An engineering geological assessment of the Mangapoike Landslide, Gisborne, New Zealand and evidence for fluid overpressurisation as a triggering mechanism

Abstract

This project describes the application of field-based and remote sensing techniques used to study a recently occurring large dam-forming slope failure, Southwest of Gisborne New Zealand. The overall objective of this study was to determine the geological conditions, discontinuities and rock mass properties that led to the triggering of the Mangapoike Landslide. In summary, the study included a detailed field investigation, laboratory testing, numerical modelling as well as an in-depth review of literature and discussions about possible triggering mechanisms. An initial desktop study indicated extensive evidence of large-scale block detachments and planar land sliding in the region. A field investigation, unmanned aerial vehicle (UAV) survey and laboratory analysis followed to provide a detailed dataset to better assess the nature and extent of the landslide study area. The ground conditions that lead to ~ 8 million m3 of debris from the slip to block the river downslope and form a new reservoir were found to include Tertiary sediments that contain bedding plane shear failures which produced a low friction slide plane when heated from shearing within the deeply incised hill county. Remotely sensed data collected via UAV equipped with a high resolution digital camera was used to capture airborne photogrammetry. The imagery was processed and modelled with structure from motion (SfM) software to provide a digital elevation model and point cloud data. Geological features were mapped from three-dimensional models created from point cloud and photogrammetry data, which later were fed into the slope stability analysis. Following analysis of the seismic and precipitation records, further investigation of the geodetic data was undertaken. Global Navigation Satellite System (GNSS) slow slip monitoring stations, approximately 5 km from the site, which track ground movement deformation coinciding with slow slip seismic events, recorded significant movement that matched the temporal record for the ongoing slope failure. A resistivity model of the subduction plate along the slide plane interface suggested conductors, interpreted as fluid rich sediments, are present at the study site. The presence of overpressurised fluids, which accumulate below shallow low permeability strata may have reduced the effective strength of the rock and triggered the slope failure is hypothesised.