Assessment of Liquefaction from New Zealand Case Histories: With insights from liquefaction-induced land damage, over the Lower Wairau Plains, caused by the 2016 Kaikoura earthquake

Abstract

Liquefaction-induced land damage has been identified in more than 13 notable New Zealand earthquakes within the past 150 years. The government, local councils, insurers, and many stakeholders are now looking to research and understand their exposure to this natural hazard after recent devastating events such as the Canterbury Earthquake Sequence. This study, operating within a collaborative research focus, aims to deepen the understanding of liquefaction through compiling all known case histories of liquefaction manifestation across New Zealand into a centralised geospatial database. The collation of these observations provides insights into the temporal, spatial, and geomorphological conditions under which liquefaction has occurred. Insights are then drawn from the widespread manifestations of liquefaction that resulted from the 2016 Kaikoura earthquake and other historical events in the region of Marlborough. Groundwater and ground motion models are developed such that the state of the art in simplified liquefaction assessments can be employed. Combining these assessment procedures with observational datasets provides a greater understanding of the predictive efficacy of the frameworks. Areas or conditions where predictions are consistent with the observations are readily identified, along with root causes for false-predictions. This case study, along with the wider case histories, have identified a very strong correlation between manifestations of liquefaction and inner bends of paleo and active river channels, attributable to the shallow groundwater and loosely deposited fine-grained, cohesionless sediments. This correlation is prevalent in all case histories explored through this research. High-quality remote sensing data and aerial imagery before and following the 2016 Kaikoura earthquake has enabled precise detailing of the manifestations that occurred along the Opaoa River, Marlborough, and is a practise that should continue following earthquakes to refine assessment procedures. The simplified liquefaction assessment frameworks have compounding conservatisms and assumptions which commonly contribute to over-prediction. Accurate estimates of groundwater are imperative to achieving correct predictions, however, inadequacies in the simplified methods ultimately reduce their efficacy in highly interlayered soils. The hydraulic continuity between layers is fundamental to the system response of the soil profile with inhibition of vertical dissipation of pore water pressures greatly reducing the likelihood of surficial manifestations.