Engineering geology of the Moawhango to Tongariro tunnel

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Hegan, B.D. 1980 Engineering geology of the Moawhango to Tongariro tunnel. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 343 47 p.

Abstract: The 19.2 km long Moawhango to Tongariro tunnel, a key part of the eastern diversions of the Tongariro Power Development, diverts water north from both the Rangitikei and Whangaehu catchments into the Tongariro river. The tunnel intake (N122/253540, invert level 836 m) is located on the northern side of a shallow basin, near the junction of the Mangaio stream and the Moawhango river. This basin is now the site of the Moawhango reservoir formed by water impounded by a 64 m (210 ft) high arch-gravity dam, constructed at the upstream end of a narrow gorge in the Moawhango river, about 1.6 km downstream from the Mangaio stream confluence. Maximum storage in the reservoir is 97 x 106 m3 with a normal top water level of RL 851 m. The reservoir has a draw-down capacity of about 15 m giving it a live storage of about 61.5 x 106 m3. A mean flow of 13.6 m3/sec (479 cusecs) will be diverted into the Tongariro river. The tunnel outfall (N112/261740) has an invert level of 816 m, and is sited immediately upstream from the Rangipo intake structure, about 1.3 km upstream from the Waihohonu stream confluence. With a fully concrete lined diameter of 3.3 m, the tunnel has a design capacity of 16.7 m3/sec (591 cusecs) and a maximum capacity of 20.9 m3/sec (740 cusecs). Excavation commenced in late 1969 at the Tongariro end, and in early 1970 at the intake using conventional drill and blast methods. Except for the 800 ft of tunnel nearest the intake, the tunnel lies wholly within rocks of the Kaimanawa Greywacke and Argillite of Permo-Triassic age. These rocks consist of indurated, complexly deformed sedimentary rocks which include sandstone, argillite, and interbedded mixed autoclastic breccias. All are closely jointed and contain numerous crush, shear and shatter zones formed by fault movements, the wider zones being usually parallel to the north-easterly trending bedding. Progress was hampered in many places by poor rock conditions and high groundwater inflows, especially beneath Paradise Valley. The tunnel was holed-through in mid 1976 and commissioned during December 1979. The only significant post-excavation problem has been invert failures in an area of high ground cover where groundwater pressures have also been high. There was probably no single reason for the failures but the groundwater pressure the most likely driving force. (auth)