Ponui Landslide: A deep-seated wedge failure in Tertiary weak-rock flysch, Southern Hawke's Bay, New Zealand

Abstract

Abstract The Ponui Landslide (September 1976), in Southern Hawke's Bay, is a wedge failure in an Upper Miocene weak-rock alternating sandstone-mudstone and amalgamated sandstone succession. The planimetric area involved with failure covers 25 ha and represents the reactivation and enlargement of a previous landslide. The Ponui Landslide has a calculated volume of approximately 2.5 × 106 m3. Morphologically, the Ponui Landslide consists of a lateral escarpment, exhumed slide surface, translational slide block, and slide debris. Rock-mass defects have controlled the landslide block geometry. The lateral escarpment has been propagated on nearly vertical intersecting fractures (joints and faults). The slide surface is coincident with bedding in an alternating succession of friable, porous sandstone and weak, slake-prone mudstone. Slide-plane dip is variable, ranging from 10° to 36° and this intersects the escarpment fractures, so isolating the rock wedge. The slide debris is comprised predominantly of silty sand. The rupture surface propagated on a thin montmorillonitic clay gouge zone parallel to bedding. The gouge represents atectonically formed shear zone. Block movement occurred subparallel to strike. The partially disintegrated slide block dammed Ponui Stream, creating a lake with maximum depth of 35 m, and a total calculated storage capacity of over 2.89 × 106 m3. The principal failure mechanism was sliding, involving essentially one large wedge-shaped block. Rapid movement and disintegration quickly reduced much of this block to debris. Other factors contributing to failure include: (1) high rainfall over a prolonged period prior to failure; (2) presumed high ground-water levels with excessive pore-water pressures generated within the porous, fractured sandstones resting on impervious mudstones; and (3) reactivation and enlargement of an existing failure which occurred during the 1931 Napier Earthquake.