Slope Failure in a Complex Volcanic Terrain
Author: Steven Price
Presented/Published: Proceedings of the 5th Australia-New Zealand Young Geotechnical Professionals Conference, Rotorua, 2002
Keywords: slope failure, headscarp, deep seated failure, hydro-thermally altered andesite, basalt.
Abstract:
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 170m from headscarp to toe with an estimated maximum width of 130m. It comprises both shallow seated and deep seated failure mechanisms to a maximum depth of approximately 20m.
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 hydro-thermally 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 2 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.
