Lower Clutha power investigations : stability assessment of the Beaumont Gorge landslides

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Johnson, J.D. 1986 Lower Clutha power investigations : stability assessment of the Beaumont Gorge landslides. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 403 46 p.

Abstract: Two deep seated, complex dip slope mass movement areas in textural zone III schist have been identified on the left bank of the Beaumont Gorge. All hydroelectric development options under consideration on the lower Clutha River would result in the formation of a RL 69m reservoir through the gorge inundating the toes of both landslides. Using geological and groundwater information, and field strength parameters derived for the various slide materials, the existing stability ot the slides has been assessed, and the effects of reservoir formation on the stability of the two landslides considered. The stability of both the Beaumont Downstream and Beaumont Upstream Slides is found to be dependent on artesian pressures confined beneath the base of the slide masses. The Beaumont Upstream Slide is considered to be creeping at rates in the order of a few millimetres per year and is regarded as marginally stable. There is some doubt as to whether the Beaumont Downstream Slide is moving. It is concluded that reservoir formation across the toe ot the Beaumont Downstream Slide will result in a 7% reduction in factor of safety. If the slide is presently creeping, this reduction in factor of safety is likely to result in an increased creep rate, possibly to as much as several tens of millimetres per day. If the slide is stationary under present conditions, such a reduction in factor of safety may not have a significant effect on the slide mass. A 13% reduction in factor of safety is expected for the Beaumont Upstream Slide following reservoir filling. This could result in accelerated slope movements and rapid slope failure is considered possible. Although it is unlikely that the proposed damsites, or nearby roads and dwellings, would be endangered by a rapid failure of the slide due to the narrow sinuous shape of the reservoir through the Beaumont Gorge, there is potential for reservoir blockage from a major slope failure of this slide. It is considered that remedial works (involving drainage and buttressing) could be undertaken to eliminate the risk of rapid movement, although more work will be required to allow appropriate measures to be designed. Surficial failures are anticipated on the steeper slide debris slopes in the toe slopes of both landslides and on other steep slopes of similar material affected by the reservoir through the Beaumont Gorge. These surficial failures, although they are likely to be relatively rapid, would not present a significant hazard to the safe filling and operation of the reservoir. Further investigations are recommended to prove the confined aquifer beneath the Beaumont Downstream Slide, and to monitor the rates and direction of slope movements for both the slide areas. Reassessment of the slide stability under earthquake conditions, and damage assessment to see what effect failure of either slide would have on the reservoir and surrounding countryside, are also reccomended. (auth)