Relationship of geology to slope failures in soft rocks of the Taihape-Mangaweka area, central North Island, New Zealand

Abstract

Numerous slope failures, some of which are active, occur in Upper Tertiary - Lower Quaternary sandstones and mudstones of the Taihape - Mangaweka area. Relationships with volcanic ashes show that landsliding has continued for at least the last 11,000 - 20,000 years, and possibly longer. Large scale failures are mainly restricted to the weaker Taihape Mudstone and Utiku Sandstone formations. Deep-seated failures in the stronger and relatively impermeable Mangaweka Mudstone are much less common, although small scale near-surface failures show no such restriction in their occurrence. Clay seams occurring in the sandstones and mudstones form, or can be inferred to form, the basal slide-planes for several of the large scale failures. Large irregular, often actively opening fractures, caused by active tectonic uplift, provide side shear planes for some failures, and faulting and earth-quakes associated with the uplift may help initiate slide movement. Small scale failures are often related to steeply dipping, stress relief exfoliation planes. A large number of porosities, densities, permeabilities, and field-measured strengths of the mudstones can be related to the microfabric which is in turn dependant on presumed depth of burial, whereas properties for the sandstones are far less dependant on degree of compaction. Slope failure commences as block sliding but may later evolve, by breakdown of the slide blocks, into creeping debris slides and creeping debris flows. Once bedrock has been broken down into landslide material, degree of saturation determines further movement. The remnants of ancient landslides are possible sites of renewed movement, and many more modern failures are in a quasi-stable sensitive state, requiring only a small increase in moisture content to reduce their shear strengths to zero.