Insights from liquefaction ejecta case histories for the 2010–2011 Canterbury earthquakes

Abstract

A database of detailed liquefaction ejecta case histories for the 2010–2011 Canterbury earthquakes is interrogated. More than 50 mm of ejecta-induced settlement occurred at thick, clean sand sites shaken by PGA6.1 = 0.35–0.70 g (wherein PGA6.1 is the peak ground acceleration for a Mw 6.1 earthquake), whereas ejecta-induced settlement at highly stratified silty soil sites did not exceed 10 mm even when PGA6.1 exceeded 0.45 g. Cone penetration test-based liquefaction-induced damage indices that do not consider soil-system response effects, such as post-shaking hydraulic mechanisms, overestimate the severity of ejecta at stratified silty soil sites. Considering post-shaking hydraulic mechanisms captures the lack of ejecta at stratified silty soil sites. It also improves the estimation of ejecta severity at clean sand sites with severe-to-extreme ejecta. Strongly shaken clean sand sites that did not produce ejecta typically had thick strata with high tip resistances, thick non-liquefiable crusts, or deeper non-liquefiable strata overlying liquefiable strata. Ejecta-induced fissures formed in the non-liquefiable crust during the Feb 2011 earthquake which liquefied soil at depth could exploit to produce ejecta during the Jun 2011 earthquake. When significant ejecta formed on the roads, elevated adjacent ground with houses typically had negligible ejecta.