Kawarau River power investigations: engineering geological investigations at the proposed KW29 powerhouse site

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Macfarlane, D.F. 1985 Kawarau River power investigations: engineering geological investigations at the proposed KW29 powerhouse site. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 391 13 p.

Abstract: The proposed KW29 powerhouse site on the Kawarau River lies within (and close to the eastern margin of) the Nevis-Cardrona Fault System, and is located immediatey downstream of the potentially active Gentle Annie Fault. Site investigations (geological mapping, diamond drilling and seismic profiling) have shown that a 100 m wide band of greenschist strikes across the site area within grey quartzofeldspathic schist and is truncated against the gentle Annie Fault. The greenschist has high intact rock strength and will provide a strong, stable foundation for the powerhouse. A broad zone of incipiently to variably sheared greyschist strikes parallel to (and follows the downstream edge of) the greenschist band and would be a fair to poor foundation material. The basement schists at the site are overlain by up to 40 m of outwash gravel with degraded terrace surfaces at two levels. Site layout proposals would result in the penstock intake structure being founded on gravel overlying the sheared and crushed schists present within the zone of the Gentle Annie Fault, with the powerhouse founded within the zone of cariably sheared greyschist. In the event of a large earthquake located on the Nevis-Cardrona fault System potential seismotectonic hazards at the site could include ground rupture affecting the intake structure foundation or the penstock slope, which would be constructed across the fault margin. To minimise this hazard and to take full advantage of the foundation qualities of the greenschist, it is recommended that the power-house be slightly relocated and re-oriented.