Lower Clutha power investigations : engineering geological feasibility assessment of the proposed Beaumont damsite

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Macfarlane, D.F. 1986 Lower Clutha power investigations : engineering geological feasibility assessment of the proposed Beaumont damsite. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 405 57 p.

Abstract: Engineering geological investigations have been undertaken to assess the feasibility of constructing a dam/powerhouse complex at the downstream end of the Beaumont Gorge. The investigations indicate that the site will be suitable for a concrete gravity dam but that there are potential problems with: 1. foliation shear-controlled instability in the left bank area which will limit slope angles in unsupported cuts; 2. zones of relaxed and/or sheared rock beneath the river which will require extensive over-excavation and/or remedial treatment (consolidation grouting) to ensure foundation integrity; 3. an existing zone of oversteepened foliation dips and relaxed rock in the lower part of the left abutment, possibly indicative of incipient gravitational displacement. Each of these aspects will require detailed assessment at the design nvestigation stage. Downstream sliding of possible shear-controlled wedges in the foundation rock is considered improbable but will require further investigation. There is some potential for instability of existing landslides along the reservoir and further stability analyses are recommended for these, in particular under earthquake conditions. The proposed damsite is within the northwest-trending Tuapeka Fault Zone and the north-trending Beaumont Fault crosses the river immediately downstream from the site. No Late Quaternary fault traces are known within 13 km of the proposed damsite and the seismotectonic hazard is assessed as low. However MMVII intensity shaking with peak ground accelerations of up to 0.3 g vertical and 0.2 g horizontal should be anticipated within the design life of the structure. Construction materials are readily available within a few km of the site area but gravel quality is variable and borrow areas will need to be selected with care. The best concrete aggregates will be obtained from the gravels within the existing river channel. (auth)