Abstract: Due to the complex fracture distribution,significant scale effect and variable unloading response mechanism, the slope rock mass in the mining area is faced with the problems that the mechanical parameters are difficult to be equivalently characterized and the failure mechanism is difficult to be accurately described.The discrete element software PFC was used to establish the DFN-DEM coupling calculation model.The influence of sample size on the stress-strain curve,failure process and mode of fractured rock mass and representative elementary volume(REV) size and mechanical parameter values under DFN distribution was analyzed.The unloading failure process of the sample under REV size was explored.The results show that with the increase of sample size,the number of cracks increases the heterogeneity of the structure,and the failure characteristics gradually change from ductile failure dominated by plasticity to brittle failure dominated by elasticity.With the increase of sam ple size,the failure mode of fractured rock mass gradually evolved from concentrated brittle failure to multi-zone synergistic progressive failure.The uniaxial compressive strength decreases exponentially with the increase of size,while the elastic modu lus fluctuates first and then tends to be stable.Under this DFN distribution,the REV size of fractured rock mass is 11 m×22 m,the uniaxial compressive strength of REV is 9.21 MPa,and the elastic modulus is 27.9 GPa.The unloading process under goes four stages:loading,stress maintenance,progressive failure and final fracture.The larger the confining pressure is,the more unstable the structure is after unloading,and the faster the crack propagation and force chain collapse are.The research results can provide some help for the mechanical parameter value and stability analysis of discrete fracture slope in mining area.

Key words: discrete fracture network(DFN),fractured rock mass,size effect,discrete element method(DEM),unloading mechanical property