Geological report on Tekapo B powerhouse site, covering investigations to mid-March 1971

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Mansergh, G.D. 1971 Geological report on Tekapo B powerhouse site, covering investigations to mid-March 1971. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 163 24 p.

Abstract: The proposed Tekapo B powerhouse site is to be located on glacial and fluvioglacial deposits on the eastern shore of Lake Pukaki. From a geological viewpoint the site can be divided into two zones, the boundary approximating to the 1770 ft contour. West of this contour the ground surface falls, with minor undulations, to lake level (1620 ft) 2500 ft away, while to the east the land rises more abruptly to 2200 ft 3500 ft away and the surface is both hummocky and stepped. To the west alluvium deposits form a unit with an average thickness of about 60 ft and comprise moderately well sorted gravel, sandy gravel and silty gravel. The alluvium appears to lie on lacustrine beds of unknown thickness, and these beds are dominantly sands, silty sands and sands with gravel. East of the 1770 ft contour, till, ice contact and alluvial deposits interdigitate and the three probably lie on the extension of the lacustrine bads. The till and ice contact are a poorly sorted assemblage of gravel and silt sand and the alluvium moderately sorted gravel and sand, but with more sand than the western alluvium. There is insufficient data available to give details of the lacustrine beds in this zone. Subsurface water was encountered in the base of the alluvial unit and regularly in the lacustrine unit where it appears that positive pore pressures can develope. Problems may be encountered in excavation due to the lack of cohesion in the surface alluvial gravel, , and from the presence of water under pressure in the lacustrine beds. Further investigations of the groundwater conditions and stratigraphy are requested. (auth)