Engineering geological investigations of the upper Otira bridge site

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Paterson, B.R. 1978 Engineering geological investigations of the upper Otira bridge site. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 302 17 p.

Abstract: The Upper Otira bridge on SH73 between Arthurs Pass and Otira is due for replacement, and the Ministry of Works and Development have proposed realignment of the southern approach to remove part of a rock spur which create a blind approach to the bridge. A geological survey of the area has shown that the new bridge will be founded on rock on the right bank and alluvium on the left bank – subsurface investigation will be needed to prove the depth to bedrock on the left bank. Two alternative alignments of the southern approach have been proposed which involved excavation of a steep rock spur consisting of hard, joined and unweathered greywacke sandstone. Stability of the rock slopes depends mainly on the strength of the rock defects or natural planes of weakness which divide the rock into segments. A kinematic analysis has been carried out to predict the potential stability of these segments in relation to proposed cut slopes, and a number of potentially unstable rock wedges have been identified. By comparison with the existing steep slopes in the area these wedges should not fail provided the strong natural cohesive forces along the defects are not seriously affected during the excavation. It is recommended that the alignment be chosen which reduces the amount of rock to be removed from the spur at the southern end of the bridge. This will reduce the risk to a power pylon sited on the spur, and allow a stable slope to be formed. Batter slopes of less than 80 degrees (6 on 1) are recommended and precautions should be taken to ensure minimum disturbance of the adjacent rock during excavation. (auth)