Geology of a proposed cut slope, Kaiwharawhara Interchange, Wellington

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Riddolls, B.W. 1973 Geology of a proposed cut slope, Kaiwharawhara Interchange, Wellington. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 165 18 p.

Abstract: An engineering geological investigation has ben carried out to determine the rock condition of natural slopes that are to be cut to a steeper angle for the construction of the Kaiwharawhara Interchange of the Wellington Urban Motorway. Following geological mapping of the area, two continuously cored drill holes were sunk to obtain a better understanding of subsurface conditions in the vicinity of the proposed cutting. Much of the ground is formed by a weathered succession of sediments dominated by gravel, overlying closely jointed sandstone and siltstone bedrock has been weakened locally by the effects of weathering. The profile of the sedimentary contact between gravel and bedrock is not certain. The proposed cutting will intersect a steeply dipping contact between the two lithologies that appears to be a fault. The western part of the cutting will be formed by moderately weathered to slightly weathered closely jointed bedrock, while the eastern part will consist of moderately weathered to highly weathered bedrock overlain by gravel. The present investigation has not defined the geology of the site in sufficient detail for a slope stability analysis. However the available data enable assessment of some factors that are relevant to the stability of the cut slope. Slopes cut in bedrock in the immediate vicinity show that bedding could dip directly out of the slope at angles as low as 35 degrees, but the strength along this feature may not be low enough to be a major hazard to stability. Narrow clayey fault zones at several different orientations have been observed, some of which could induce instability. Because they dip at more than 50 degrees, an overall cut-slope angle that does not exceed this figure is desirable. Judged from exposures elsewhere in the Wellington area, a 50 degree slope will provide a good margin of stability in the gravel. Construction of the slope in a series of benches will assist stability by providing adequate drainage, and will collect loose debris that otherwise might fall on the road. Excavation should not be difficult, but the bedrock that will form the western part of the cutting may require ripping. (auth)