Variability of the engineering properties of rock masses quantified by the geological strength index: the case of ophiolites with special emphasis on tunnelling

The paper presents a quantitative description, using the Geological Strength Index (GSI), of the rock masses within an ophiolitic complex including types with large variability due to their range of petrography, tectonic deformation and alteration. This description allows the estimation of the range of rock mass properties and the understanding of the dramatic changes in behaviour which can occur during tunnelling, from stable conditions to severe squeezing within the same formation at the same depth. The paper presents the geological model in which the ophiolitic complexes develop, their various petrographic types and their tectonic deformation, mainly due to overthrusts. The structure of the various rock masses includes all types from massive strong to sheared weak, while the conditions of discontinuities are in most cases fair to poor or very poor due to the fact that they are affected by serpentinisation and shearing. Serpentinisation also affects the initial intact rock itself, reducing its strength. Associated pillow lavas and tectonic mélanges are also characterised. Based on the GSI, a classification of the behaviour in terms of tunnelling is presented, including stable conditions, structural instability, mild overstressing, stress dependant instability, squeezing and ravelling.RésuméUne description quantitative des massifs rocheux des complexes ophiolitiques est présentée par le moyen de l’index GSI. Les ophiolites forment un cas particulier à cause de leur variété pétrographique, leur déformation tectonique et leur altération. Cette description permet l’estimation des propriétés géotechniques et la compréhension des différents types de comportements souvent très variables rencontrés lors du creusement de tunnels. L’article discute brièvement le modèle géologique de ces formations, leurs variétés pétrographiques et leur déformation à cause surtout des charriages. La structure des massifs rocheux ophiolitiques inclut tous les types, (du milieu continu au cisaillé), tandis que l’état des joints est toujours faible à cause de la serpentinisation de leurs épontes. La serpentinisation peut aussi affecter la masse entière de la roche saine. Une classification du comportement en tunnel est présentée basée sur l’index GSI: conditions stables, instabilité structurale, instabilité due à des convergences.