system.