Modification of the liquefaction potential index to consider the topography in Christchurch, New Zealand

In recent years, many have mapped the liquefaction potential index (LPI) to describe the liquefaction hazard at regional scale. Several investigators have calibrated the LPI to field observations of liquefaction-induced ground failure after a number of major earthquakes; the significance of LPI values and their correlation with the severity of liquefaction-induced ground failure can vary greatly depending on the calibration. In this study, the LPI was computed at more than 1200 cone penetration test soundings across the Christchurch area, New Zealand, using peak ground accelerations from the 2011 Christchurch Earthquake. Based on detailed field observations of liquefaction-induced ground failure after the earthquake, it was shown that the LPI has potential for discriminating between areas with no liquefaction-induced ground failure hazard and areas that may experience liquefaction-induced ground oscillations and settlement; however, the LPI performed poorly at sites where severe ground failures occurred, especially at sites that experienced lateral spreading. As many researchers have found a positive correlation between lateral spreading and the proximity and depth of a nearby free-face (i.e., a steep topographic depression, river channel, etc.), a new LPI framework was proposed that includes a parameter named the free-face ratio (FFR). FFR was shown to have a significant correlation with field observations of lateral spreading in Christchurch. It was shown that by incorporating FFR into the LPI framework, modified LPI values are positively correlated with the severity of field observations of liquefaction-induced ground failure in Christchurch. It was also shown that maps can be produced based on LPI and FFR, and such maps rarely underpredict the liquefaction-induced ground failure hazard, unlike maps based on the unmodified LPI.