Slope Failure In A Complex Volcanic Terrain, Opito Bay, Kuaotunu, Coromandel Peninsula

The Ohinau Drive slope failure has occurred at the northern base of the volcanic Tahanga Hill, Opito Bay. The failure has affected a recent subdivision on Ohinau Drive situated immediately adjacent to the hill. The slide is a complex, variable depth failure encompassing several differing geological units. It extends a distance of 170 m from headscarp to toe with an estimated maximum width of 130 m. It comprises both shallow-seated and deep-seated failure mechanisms to a maximum depth of approximately 20 m. In the winter of 1996 slope instability was recognised following development of a headscarp and ongoing disturbance to kerbing and manholes.

Investigations undertaken revealed complex geological conditions generally comprising hydrothermally altered andesite partially overlain by basaltic debris and weathered basalt lava. Artesian water pressures were encountered within the andesite. The investigation results indicate that both a deep-seated failure through the underlying andesite and a shallow-seated movement involving the basalt debris were recently active. A geotechnical model was constructed along two cross sections with computer aided stability analyses undertaken. Target groundwater levels were determined to achieve a satisfactory Factor of Safety to allow future subdivision development. Drainage installation and monitoring is yet to be established following liaison with Council.