Abstract:The geological strength index (GSI) of jointed rock mass is an important parameter for rock mass engineering design and stability analysis. It is of great significance to study the failure mode and GSI of jointed rock mass. The compression simulation test of blocky jointed rock mass with two sets of orthogonal joints was carried out by using the granite numerical model calibrated by laboratory test. The test results show that the strength of intact rock increases by 40.68% and the strength of jointed rock mass increases by 166.01% when the confining pressure is from 3MPa to 10MPa, and the strength of jointed rock mass increases more obviously by confining pressure. The simulation results show that the strength and failure of blocky jointed rock mass show obvious anisotropy with joint inclination angle. The compressive strength and GSI of blocky jointed rock mass decrease first and then increase with the joint inclination angle. The blocky jointed rock mass mainly undergoes shear failure, tensile failure and shear-tensile mixed failure under confining pressure. Finally, based on the analysis of the numerical results, a prediction model for GSI of blocky jointed rock mass is proposed based on the generalized Hoek-Brown criterion. The GSI of blocky jointed rock mass has a nonlinear relationship with joint spacing. The model can provide relevant basis for the determination of GSI of blocky jointed rock mass in engineering.