Moeraki township: Instability assessment

Abstract

Moeraki township has within its boundaries active mass movement jeopardizing housing and roading which necessitates comprehensive instability investigations for hazard mitigation. Geomorphological research identified salient deep-seated slope failure encompassing the entire township area developed within Tertiary mudstones; subsequent shallow instability amidst juxtaposed slide blocks has been accelerated by man and poses a major threat to residential development. Tertiary mudstone units in the field area are subject to strain "softening'' and were characterized as over-consolidated, blue-grey, silty clays containing an abundance of smectites. In-situ softening appeared expedited by high ground water pressures and yields a cohesive plastic clay of low strength prone to slope failure. Investigations were concentrated in two areas where active mass movement was threatening housing and roading. Severe damage in the two study areas was caused by well delineated cohesive flow-slide behaviour moving at rates in excess of 15 cm per year principally via "drained" failure in response to rising ground water levels. Comparatively stable mudstone regolith/colluvium at low slope angles exhibited "creep" deformation in the order of 2 cm to 4 cm per year, sufficient to damage building foundations. Incipient slope failure was assessed utilizing classical limit equilibrium analyses (Janbu, Generalised Procedure of Slices) and highlighted the inadequacies of such methods in dealing with real world scenarios such as "progressive slope failure". Data was collated to develop a model for evolving slope failure within mudstone; principle criteria controlling shallow instability were concluded as: 1) extent of terrain activity 2) ground water pressure. Mass movement susceptibility plans (scale 1:1000) were compiled for the township area based on the preceding failure model. Terrain evaluation was by subjective assessment of three factors; geology, geomorphology and hydrology. Finally, concise recommendations on land use zoning are outlined.