Controls on slope failures at the Te Toto Amphitheatre, Raglan, New Zealand

Abstract

The Te Toto Amphitheatre, on the north western slope of Mount Karioi, Raglan, New Zealand presents the geomorphic expression of a large (estimated 25-30Mm3) ancient slope failure. The amphitheatre has a low angle floor, with near vertical back and side walls. Modern rockfall processes are observed as scree slopes near the side walls. Using discontinuity orientation measurements (N=301), a kinematic analysis suggests that toppling failure is a feasible failure mechanism for the rockfall. The Geologic Strength Index (GSI) is used to estimate the rock mass quality of threegeotechnical units identified during field mapping. These include a visually extensive Flow unit (GSI 60-65), scattered pockets of a Pyroclastics units up to 6m in thickness (GSI 40-45) and a 2m thick Volcanic sediment (GSI 40-45). Field estimates, Schmidt Hammer and point load tests suggested that the units have unconfined compressive strengths (UCS) of 60-200MPa (Flow), 20-40MPa (Pyroclastics) and 7-20MPa (Volcanic sediment). Limit equilibrium modelling was performed to determine the mechanism and contributing factors for failure of the rock masses present. The Southern wall of the amphitheatre was used as a slope profile (average slope angle of 20°, steep co astal front of 50°). Modelled rock mass properties were determined using the Hoek-Brown criterion. A low angle anisotropy was introduced to represent the structural element of bedding. FOS < 1 values were achieved using lower bound material strength values and high porewater pressures (represented using Ru=0.4)