Surficial liquefaction manifestation severity thresholds for profiles having high fines-content, high-plasticity soils

The severity of surficial liquefaction manifestation was significantly over-predicted for a large subset of case histories from relatively recent earthquakes that impacted the Canterbury region of New Zealand. Such over-predicts generally occurred for profiles having predominantly high fines-content (FC), high-plasticity soil strata. Herein, the liquefaction case histories from the Canterbury earthquakes are used to investigate the performances of three different manifestation severity index (MSI) models. The prevalence of high FC, high-plasticity strata in a profile is quantified through the soil behavior type index averaged over the upper 10 m of a profile (Ic10). It is shown that for each MSI model (1) the threshold MSI value distinguishing cases with and without manifestation increases as Ic10 increases and (2) the ability of the MSI to segregate cases with and without manifestation decreases with increasing Ic10. Additionally, probabilistic models are proposed for evaluating the severity of surficial liquefaction manifestation as a function of MSI and Ic10. The approaches presented in this study allow better interpretations of predictions made by existing MSI models, although their efficacy decreases at sites with high Ic10. An improved MSI model is ultimately needed that better accounts for the effects of high-FC, high-plasticity soils more directly.