A quantitative approach to estimate the peak and residual strength parameters and deformation modulus of jointed rock masses is presented in this paper. The method is based on the characterization of rock block volume and joint surface condition from field mapping data and applying the GSI system. It provides a complete set of mechanical properties (Hoek-Brown peak (m b s, a) and residual (m r s r, a r) strength parameters, or the equivalent Mohr-Coulomb peak (c and φ) and residual (c r and φ r) strength parameters as well as deformation modulus (E) for design purpose.
Numerical tools are widely used in design ofmany engineering structures in or on rocks such as foundations, slopes and open pits, tunnels, underground caverns, mining developments and stopes. Knowledge of the rock mass strength and deformation behaviors is required for conducting the numerical modeling. Unfortunately, the determination of the global mechanical properties of a jointed rock mass remains one of the most difficult tasks in rock mechanics.Traditional methods to determine these design parameters include field tests (plate-load tests for deformation modulus and in-situ block shear tests for strength parameters). These tests can only be performed when the exploration adits are excavated and the cost of conducting in-situ tests is high. Alternatively, parameters are obtained by back-analysis but this again requires established excavations. Furthermore, the resulting parameters are largely dependent on the adopted constitutive models. Few systematic attempts have been made to develop methods to characterize the jointed rock mass to estimate the deformability and strength indirectly.The Geological Strength Index (GSI), developed by Hoek et al. (1995), is one of them. It uses properties of intact rock and conditions of jointing to classify the rock but most importantly to arrive at an estimate of rock mass deformability and strength parameters. It provides a set of mechanical properties (Hoek-Brown peak strength parameters m b and s, or the equivalent Mohr-Coulomb peak strength parameters c and φ, as well as elastic modulus E) for design purpose. A method to obtain a compete set of design parameters using the GSI system is presented in this paper. Based on results from numerical simulation and laboratory and field tests, the GSI system is extended to cover the residual strength of jointed rock masses. The idea is to obtain peak GSI value from field mapping and adjust the peak GSI to the residual GSI r value based on the two major controlling factors in the GSI system, i.e., the residual block volume V r b and the residual joint surface condition factor J r c. Methods to estimate the residual block volume and joint condition factor are presented. The peak and residual strength parameters as well as deformation modulus thus determined from the GSI system can be used in the numerical analysis for safe and cost-effective design.
