Peak and residual strengths of jointed rock masses and their determination for engineering design

Knowledge of the rock mass strength is required for the design of many engineering structures in or on rocks. GSI system, proposed by Hoek et al. (1995), is now widely used for the estimation of the rock mass peak strength and the rock mass deformation parameters. 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 the present study, numerical simulations of laboratory strength tests are carried out to investigate the residual strength behavior of rock masses. Based on the results from numerical simulation and laboratory and field tests, the GSI system is extended to cover the residual strength of jointed rock masses. It is proposed to 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-b(r) and the residual joint surface condition factor J(c)(r). Methods to estimate the residual block volume and joint surface condition factor are presented. The peak and residual strength parameters determined from the GSI system are compared to the data from field block shear tests and the validity of the proposed method is verified.