The Coronet Peak Landslide (CPL) is located on the western edge of the Arrowtown Basin in Central Otago, in the lower South Island, New Zealand. Urban growth in key towns within Central Otago, like Queenstown is expanding, with the use of flat glacial-alluvial terraces being used up. The expansion has now reached areas characterised by new developments being located on steeper schist slopes, with increased geotechnical issues. The slopes within the Queenstown area are susceptible to instability due to a number of factors, most importantly lithology, foliation attitude, and rock/soil mass weakening linked to deglaciation. There are numerous documented large landslides in the region, and pressures for urban expansion have forced developments onto the lower slopes of these large landslides, such as the CPL. Understanding the associated hazard and risk of developing these large landslides is critical, as urban growth in the region is expected to continue for the foreseeable future.

The primary objective of this thesis is to characterize the geomorphology of the CPL, specifically focusing on Arthurs Point. An additional objective is to characterize the slope debris of the CPL at Arthurs Point, referred to as chaotic schist debris. This research investigates the geomorphology and geotechnical properties of the landslide debris through geomorphic mapping using high-resolution imagery from lidar (2016 & 2021), recent aerial imagery (2021), and historical imagery (1956), alongside geotechnical subsurface investigations. This study uses eight 15 m deep boreholes, five 3 m deep test pits, and 12 piezometers, that provide the opportunity to characterise the slope material in detail. This research provides a detailed understanding of the slope material within the lower slope of the CPL at Arthurs Point, as previous investigations did not have access to high-resolution imagery.

The CPL is a deep-seated gravitational slope deformation (DSGSD) characterized by multiple geomorphic zones with distinct features that result in differential instability of the slope. Although these large geomorphic features are considered to be relatively stable, under specific conditions, such as climatic changes, fluvial-glacial processes and human activity, secondary landslides associated with DSGSD can transform into catastrophic failure. There have been no recent documented large-scale failures within the CPL, but it is evident from the geomorphology that secondary landslide events have occurred on the toe in the past.

Based on distinct surface morphologies across the CPL at Arthurs Point revealed by geomorphic mapping, the landslide is divided into four geomorphic zones: (A) the headscarp zone, characterised by distinct sackung features; (B) the main landslide body comprised of a hummocky topography; (C) the Shotover River reactivation, interpreted to be the most recent reactivation; and (D) the eastern reactivation with complex drainage and undulating, hummocky topography. No significant changes were identified between 1956 and 2021, however, comparison of the images over this time confirmed that zone C (the Shotover reactivation zone) is the most recently active zone due to current river scour and erosion of the toe of the zone. This study re-zones the previously mapped zone B of the CPL, and splits zone B1 into zone A (the headscarp zone) and zone B (the main landslide body zone).

While no significant movement was observed over the 65 years, different geomorphic features are identified with the different sets of data between the recent lidar (2016/2021) and the 1956 aerial image. Due to changes in land cover and human modification of the slope, geomorphic features such as scarps, drainage paths, and other linear features, interpreted to be soil creep features, are more easily identifiable in the 1956 aerial image as opposed to in the lidar imagery.

Geotechnical subsurface investigations of the chaotic schist debris at Arthurs Point show significant geotechnical variations of the material, with a large grading distribution of the material as well as a complex shallow groundwater system. Displaced competent schist blocks >10 m were identified in the field within a finer gravelly and silty micaceous matrix. Particle size distribution tests undertaken on one borehole (BH08) show the chaotic debris soil to be well and gap graded, with an average curvature coefficient, Cc, of 0.7 [ranging from 0.1–1.8] and an unconformity coefficient, Cu, of 27.6 [14–36]. Shallow perched groundwater is confined to the northwestern portion of Arthurs Point Woods development, with an average depth to groundwater of 2.32 m and an average pressure head of 9.7 m.

Altogether, this study suggests that the Arthurs Point Woods residential development is placed in an unfavourable location within the toe of CPL based on the surface morphology, within a chaotic debris material that is weak and has a complex perched shallow groundwater system. Highly fissile, steeply overturned, and deformed schist was identified within the lower portion of the development, within the toe zone of CPL at Arthurs Point, suggesting stress relief and deep-seated gravitational processes.

To ensure a comprehensive understanding of the subsurface conditions, additional geotechnical investigations should be carried out inside and beyond the boundaries of the development to fully understand the characteristics of the entire slope as well as the groundwater conditions. The combination of geomorphic mapping using current and historical imagery demonstrates that portions of DSGSDs can be identified and zones based on surface morphology and activity. However, ensuring the long-term stability for safe development on these slopes requires extensive site-specific geotechnical investigations due to their complexity.