Determination of residual strength parameters of jointed rock masses using the GSI system

The Geological Strength Index (GSI) system, proposed in 1995, is now widely used for the estimation of the rock mass strength and the rock mass deformation parameters. The GSI system concentrates on the description of two factors, rock structure and block surface conditions. The guidelines given by the GSI system are for the estimation of the peak strength parameters of jointed rock masses. There are no guidelines given by the GSI, or by any other system, for the estimation of the rock mass’ residual strength that yield consistent results. In this paper, a method is proposed to extend the GSI system for the estimation of a rock mass's residual strength. It is proposed to adjust the peak GSI to the residual GSIr value based on the two major controlling factors in the GSI system—the residual block volume Vbr and the residual joint condition factor Jcr. Methods to estimate the residual block volume and joint condition factor are presented. The proposed method for the estimation of rock mass's residual strength is validated using in-situ block shear test data from three large-scale cavern construction sites and data from a back-analysis of rock slopes. The estimated residual strengths, calculated using the reduced residual GSIr value, are found to be in good agreement with field test or back-analyzed data.