Riddolls, B.W.; Perrin, N.D. 1972 Engineering geological investigations for proposed motorway near Plimmerton. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 136 25 p.
Abstract: Deep cuttings are planned in the construction of the motorway in the vicinity of Gray's Road, and the rock condition has been investigated there from eight drillholes. The rock comprise greywacke and argillite, generally weak owing to the existence of several faults and the effects of prolonged weathering. Excavation to formation level will be achieved without difficulty. Although the physical properties essential for determining stability accurately have not been obtained, it is unlikely that the slopes in the weathered bedrock will have an adequate factor of safety if cut steeper than 50 degrees. The stability of cut slopes also depends on the orientation of faults, joints and bedding, along with planar failures are possible. The steep dips typical of this area minimise this possibility, but a number of trenches are necessary to confirm this, and to enhance the value of the field and drillhole data. The superficial clayey material overlying bedrock will probably be adequately stable only in slopes cut at low angles, especially where highly plastic clay occurs. (auth)