A new rational method for calculating the GSI

In this paper, a new approach for a quantitative assessment of the Geological Strength Index (GSI, Hoek, E., Kaiser, P.K., Bawden, W.F., 1995) is proposed. In particular, on the basis of the conceptual affinity of the GSI with the Joint Parameter (JP) used in the RMi (Rock Mass index, Palmstrom, 1996), a relationship between the two indexes is derived and exploited in order to obtain a reliable, quantitative assessment of the GSI by means of the basic input parameters for the determination of the RMi (i.e. the elementary block volume and the joint conditions). In this way, the user has the possibility of applying and comparing two truly independent approaches for the determination of the GSI: the traditional qualitative “Hoek’s chart”, mainly based on the degree of interlocking of rock mass, and the proposed quantitative assessment method, mainly based on the fracturing degree of a rock mass. On the basis of such a double-estimation, a definitive “engineering judgement” can be made more rationally. The new approach facilitates as well the implementation, from one side, of the probabilistic approach for managing the inherent uncertainty and variability of rock mass properties and, from the other, of the RMi system as empirical method for tunnel design. Given the complementarities of the two indexes, the proposed approach appears to be very promising. An example application is presented to illustrate the high potentiality of the new method.