Engineering geological investigations of landslides in Auckland, New Zealand

Summary The undulating topography of the Auckland urban region of New Zealand is susceptible to landslides of varying process-mechanisms, including: (1) earthflows of saturated Pleistocene Tauranga Group sediments, tephra and residual soils flowing off more competent underlying rock; (2) rotational slumping of man-made fill or Tauranga Group sediments; (3) block-slides of weak Miocene Waitemata Group sedimentary rock, dipping out of slope. Such landslides are often triggered by intense short periods, or prolonged periods of rainfall, such as the ‘Tasman Tempest’ and ex-Tropical Cyclone Debbie storms of 2017. Typically, rainfall infiltration results in a rise of the groundwater table and an increase of the pore water pressure, causing a reduction in effective normal stress and thereby soil strength, leading to landslides. Here, some case studies are presented of landslide investigations in Auckland using geophysics such as ERT and EM to monitor subsurface hydrogeology, in conjunction with Unmanned Aerial Vehicles (UAV) to monitor topographic change.