Engineering geology assessment of slope instability on forest lands in South Westland (1987)
AuthorsEggers, Mark Jshow all
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 basal shear in grey mud material derived from faulting in lower Otumotu Formation bedrock. Surface movement monitoring investigations measured a high rate of movement of up to 419cm/year. Unloading of toe support in December 1984 caused an instantaneous acceleration in movement rates which subsequently declined during the monitoring period of March 1985 to June 1986. This deceleration process obscured any climatic influences on movement. Slope failure potential in South Westland slopes is governed by the fundamental causes of slope instability: the distribution of weak fault zone materials and crushed bedrock, and the steep topography (20-65°). Aerial photograph evidence suggests that slope instability features develop very rapidly (within one rainstorm event) and active instability periods occur in short-lived episodes (less than 50 years) which are coincident with the occurrence of heavy precipitation periods. Recommendations for future management of South Westland forest lands are:- 1. Alpine Fault Zone slopes should be limited to protection forestry management practices. 2. Greenland Group Hill Country and Cretaceous-Tertiary Hill Country should be assessed by more detailed phases of evaluation for small-scale bush-mill harvesting methods only. 3. Engineering geology methods of investigation and data presentation have been demonstrated to make a valuable contribution to forest resource allocation studies and it is recommended that these methods be applied to all levels of evaluation of slope movement potential on forest lands.