Geology of the DG7 damsite area. Progress report

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Thomson, R. 1975 Geology of the DG7 damsite area. Progress report. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 216 29 p.

Abstract: Two damsites are currently being investigated - the A site at Gibraltar Rock and the B site 1.5 kilometers further downstream. The B site is currently favoured for reasons of construction space and economics. Limited foundation drilling has been completed at the A site and drilling of the abundant areas is in progress at the site B. An inconclusive angle hole at the A site is the only attempt so far to investigate within the river channel. Geological mapping has indicated numerous faults within the site areas that principally trend along the valley and have high angles of dip. The most important of these forms an apparent broad zone at the base of the steep slopes on the NE side of the valley. Outcrop and drill hole data so far indicate considerable fracturing in the zone and excavation within it may encounter slope stability problems. Low-angle crush zones are being encountered in drill holes between the above zone and the river. Other faults do not appear to create appreciable construction and stability difficulties. Jointing is general within the schist basement but variable in intensity. Unweathered joints are generally tight and rock is competent. Overburden consists of terraces of gravelly to bouldery alluvium and various forms of mass movement deposits. The largest of the latter is located above and to the SW of the right abutment of the A site. This landslide appears to have and accurate basal slip surface and is being reactivated, at least in part, at the present time. Smaller landslides occur to the NE of the B site and are largely unaffected by proposed construction. Limited potential rockfall areas have been observed on the right bank of the river. (auth)