STUDY ON FRACTURE MECHANISM AND CRITERIA OF FAILURE STRENGTH OF ROCK MASS CONTAINING COPLANAR CLOSE DISCONTINUOUS JOINTS UNDER DIRECT SHEAR

Rock masses containing coplanar close discontinuous joints are commonly found in nature,and their failure mechanism and strength strongly depend on the properties of joints and rock bridges,which are defined as the areas between joints.Several shear failure criteria of the rock mass are viewed and compared,and their weaknesses are pointed out.The mechanical behaviors of brittle rock mass containing coplanar close discontinuous joints under shear condition are analyzed,and the failure mechanisms of the rock mass are proposed.The rock mass may fail in three ways,failure in tension,failure in shear,and failure in mixed tension and shear.A modified criterion of failure strength of the rock mass is proposed for failure in mixed shear and tension.The equation of the shear strength contains the properties of geometry and mechanics of joints and rock bridges.Shear initiation angle as a new parameter is introduced firstly and also is contained in the equation.The failure mechanism can explain the phenomenon in direct shear test,and the calculated results according to the modified criterion of failure strength agree well with experimental results.