Ohau A power project, Ohau canal : slope debris at Ohau terrace

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Macfarlane, D.F. 1977 Ohau A power project, Ohau canal : slope debris at Ohau terrace. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 284 41 p.

Abstract: Four slope debris units are along the base of the Ohau Terrace. Two of these (fan alluvium and slopewash 1) are mixed silts and gravels derived from the Mount John Till forming Manaka Ridge and the Basal Mount John Till (a homogeneous silty sandy gravel up to 50 metres thick resulting from the thrusting of the till into outwash gravel). Both have been largely removed during stripping operations. Underlying both the above slope debris units, and exposed in its own right at the eastern end of the terrace, is the gravel deprives slopewash 2 unit charac terised by segregation. The numerouse open, loose gravel layers of this unit, dipping up to 30 degrees South, result in a poor foundation material which should be removed from the foundations. An exception to this requirement is under the left berm of the fill section along the base of the terrace provided that (1) the slopewash 2 gravel in the foundation are compacted by at least six passes of a 10 tonne vibrating roller and are protected by a surface layer of preferably not less than 50cm of well graded, compacted material, and (2) the fil embarkment is abutted against the vertical face defining the edge of the stripped zone. The shape of the cut to fill interface at foundation level defined in Specification HD 1065 has been used to define the critical point for excavation of this plane necessary to remove the slopewash 2 gravels from the foundation zone in this area. The fourth slope debris unit is a colluvial soil layer up to 1.5 metres thick which has been left unsupported by stripping. This amterial is potentiallu unstable under saturated conditions unless support is reintroduced by abutting the fill embarkment against it. Ground water levels under the canal co-incide with present river level. Some problems with water inflow have been encountered during stripping below the interface plane where old river channel deposits are being removed. (auth)