A reappraisal of the Cromwell slide in relation to reservoir and roading proposals

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Salt, G.A.; Thomson, R. 1979 A reappraisal of the Cromwell slide in relation to reservoir and roading proposals. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 338 42 p.

Abstract: The Cromwell slide is a marginally unstable mass movement deposit that mantles the steep slopes east of Cromwell. Its toe lies beneath the surface of the Clutha River, which bounds it on the west. Initial failures may have caused by gravitional collapse of over-steepened schist slopes formed during an early degradation period of the river as it flowed along the eastern margin of the valley; multiple rotational shear surfaces and associated moderate to intense fracturing were probably generated in the rock at this time. Physical weathering produced a surface zone of chaotic debris and slope movements have occurred intermittently to the present day. Although proposed roading associated with hydro development will modify the existing slope, the main concerns are the effect of the proposed reservoir (Lake Dunstan) on the stability of the slide and the consequent effect on Cromwell Township of sudden collapse of part of the slide into the lake. A stability analysis of the active segment of the slide has been made, using a model derived from both surface mapping and subsurface investigations; this analysis indicates that there will be a reduction in the factor of safety of the slide after saturation by the proposed RL 195 m reservoir. Rapid drawdown will further reduce the slide stability. The stability can be enhanced, however, by buttressing the toe, reducing the angle of the lower slopes, removing material from the slide head, or by a combination of these techniques. Removal of slide debris from the above the proposed route of the relocated highway should similarly improve its stability in sections of cut, but roading should be considred as part of the overall slide assessment and not as a separate issue.