Engineering Geological Investigation of Te Kopia Geothermal Area, New Zealand

Abstract

Slope instability is common in many geothermally active environments. This study has aimed to identify the causes and nature of slope failure in the hydrothermally-altered rocks along the Paeroa Fault scarp in the Te Kopia Geothermal Field, Taupo Volcanic Zone, New Zealand. The project involved a series of field and laboratory-based approaches with the aim of investigating the geotechnical properties of the various exposed rock facies, and modelling slope behaviour to develop a factor of safety (FoS) for the site. The geology of the Te Kopia area is dominated by variably weathered and altered Pleistocene ignimbrites. Various rock types are found in the region; these range from intact massive ignimbrite to ‘rotten’ barren ground heavily altered by acid steaming fumaroles, acid springs and meteoric waters. Six study sites were chosen and represent a range of facies types: silicified ignimbrite (SI), indurated ignimbrite (II), acidic altered rotten ignimbrite (AARI), altered host rock (AHR), remnant sinters (RS) and weathered ignimbrite (WI). Combining laboratory-based data (porosity, point load strength, slake durability, rebound hammer or unified compressive strength, UCS) and field-based data (to classify rock strength) revealed that WI, AARI and SI are more likely to fail than SI, II and AHR. Historical images of the study area from 1964 to 2013 were compared to examine possible changes in geomorphology due to slope movement. This comparison did not show significant slope displacement, although surface geomorphological changes had occurred, likely due to alteration, weathering and erosion. Results from an unmanned aerial vehicle (UAV) survey were processed to provide a photomosaic of the study area, from which a digital terrain model (DTM) was constructed to image long-sections of the topography as an input into slope stability modelling. Kinematic analysis of likely failure mechanisms was undertaken using Rocscience Dips software at three selected sites, which showed a higher probability of wedge failure than planar or toppling failure. Limit Equilibrium Modelling using engineering software (Slide) revealed the slope is at or close to failure (i.e. Factor of Safety, FoS ≤1.0), irrespective of either (1) the modelling scenario chosen (e.g. static/dry conditions; low groundwater; high groundwater), or (2) the limit equilibrium method that was applied to the calculations. This study has led to an improved understanding of the variability in the rock mass condition at Te Kopia, as well as an appreciation of the susceptibility to failure of the slopes in the area. In a broader sense, this may have implications for understanding material properties and failure mechanisms that prevail in other geothermal areas.