The integration of site geology with engineering requirements is the basis of Engineering Geology, and there continues to be a need to describe site geology in terms appropriate for the analyses of deformation and stability of ground in situ. Two developments hold the potential to improve such description: the characterization of soil and rock, and the wider use of numerical modeling. Methods of characterization can now be tested with the aid of numerical analyses, and the suitability of the predictions they lead to can be tested with site instrumentation.
Since the attempt by Terzaghi in 1946 to describe the characteristics of rock masses, numerous rock mass classifications have been developed. Hoek and Brown (1980) set out to develop a failure criterion and a classification system, the Geological Strength Index (GSI), specifically for the purpose of designing tunnels, slopes or foundations in rocks. Here the geological character of rock material, together with a visual assessment of the mass that forms, are used as a direct input for the selection of parameters relevant for the prediction of rock mass strength and deformability. GSI has considerable potential for use in rock engineering because it permits the manifold aspects of rock to be quantified, enhancing geological logic and reducing engineering uncertainty.
A detailed description of GSI is presented with suggestions for its use and discussion on its limitations. A number of examples from designs of engineering structures conclude the presentation.
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