Engineering Geological Investigation of the Pamoa Landslide, East Coast, New Zealand

Abstract

Shallow landslides occurring on deforested slopes in the Gisborne Region have been rather prevalent for decades, principally due to weak soils, steep slopes, land use change and rainfall effects. These effects contribute to the extreme erosion rates and exceptionally high sediment yields for East Coast rivers. The Pamoa Forest Block ~25 km southwest of Gisborne (the focus of this study), is one of many areas of pinus radiata plantation in the region. As with other East Coast areas over recent decades, post-harvesting erosion and slope failure through shallow landslides occur. Slope instability in the Pamoa area is of particular significance because the Gisborne town water supply pipeline crosses the valley from the Waingake Waterworks reservoir. Second, residential properties are within the broader area, and so the access roads and driveways to the properties are occasionally affected by overslips and underslips, due to erosion of slopes above and below roads. Third, the unsealed forest haul roads themselves need to be kept open for safe egress of forestry workers and their logging cargos. Thus, understanding slope instability is important for various infrastructure integrity reasons. The landslide studied here is henceforth referred to as the Pamoa landslide, and it occurred on a steep slope between two haul roads at the only entrance of the Pamoa Forest Block. Thus, it is broadly characterised as an overslip relative to the lower haul road, and an underslip or “dropout” in the context of the upper haul road. The failure appears to have initiated between September-December 2018, but was first discovered in February 2019, during inspection. Historical images suggest the failure occurred following the harvesting of pinus radiata on steep slopes. Slumping at the headscarp was first evident in April 2019 aerial imagery and subsequent unmanned aerial vehicle (UAV) imagery from inspections by Gisborne District Council (GDC). Direct testing included soil augering, shear vane and scala penetrometer, as well as sampling for laboratory testing of index properties. Remote sensing was in the form of ground penetrating radar (GPR) and UAV surveys, along with LiDAR. In summary, the landslide is in soils and is a slump that transitions downslope into an earthflow. Evidence suggests it is extending upslope by retrogressive failure. Recommendations for remediation of the landslide include re-installation of drainage pipes, slope engineering through buttressing of the toe, lowering of the toe slope angle, benching, a palisade wall, and bioengineering such as the manuka or kanuka species. Monitoring using inclinometer, extensometer, tiltmeter and piezometers may prove useful, as well as UAV Structure-from-Motion photogrammetry and Digital Terrain Modelling, to monitor surface change.