Geology of Moawhango tunnel - Tongariro heading buried channel crossing T.F. 13 670-13 90

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Hancox, G.T.; Grammer, T.R. 1973 Geology of Moawhango tunnel - Tongariro heading buried channel crossing T.F. 13 670-13 90. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 155 13 p.

Abstract: During investigations for the crossing under the Waipakihi River, the possible existence of an old buried channel, much deeper then the present Waipakihi Valley, was suggested by Dr Muller - Eng. Consultant to Codelfa-Cogefar, the tunnel Contractor. In order to check the possibilities of such a deep buried channel, two drill holes (W8 and W9) were drilled, about 1000' west of the tunnel line, and the presumed junction of the Waipakihi River with the buried channel. The results of these drill holes apparently confirmed the presence of gravel filled channel with a base level about 100 ft lower than the present Waipakihi and that rock level on tunnel line could be below invert level (2,700 ft). As an additional check, Geophysics Division, D.S.I.R. carried out a seismic refraction survey in this area (Ingham 1972). This showed that although there was a trough like depression in the greywacke bed rock, (the lowest point was at T.F. 13,500 ft) it appeared to be no deeper than the present valley. To correlate these findings a hole (W10) was drilled at the deepest part of the trough (T.F. 13,500). Greywacke bed rock was encountered at RL 2,821 ft which was in close agreement with the level indicated by the seismic survey. A full report of geological investigations up to this stage was found in Hegan, 1972. In a report on the Geology of the Moawhango-Tongariro Tunnel line (Hegan 1971), a prominent lineament was mapped from aerial photographs suggesting that either a fault, shear, crush or shatter zone intersected the tunnel line at approximately T.F. 13,400 (+/- 200 ft). Although the rock condition in the tunnel from 13,400' to 13,600' had been excellent (apart from 20 ft of poor-fair rock at 13,200') cautious tunneling procedures were adopted with top bench excavation and exploratory drill holes to T.F. 13,578'. Full face excavation with exploratory drill holes continued from 13,578'. (auth)