A Multi-Temporal Slope Stability Analysis of Lemons Hill, Northland, New Zealand

Abstract

Landslides are a common geologic hazard becoming ever more prevalent as rising populations push development further into areas of geomorphic instability. It is unrealistic to assume that all landslide prone areas can be avoided or that mitigation measures will fully prevent instability or the consequences of such an event. Monitoring active areas may allow for a better understanding of individual slope behaviours, allowing for adequate warning or mitigation of the slope. On 13th February 2018, following prolonged rainfall from Cyclone Fehi, a significant landslide initiated at Lemons Hill above State Highway 11 (SH11), a popular tourist route through Northland, New Zealand. This occurred as a shallow (2 m deep) translational failure within completely to highly weathered greywacke and was likely triggered by elevated pore water pressure from Cyclone Fehi. Lemons Hill is naturally oversteepened (>40°), a condition aggravated by a road cut constructed to accommodate SH11. Mitigation included the construction of engineered batters, resulting in six months of traffic disruptions. This study examines the triggers, along with the past and current mechanisms of failure at Lemons Hill and in the wider catchment area. Assessment of historic data, field investigation, Limit Equilibrium Modelling (LEM) and monitoring via multi-temporal Unmanned Aerial Vehicle (UAV) photogrammetry surveys was undertaken. Results revealed that raised groundwater levels triggered the 2018 landslide, indicated by back analysis LEMs as Factor of Safety (FoS) values decreased from 1.491 to 0.250 under dry and saturated conditions respectively. Steep slope gradients in combination with loose completely to highly weathered materials were further shown to amplify instability. Engineered batters have reduced the failure risk upslope of SH11. Comparison of a saturated back analysis LEM with a saturated post batter LEM indicate a respective increase in the FoS from 0.250 to 0.714, with a significant reduction in the length of the critical failure surface. The areas downslope of SH11 however remain a stability concern. Finally, the UAV photogrammetry surveys were successful in capturing the three-dimensional condition of Lemons Hill at specific points in time, with sufficient detail for major geomorphic features to be identified. This technique shows strong potential for monitoring active slopes in challenging terrains, such as along a State Highway.