Mansergh, G.D. 1978 Geological investigations for Hendes Creek bridge site. SH6 NZMS 1 sheet S71. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 310 12 p.
Abstract: A request was received for a geological assessment of rock slope stability at the Hendes Creek bridge site, and advise sought on the siting of investigation drill holes. The site is in a valley cut in hard hornfelsic rock which is covered in places by alluvium and colluvium. Joints and shear planes, the latter forming shear zones, were recognised cutting the hornfels. The orientation of these rock defects were recorded and stereographically analysed. This analysis showed that wedge type failure is possible on both the joint planes and the shear planes. It is suggested that failure is more probable along shear planes than joints. It is recommended that to increase the stability of the batters on the site the dip of the batter angle is made no greater than 60 degrees in jointed rock and 43 degrees in shear zones. The geological mapping suggests that the relief of the bedrock surface along the bridge alignment is not great and hence there are no great problems in siting the abutments and piers on steep faces, but there is a possibility that one of the zones of shear planes may pass under the left abutment and pier C. It is therefore recommended that an accurate survey be carried out to determine the location of the shear zones, and if they do cross under the abutment site, the rock in the proposed footing be laid bare for inspection and reassessment. If the shear zone passes clear of the abutment pier, two 10m drill holes are recommended to determine the bedrock profile. (auth)