The GSI system for rockmass strength estimation based on the Hoek-Brown criterion has become the industry standard for international tunnelling and is becoming popular in slope applications. This system is based on the principle that structure within a rockmass (joints, fractures, joint surface alteration and degradation) acts to reduce both the cohesion and frictional properties of the rockmass. Evidence suggests, however, that this method overestimates the strength of some insitu rocks at high GSI (>75) and underestimates aspects of strength in certain rocks for moderate GSI values between 65 and 75. While the recognition of this problem is not new, this paper presents a new and robust approach to strength parameter estimation and non-linear plastic analysis of this class of rocks in conventional models using the modified Hoek-Brown system (a ? 0.5). Theoretical background is briefly discussed and guidance for practical application is given. Finally limitations are specified and appropriate decision support is given via a transition function between traditional GSI characterization and this new approach.


This paper deals with hard rock (including but not limited to granitoids, gneisses, quartzites and nonfoliated mafics) of high rockmass quality (as defined for example by the GSI system of Hoek & Marinos (2000), as shown in Figure 1. For hard rock materials, extensile crack initiation is the primary form of damage, even under compression (e.g. Stacey 1981, Tapponier & Brace 1976, Griffith 1921). Under low confinement, the propagation of extension cracks lead to spalling observed around hard rock openings at depth. Lower bound in situ compressive strength for excavations in hard rocks corresponds to an extension crack damage initiation threshold that is a function of the nature and density of internal flaws and heterogeneity.

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