Ohau B power project : additional lining material investigations

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Macfarlane, D.F. 1979 Ohau B power project : additional lining material investigations. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 334 39 p.

Abstract: This report describes the results of an investigation into the gully immediately west of Tomahawk Gully as a potential source of addtional lining material. There is a slight possibility that the existing Tomahawk Gully pit as developed will not yield sufficient material to complete the lining of the Ohau C canal. Either the north Cairn Gully or west Tomahawk Gully fans will be developed on a small scale to make up quantities, if required. The north Cairn Gully fan is not considered in this report. The investigation proved geological conditions very similar to those in Tomahawk Gully. Two distinct fan deposits are recognisable and material properties are comparable to those previously reported for Tomahawk Gully (Convery, 1976; Read, 1976), although the materials encountered in this investigation are finer overall and characterised by non plastic fines. Despite these differences, FAD from west Tomahawk Gully is compatible with both AX and CB gravels in terms of criteria given in Specification HD 1195. In addition, it shows a consistently lower permeability than reported during the Tomahawk Gully investigations. Results from this investigation closely parallel those obtained during test pad investigation of the Tomahawk Gully FAD (Haugh, 1978). The mean long term grading and permeability characteristics of the FAD utilised in lining the Ohau B canal are also very similar to those determined during the investigation. On this basis it is suggested that the west Tomahawk Gully FAD will be a suitable lining or dam core material. A re-assessment of previously obtained subsurface data in the area of the proposed extension of the Tomamhawk Gully borrow pit indicates that the available volume of core / lining will be reduced by thick silt layers and interfingering outwash gravel. As this may necessitate minor development of an alternative source, a potential borrow pit location on the western fan is recommended. Sited to minimise intersection with known silt beds, this pit is capable of yielding up to an estimated 255,000m3 of core / lining material, mostly young FAD, if borrowed to RL 440 with 2:1 batters and assuming an average 2m of surface stripping / waste material. This is considerably in excess of anticipated requirements.