Engineering geological investigations of the Maniototo combined irrigation and power scheme

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Paterson, B.R. 1979 Engineering geological investigations of the Maniototo combined irrigation and power scheme. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 322 100 p. 1 map.

Abstract: The Maniototo combined irrigation and power scheme involves diversion of most of the Upper Taieri River through a canal, tunnel and penstock system which includes 2 powerstations with a generating capacity of 12.3 MW. Engineering geological investigations for the scheme were concentrated along the tunnel line and the adjacent sections of the canal where cuts of 20-30 metres are planned. Investigations consisted of surface geological mapping, diamond drilling (21 cored holes) and a series of analytical and geotechnical laboratory tests on core samples. Drilling proved that apart from a discontinuous, superficial layer of loess and terrestrial sediments, all the tunnelling and most of the canal excavation would take place in the basement rock, schist. The latter is a complexly-deformed, anisotropic rock consisting of relatively massive, moderately-hard, quartzo-feldspathic schist and foliated, less competent pelitic schist, both of which have different physical properties and a random distribution. The most important factors to be investigated were the stability of the canal and tunnel portal batters, as well as the likely behaviour of the rock mass and groundwater during tunnelling. These factors are strongly influenced by the main discontinuities in the schist which include joints, schistosity and shear zones. The orientation of these features were examined kinematically in relation to the proposed batters, and to allow a stability analysis to be carried out, the geotechnical properties of the discontinuities were determined by laboratory tests. These data were also used to predict the rock and groundwater conditions that could be encountered during tunnelling, and the effects these features could have on excavation and tunnel support.