Geology of test drives for Terrace tunnels

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Perrin, N.D. 1971 Geology of test drives for Terrace tunnels. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 108/A 23 p. + 2 maps

Abstract: This report is a summary of the geology and engineering geological problems encountered in the test drives for the proposed motorway tunnels between Shell Gully and Ghuznee Street, Wellington. The zone of an old major fault was encountered, consisting of fairly loos clay-bonded rock fragments which will require close supporting during construction of the proposed tunnels. Movement on this fault is considered unlikely within the life of the tunnels. Besides the fault zone, much of the material exposed in the test drives is of poor quality, except for the zone of hard rock at the northern end. Overbreak should be slight in most of the rock, but more overbreak can be expected in the more coarsely jointed, harder rock. Ground water could cause washing-out of the loose fault breccia or the soft clayey material, and failures may occur on the many, plastic fault gouges unless close supports are used. The northern, hard zone requires less support, and rock bolts may be able to be used to advantage. The results of the instrumentation and testing can be applied with confidence to only about 50% of the proposed tunnel. (auth)