Abstract: The size effect of mechanical parameters of fractured surrounding rock mass is a key factor in deep mining engineering. The widely distributed joint structure in natural rock mass will make the rock mass show significantly different mechanical properties with the increase of scale，which directly affects the safety of underground construction and brings severe challenges to the safety of deep mining. In order to clarify the size effect law of jointed rock mass and the size of characterization unit，and to solve the prob⁃ lem of evaluating the mechanical parameters of deep broken surrounding rock，combined with the engineer⁃ ing practice of a kilometer-level deep lead-zinc mine in Yunnan，the SIROVISION three-dimensional te⁃ lemetry system combined with artificial compass measurement is used to systematically count the distribu⁃ tion characteristics of joints in the field. A total of 47 joints are depicted and three groups of dominant joints are divided. Using PFC particle flow software，combined with the microscopic parameters calibrated by indoor uniaxial compression test ( simulation and test error ≤ 5% )，a three-dimensional joint network model is established，and eight different sizes of synthetic rock masses are constructed. The size effect is analyzed by uniaxial compression test. The results show that the elastic modulus is basically stable at first，and then tends to be stable after a sudden drop. The uniaxial compressive strength gradually decreases with
the increase of size and then remains stable. Through comprehensive analysis，the size of the characteriza⁃
tion unit of the jointed rock mass is determined to be ϕ0.8 m×1.6 m. The research results can provide reli⁃
able theoretical support for the design and safety control of deep surrounding rock engineering.

Key words: deep broken surrounding rock, size effect, jointed rock mass, PFC, REV, rock mass me? chanical parameters