Abstract:
The stability of natural rockslide dams with respect to sudden breaching is a major safety issue in mountain areas, although unbreached rockslide-dammed lakes such as Lochnagar in Central Otago, New Zealand, may persist in the landscape for millennia. Consequently it is important to attempt to understand the mode and drivers of these failures as they may impact on our expanding populations. The Lochnagar landslide-dam is located within the steep schistose mountains of the Southern Alps of New Zealand. During an extensive study of the site, structural and Schmidt hammer measurements were taken from the immediate area of the failure, as well as samples collected for laboratory analysis. Point load estimates of the uniaxial compressive strength were of 85 MPa perpendicular and 18 MPa parallel to the schistosity. The rock mass quality was estimated using the Geological Strength Index (GSI) with values of 35-45 observed. These results were used in a series of numerical modelling techniques: kinematic analysis (DIPS), limit equilibrium (Swedge) and distinct element modelling (3DEC). The results of the numerical modelling suggest that a wedge failure with toe buckling or ploughing through its rockmass is the likely failure mechanism. An observed fault zone at the base of the landslide may have preconditioned the slope to failure by weakening the toe. Preliminary ages from a program of Terrestrial Cosmogenic Nuclide (TCN) 10Be exposure age dating, indicate that the slide mass that formed the dam is of at least early Holocene age and therefore glacial retreat after the LGM could also be a contributing factor.
