Deep-Seated Gravitational Slope Deformation in Greywacke Rocks of the Tararua Range, North Island, New Zealand

Abstract

The Tararua Range is located in the southern part of the North Island, New Zealand. It contains a large number of ridges with geomorphic features typically associated with deep-seated gravitational slope deformation (DSGSD). The bedrock in the study area consists of alternating sandstone and mudstone beds that are highly folded and contain a large number of faults. The key components of this study included: the compilation of an inventory of DSGSD features within the Tararua Range and the characterisation of the rock mass and geomorphology of selected field sites. The results of the fieldwork along with the data from laboratory index testing on collected rock samples where used to conduct kinematic and finite element analyses. These analyses assessed potential factors influencing the stability and deformability of slopes within the study area. Over thirty DSGSD features were recognised in the Tararua Range (~440 km²) with a number of different surface geomorphic expressions. Field investigation was conducted at selected sites with very well developed geomorphic expression (10–20 m high scarps). The rock mass quality at these sites was estimated using the Geologic Strength Index (GSI). The overall rock mass was very blocky to blocky/disturbed/seamy with good to fair surface conditions, yielding GSI values between 30 and 55. Uniaxial Compressive Strength (UCS) estimates of the sandstone using the point load test yielded values between 90 and 150 MPa. Both the sandstone and mudstone lithologies were found to have a high resistance to slaking. Kinematic analyses indicated that while structurally controlled failure mechanisms were feasible along a limited number of discontinuities, they could not satisfactorily explain the formation of the geomorphic features observed. Finite element numerical models were generated incorporating a range of rock mass strength values based on field and laboratory observations, along with site-specific discontinuity set orientations. In all modelled cases, the calculated critical strength reduction factor was >1, suggesting that catastrophic failure is unlikely without external triggering forces. The results of the numerical modelling also suggest that a combination rock mass strength and favourable localised discontinuity set orientation is controlling the deformation of the rock mass that lead to the DSGSD features.