Mt Hector water utilization and conservation scheme: interim report on reconnaissance engineering geological investigations

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Hancox, G.T. 1977 Mt Hector water utilization and conservation scheme: interim report on reconnaissance engineering geological investigations. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 282 17 p.

Abstract: The Hutt Valley Power and Gas Board's proposed ''Mt Hector Water Utilisation and Conservation Scheme'' is a large diverse project spanning a large part of the southern Tararua Range. The scheme involves the construction of about six dams varying from 40-120 m high, and the excavation of four major tunnels with a total length of more the 30 km. Also planned are a surge chamber, penstocks, powerhouse and a number of other structures. The rocks in the project area are monotonous, interbedded greywacke sandstone, argillite, and breccias that are characteristic of the Wellington region. Almost all of the rocks are closely jointed of fractured, and there are extensive belts of sheared and crushed rocks. The geological structure, topography and to a lesser extent slope stability are essentially controlled by a number of major faults that run NE-SW through the range. The engineering geology of the proposed dam sites, reservoir areas, and tunnel lines has been briefly assessed by means of detailed office studies of literature, aerial photographs and topographic maps, with field investigations in a number of key areas in the Tararuas. Most of the dam sites appear to be geologically sound, some appear to be less suitable. Detailed assessments involving exploratory drilling, seismic surveys, abutment stripping, trenching and geological mapping at each site would eventually be necessary to prove the suitability of the sites and proposed reservoir areas if the scheme is to proceed. Slope stability could be a major problem in some of the reservoir areas. Small landslides, slumps, and scree slides into the reservoirs could be expected to occur. In some reservoirs this could result in rapid gravel and debris accumulation, leading eventually to a reduction in water storage capacity. These aspects should be carefully considered when selecting the damsites. The tunnels that are proposed intersect most of the major faults in the area. The Waiohine - Waiotauru tunnel is favourably orientated, being sub-normal to the major fault zones. The other tunnels, located within a wide zone of faulting, are aligned sub-parallel to the main Otaki faults, crossing them obliquely in several places. In these tunnels a high percentage of the tunneling is likely to be in soft, wet, weak shattered and crushed rocks possibly with high groundwater inflows. Close supports could be required and bench and heading methods of excavation could be necessary for considerable distances. The total cost of these tunnels would doubtlessly be reflected by the tunneling problems that are encountered. (auth)