Riddolls, B.W. 1971 Geology of Mangapapa hydroelectric scheme: Progress report no. 2. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 111 7 p.
Abstract: In Tunnel 5, difficulty in operating the tunnelling machine was experienced after 1500 feet because of expansion of the bored rocks. This is probably due to the high groundwater pressure and the permeability of the soft ignimbrite. A rock fall at 1750 feet halted tunneling operations, with large blocks dropping from the roof along joints. Provided groundwater can be drained from around the tunnel, it may be possible to continue tunneling along the present line. Realignment from the present heading would reduce the likelihood of further rock falls along joints, but such a change would require further exploratory work to determine the rock condition more precisely. Because of the occurrence of ignimbrite that is too hard for machine tunnelling, Tunnel 3 is now being excavated by conventional methods. (auth)