The geology and geomorphology of the Coronet Peak and Arthurs Point landslide complexes
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Coronet Peak and Arthurs Point Landslides have both formed in schist bedrock on the northwestern side of the Wakatipu Basin. The rock masses at both sites show evidence for the presence of many defects, such as foliation, schistosity, foliation shears, sheared and crushed zones, faults and joints, all which have formed as a result of the deformation and metamorphism the schist has undergone. The Arthurs Point Landslide has a volume of 2.4 x 10⁷m³, and is a wedge failure, with sliding occurring along the intersection of two master joint sets rather than foliation, the latter dipping into the slope. It is a retrogressive failure, with the eastern part having failed as a consequence of undercutting of the slope during the Last (Otiran) Glaciation. Actual failure of the eastern segment occurred after retreat of the ice left the valley sides over steepened and unsupported, whilst failure of the western part was related to the rock above the initial failure being left unsupported and incision of the Shotover River. The Coronet Peak landslide is a translational planar slide with a volume of approximately 1 x 10⁹m³ that failed along foliation and/or foliation shears, and the slide can be divided into zones (Zone A and B), based on morphological evidence. Zone A is the larger zone (it has a volume of 6 x 10⁸m), and has well-developed hummocky topography, indicating continued but slow movement (probably less than 2-5mm/yr) as it has no form of toe support removal. Initial failure of the Coronet Peak landslide was triggered by ice undercutting and subsequent retreat of the Wakatipu Basin, most probably during the Waimean Glaciation (approx 135 000 years ago), as it was during this ice advance that the Wakatipu Glacier extended all the way into the Wakatipu Basin and partially up the Shotover and Arrow Valleys. Zone B can be divided into three subzones, which are all different reactivations. Zone B1 is a reactivated part of the Coronet Peak landslide and was triggered by the ice erosion related to the Otiran Glaciation. Evidence for this is seen on the valley floor at the toe of Coronet Peak landslide, with the Old Ben Lomond moraine situated adjacent to the end of the reactivated part. Most surficial glacial deposits in the Wakatipu Basin are related to the Otiran advance. Zone B2 is a reactivation within this, and was triggered by incision of the Shotover River. Zone B3 consists of two reactivations, both possibly related to flood events of the Shotover River. Back analysis of the Coronet Peak Landslide were performed, to ascertain whether the model used for the landslide is appropriate. The results showed that the model is a reasonable one. It is feasible to build on the Arthurs Point and Coronet Peak landslides, as long as it is not at the usual densities. With landslide debris, the foundation materials are not always compacted well, and therefore injudicious excavation could cause reactivations, like at the Coronet Plaza, where trimming a small amount of rock off the toe of the CPL to allow room for extensions caused some of the toe area to reactivate. The other important factor when building on landslides is surface and subsurface water, as more water in the slope than normal will increase the instability. Building at the usual densities would increase the area being excavated, and so the disruption to the foundation materials, and it would increase the amount of water to be controlled.