The Cromwell landslide

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McKellar, I.C. 1974 The Cromwell landslide. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 199 16 p.

Abstract: The rock slide on the east bank of the Clutha River, upstream froom Cromwell S.H.8 road bridge, will have its toe flooded with DG7 lake at 640 ft. It is a form of surface failure not suitable for accurate slope stability analysis; and it is not possible to predict safety factor from slide geometry and soil properties. Movement across one crack at the top of the slide, measured between 1965 and 1970 was very small, but there are local reports of earlier movements of several feet in one detached block. An investigation drive at the proposed water level shows a thin cover of slide debris on a stretched schist layer on undisturbed schist. Routine laboratory tests on selected soil samples is suggested, together with a measuring programme to detect small movements of blocks near the top of the slide, surface contouring, and possibly a second investigation drive at another site. Geophysical measurements may also be helpful. Beyond this, judgement and experience will have to be relied on for decisions on stabilization measures. Reduction in the masses of the slide near 750 ft level could improve stability and possibly be combined with realignment of Partridge Road at this higher level. (auth)