The presentation deals with the integration of site geology with engineering requirements. Methods of rock mass 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.
Rock mass classification systems (such as RMR and Q) played an important role in tunnel design and also in estimating rock mass properties. With the development of extremely powerful microcomputers and of user-friendly software there was a higher demand for reliable input data related to rock mass properties required as input into numerical analysis or close form solutions for designing tunnels. This necessity led to the development of a different set of rock mass classification. The Geological Strength Index is such a classification. The geological character of rock material, together with a visual assessment of the rock mass are used as a direct input for the selection of parameters relevant for the prediction of rock mass strength and deformability.
A description of the Geological Strength index (GSI) will be presented with suggestions for its use and discussion on its limitations. One of the advantages of the index is that the geological reasoning it embodies allows adjustments of its ratings to cover a wide range of rock masses and conditions including complex rock masses with lithologic variety.a
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