Engineering geology of a cut slope in Shell Gully, Wellington urban motorway

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Riddolls, B.W. 1973 Engineering geology of a cut slope in Shell Gully, Wellington urban motorway. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 162 11 p.

Abstract: A cut slope with an overall inclination of 41 degrees and height of 30 m was formed recently in Shell Gully as part of the construction of the Wellington Urban Motorway. Detailed observations of the geology have shown that interbedded sandstone and argillite dip steeply into the slope. The rocks are closely jointed and faulted and have been variously affected by weathering. Petrographic studies of the sandstone show that the weathering breakdown occurs principally through alteration of the matrix, and that the destruction of larger particles occurs only in the most advanced stages of weathering. Clay mineral analyses suggests that illite is more abundant with greater intensity of weathering, but hallosite is present locally in large amounts in completely weathered rocks that occur close to the surface. Microfacturing of mineral grains may be related to expansion of clay minerals in fractures and along intergrain boundaries, but further work is necessary to establish the origin of this feature more clearly. Defects such as clayey fault zones are present in the rock mass, but as none is adversely orientated, the stability of the slope should not be affected. Several small failures extending over one or two benches have occurred along the most persistent joint plans. (auth)