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A Review On The Limitations And Applications Of Geological Strength Index

Writers : S. V. Alavi Nezhad Khalil Abad, Edy Tonnizam Mohamad, Ibrahim Komoo, R. Kalatehjari & M. Hajihassani

Date : 23-24 June 2012

Publisher : National Geoscience Conference, 2012 (NGC 2012)

Location : Pullman Hotel, Kuching

Abstract :

Reliable estimates of the strength and deformation characteristics of rock masses are required for almost any form of analysis used for engineering design. A method for obtaining estimates of the strength of jointed rock masses, based upon an assessment of the interlocking of rock blocks and the condition of the surfaces between these blocks were proposed and over the years, it was being modified in order to fulfill engineering requirements that were not considered when the original criterion was developed. In addition, as it is basically impossible to determine the strength and deformation characteristics of the rock mass through direct in situ testing except for back-analysis of already constructed tunnels, a need for some method to estimate the rock-mass properties from the intact rock properties and the characteristics of the discontinuities in the rock mass was required (Hoek et al., 1998). Therefore, the new classification called Geological Strength Index (GSI) was developed; extended from the original criterion. The GSI classification system greatly respects the geological constraints that occur in nature and are reflected in the geological information. Currently, in order to meet the need for dependable input data, especially when rockmass properties are required as the input for numerical analysis or closed form solutions, the GSI system has been proposed (Marinos & Hoek, 2000). As a direct input to the selection of parameters related for the prediction of rock deformability as well as rock-mass strength, the geological character of rock material and assessment through observation of the mass it forms, have been applied. This approach has the ability to be considered a rock mass as the mechanical continuum without losing the geological influence on its engineering properties. In order to characterize rock masses which are complicated to describe, a field method has been provided by GSI. This paper tries to review some critical questions that exist about the correct selection of the index for a range of rock masses under different conditions after years of GSI application and its quantitative characterization of rock mass variations. Given cases where the GSI is not appropriate to be used, additional recommendations on the GSI usage are provided. Specifically, a suggestion and discussion which are related to issues such as the presence of ground water, the influence of great depth, size of the rock mass, its anisotropy, the properties of weathered rock masses and soft rocks and the aperture and the infilling of discontinuities are presented. The GSI has considerable potential for use in rock engineering because it permits the manifold aspects of rock to be quantified thereby enhancing geological logic and reducing engineering uncertainty. Its use allows the influence of variables, which make up a rock mass, to be assessed and hence the behaviour of rock masses to be explained more clearly. 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 but it also allows us to understand the limits of its application. Finally, there were some suggestions for using GSI.