Abstract:
A field, laboratory and numerical modelling methodology was developed to investigate the likely mode of failure, and triggering event, for the Lochnagar landslide-dam, Central Otago, New Zealand. The key components of the methodology include: engineering geological mapping of the surrounding site, systematic description and characterisation of the rock mass, geomorphological mapping of the failure scarp and debris deposit, laboratory analysis of field samples (strength, weathering and exposure-age-dating), along with kinematic analyses and numerical modelling (limit equilibrium and distinct element). All three slope stability analysis techniques suggested a similar wedge-sliding mode of failure, with the addition of a unique toe buckling failure mechanism from the more complex distinct element modelling. This geomechanical model is the first of its kind to demonstrate the feasibility of toe buckling in Otago Schist. The pre-conditioning of the slope from seismic deformation and glacial retreat was analysed, and results suggest that a combination of slope pre-conditioning and a large earthquake triggering event were the driving force behind the failure. Exposure-age-dating of the landslide debris, using 10Be terrestrial cosmogenic nuclide dating, suggest that the Lochnagar landslide-dam was constructed over multiple slope failure events. The oldest date suggests an initial failure in the early Holocene which coincides with glacial retreat after the Last Glacial Maximum (LGM).
