Field reconnaissance of the Rawene Slip urban landslide, Auckland, New Zealand

Abstract

A brief investigation of an ongoing urban landslide complex in Auckland, New Zealand, is presented. After progressive deformation of a carpark surface in the centre of Birkenhead over several months, two landslides occurred, in October and November 2017. The landslides occurred adjacent to each other at the head of the Rawene Reserve area, with rotational slumping forming headscarps and each landslide developing an earthflow component with a runout over 100 m. The failures appear to be formed of fill and residual soil. While the residual soil is formed on Miocene East Coast Bays Formation sands and silts, historical aerial images reveal that the fill was emplaced on the slopes in the 1970s and 1980s within gullies, to allow the carpark to be built. The precise triggering mechanism of the landslide activity is unknown. Although rainfall-triggered landslides occurred over much of the Auckland region in early 2017 due to the passing of ex-tropical cyclone Debbie, total rainfall for 2017 in the area (1299 mm) closely mirrored the 1980–2010 ‘normal’ of 1231 mm. Anecdotal evidence from eyewitnesses in days leading up to the first landslide highlighted resident’s concerns about the effects of vibration from piling operations at a construction site adjacent to the carpark, where cracking and subsidence was evident. Thus, given the timeline of observations recorded at the site, and local geology, progressive failure along a weak surface, affected by dynamic loading from piling-induced vibrations, may have increased pore pressure and decreased the shear strength of the residual soil and fill.