Engineering geology assessment of slope instability on forest lands in South Westland

• Main Author: Eggers, Mark J
• Format: Other/Unknown Material
• Language: English
• Published: University of Canterbury. Engineering Geology 1987
• Subjects: Greenland
• Online Access: http://hdl.handle.net/10092/9580; https://doi.org/10.26021/2109

Assessment of slope instability on forest lands in South Westland was limited to the resource allocation level of evaluating slope movements on forest lands with the prime objective of providing an overview of slope movement potential adequate for forestry development planning. Three sites were selected for detailed investigation on the three most unstable landform units which were identified by previous studies, viz:- 1. Greenland Group Hill Country: Boulder Creek; 2. Alpine Fault Zone slopes: Havelock Creek, and; 3. Cretaceous-Tertiary Hill Country: Grave Creek. Investigations were divided into three stages, engineering geology field and laboratory studies, assessment of slope movement processes and instability controls, and implications of slope instability for forest management. The dominant type of slope failure at Boulder Creek is debris slump/slide-avalanche in crushed hornfelsed sandstone and puggy tectonic breccia bedrock materials with failures typically initiated during high intensity rainstorm events and seismic events. The Boulder creek catchment is presently undergoing a period of increased slope activity which is generating a substantial quantity of sediment and is overloading the stream channel with rock debris. Boulder Creek provides an exceptional example of slope instability problems in Greenland Group Hill Country, this being explained by the oversteepened sides of the glaciated Moeraki River valley in fault-crushed bedrock. Alpine Fault Zone slopes fail most commonly by debris slide-avalanche in crushed mylonite schist and crushed garnet schist bedrock triggered by high intensity rainstorm events. Investigations at Havelock Creek also identified large-scale rock (block) slide failures in the same crushed bedrock materials which are initiated by infrequent seismic events. The Grave Earthflow, located in Cretaceous-Tertiary Hill Country, was triggered by construction of State Highway 6 in 1963-65. Failure is taking place by a complex rotational slide-earthflow type of movement along a zone of ...