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The geological strength index: applications and limitations


The geological strength index (GSI) is a system of rock-mass characterization that has been developed in engineering rock mechanics to meet the need for reliable input data, particularly those related to rock-mass properties required as inputs into numerical analysis or closed form solutions for designing tunnels, slopes or foundations in rocks. The geological character of rock material, together with the visual assessment of the mass it forms, is used as a direct input to the selection of parameters relevant for the prediction of rock-mass strength and deformability. This approach enables a rock mass to be considered as a mechanical continuum without losing the influence geology has on its mechanical properties. It also provides a field method for characterizing difficult-to-describe rock masses. After a decade of application of the GSI and its variations in quantitative characterization of rock mass, this paper attempts to answer questions that have been raised by the users about the appropriate selection of the index for a range of rock masses under various conditions. Recommendations on the use of GSI are given and, in addition, cases where the GSI is not applicable are discussed. More particularly, a discussion and suggestions are presented on issues such as the size of the rock mass to be considered, its anisotropy, the influence of great depth, the presence of ground water, the aperture and the infilling of discontinuities and the properties of weathered rock masses and soft rocks.


Le Geological Strength Index (GSI) est un système de classification des massifs rocheux développé en mécanique des roches. Il permet d’obtenir les données relatives aux propriétés de masses rocheuses, données nécessaires pour des simulations numériques ou permettant le dimensionnement d’ouvrages:tunnels, pentes ou fondations rocheuses. Les caractéristiques géologiques de la matrice rocheuse ainsi que celles relatives à la structure du massif correspondant sont directement utilisées pour obtenir les paramètres appropriés relatifs à la déformabilité et la résistance de la masse rocheuse. Cette approche permet de considérer une masse rocheuse comme un milieu continu, le rôle des caractéristiques géologiques sur les propriétés mécaniques n’étant pas oblitèré. Elle apporte aussi une méthode de terrain pour caractériser des masses rocheuses difficiles à décrire. Après une décennie d’application du Geological Strength Index et de ses variantes pour caractériser des masses rocheuses, cet article tente de répondre aux questions formulées par les utilisateurs concernant le choix le plus approprié de cet index pour une large gamme de massifs rocheux. Des recommandations quant à l’usage du GSI sont données et, de plus, des cas où le GSI n’est pas applicable sont discutés. Plus particulièrement, des suggestions sont apportées sur des questions relatives à la taille de masse rocheuse à considérer, son anisotropie, l»influence des grandes profondeurs, la présence d’eau, l’ouverture et le remplissage des discontinuités ainsi que les propriétés des masses rocheuses altérées et des roches tendres.

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Correspondence to P. Marinos.

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Marinos, V., Marinos, P. & Hoek, E. The geological strength index: applications and limitations. Bull Eng Geol Environ 64, 55–65 (2005).

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Mots clés

  • Geological Strength Index
  • Massif rocheux
  • Structure géologique
  • Propriétés mécaniques
  • Conditions d»utilisation du GSI


  • Geological Strength Index
  • Rock mass
  • Geological structure
  • Mechanical properties
  • Selection of the GSI