Characterisation of geotechnical units on Mount Taranaki and influence on edifice stability

Abstract

A field, laboratory and numerical modelling methodology were devised to investigate the edifice stability of Mount Taranaki, New Zealand. The primary aim of this thesis was to determine the limiting height of Mount Taranaki, where massive edifice failure occurs resulting in hazardous debris avalanches. A secondary aspect of this study was to establish modern edifice stability and the influence of seismicity and groundwater as external forcing mechanisms. The key components of the methodology include a thorough field (discontinuity mapping, GSI, rock mass strength estimates, Schmidt Hammer) and laboratory (Point Load, Cone Indenter) investigation using the results to derive rock mass strength parameters for each geotechnical unit to build two-dimensional models using limit equilibrium and finite element numerical modelling codes. The edifice is shown to be currently meta-stable, meaning it is gravitationally stable provided no external forcing occurs. Seismicity, groundwater and presumably volcanism are shown to have a significant influence on the edifice stability, with the potential for failure to occur in the event of a 0.3g seismic event, which is within the range of seismic activity recognised for the Taranaki region over a 475- year return period. Following a systematic approach, hypothetical growth of Mount Taranaki was modelled by adjusting edifice height and slope angle. Finite element modelling showed that there is potential for gravitational failure to occur without external forcing when the edifice itself has a height of 3000m, a slope of 35°, and jointing (SRF = 0.92), although failure almost occurs when the edifice height is 2500m, with a slope of 35°, and jointing. (SRF = 1.01). A qualitative hazard assessment shows that much of the Taranaki region is at risk if volcanic collapse were to occur initiating a debris avalanche, however, the most populated area of New Plymouth is largely protected by the remnant volcano, Pouakai.