Engineering geological investigation of landslides in Karekare, Auckland, New Zealand

Abstract

In the early months of 2023, the Auckland region was subjected to three major storm events, exTropical Cyclone Hale (10 January), Auckland Anniversary Storm (27 January) and ex-Tropical Cyclone Gabrielle (12–16 February), leading to catastrophic impacts and economic losses amounting to NZ$14.5 billion. January 2023 set the record for the wettest month in Auckland’s history, with 539 mm of rainfall recorded in the central city, resulting in the triggering of thousands of landslides, and severe flooding. Two weeks later, ex-Tropical Cyclone Gabrielle impacted the North Island, causing further devastation with hundreds of thousands of landslides and extensive flooding. On February 13 2023, the intensive rainfall of ex-Tropical Cyclone Gabrielle led to the initiation of multiple slope failures within the Karekare township area, ~50 km west of central Auckland. This was estimated to be 1-in-200 year storm event. Karekare was cut off from metropolitan Auckland, with significant damage to both infrastructure and private property. This site investigation involved in-situ shear vane and handheld penetrometer testing. Samples of the residual Piha Formation soils collected from the site were analysed further in the laboratory to investigate the soil index properties. The mineralogy of the soils was investigated using SEM-EDS methods. Pre- and post-failure LiDAR and post-failure imagery were used to map the extent of the slope failures within the study site. From site visits and GIS modelling, it was determined that the landslides were primarily shallow translational landslides and debris flows, in shallow residual soils. Slope stability modelling was conducted to investigate the current factor of safety of slopes. This highlighted that currently, slope failures are inactive, and failure is only likely with soil saturation. As future slope failure is possible, slope engineering and monitoring methods should be implemented to mitigate further landslide risk. The findings of this study were contextualised against the landslide and property risk categorisation conducted by GHD on behalf of Auckland Council in late 2023.