Geotechnical properties of a weathered granite from the Western Cape, RSA

In-situ weathering of the Cape Granite has resulted in a material with the inherent structure of a parent rock which exhibits the geotechnical properties of silty sand. This paper describes the findings of a comprehensive study made of this material at the portal zone of the pilot bore of a major road tunnel. Occurring to a depth of up to 50 m with the water table at ground level the weathered granite posed serious problems to any excavation. Mineralogical and chemical analyses were conducted on the weathered, less weathered and fresh granite to elucidate the weathering process. A full range of geotechnical tests were carried out on both undisturbed and disturbed samples. The paper concludes with some observations on the behaviour of the weathered granite during tunnelling and in open cut excavation.