Anticipated tunnelling conditions in the Rangipo tailrace tunnel

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Beetham, R.D. 1979 Anticipated tunnelling conditions in the Rangipo tailrace tunnel. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 327 20 p.

Abstract: Excavation in the 3km long Rangipo tailrace tunnel has been slowed due to the poor rock condition and moderate groundwater pressures being encountered. It has become apparent that with present rates of progress the target date set for completion for this part of the project can not be met. The geology of the tailrace tunnel area is described and engineering geological aspects of the Kaimanawa Greywacke Group rocks are discussed. With the benefit of additional information provided by the excavation of the tailrace tunnel and other excavations, it is concluded that quickest completion of the tunnel could be achieved by shifting the alignment eastwards, to take a shorter path through the broad zone of faulted rock and to take advantage of better rock indicated to be on the east side. A further seismic profile would be helpful in providing the northward extent of this zone of good rock.