Study On Shear Mechanical Properties Of Rock Mass With Discontinuous Joints Under Unloading Normal Stress

In practice,joint penetration failures caused by excavation of more slope projects are essentially shear failures caused by unloading normal stress.However,the current experimental research on the shear mechanical properties of jointed rock masses mainly adopts the method of constant normal stress and increased shear stress,while the research on jointed rock masses under unloading normal stress conditions is relatively rare.In view of this,this paper takes the discontinuous jointed rock mass as the research object,uses similar materials to prepare artificial joint specimens with different roughness,and carries out the direct shear test with constant normal stress and the direct shear test under unloading normal stress with constant shear respectively.According to the test results,the influence of roughness on the shear deformation characteristics,strength characteristics and failure mode of discontinuous jointed rock mass under constant normal stress and unloading normal stress conditions is revealed.And a shear strength criterion considering joint roughness and weakening of mechanical parameters of rock bridge under unloading normal stress condition is proposed.The main research content and conclusions of the thesis are as follows:(1)Taking the prepared discontinuous joint samples with different roughness as the research object,the conventional direct shear tests under different normal stresses of 2MPa,4MPa,6Mpa,8Mpa and 10 MPa,etc.were carried out for the samples with different roughness,and the conventional shear mechanical properties of the rock mass with five different roughness(JRC = 0,5,10,15,20)were studied.The results show that under constant normal stress,with the increase of roughness,the peak shear displacement of jointed rock mass gradually decreases,while the peak shear stress gradually increases,and the failure mode gradually changes from tension failure mode to tension shear mixed failure mode;(2)The direct shear test under unloading normal stress with constant shear was carried out on the prepared discontinuous joint specimens.The shear deformation,strength characteristics and failure mode of the discontinuous joints were analyzed under unloading normal stress conditions.The results show that the unloading normal displacement of jointed rock increases with the increase of roughness under unloading normal stress condition,the unloading tangential displacement and the normal stress decreases.The failure mode is gradually changed from shear failure to tensile failure;(3)Compared with the shear mechanical properties of discontinuous jointed rock mass under constant normal stress,in terms of strength characteristics,the cohesion of jointed rock mass under unloading normal stress condition is lower and the internal friction angle is higher;for the same failure normal stress,the failure tangential stress of samples under normal unloading condition is smaller.In the aspect of failure mode,the failure mode of the joint specimen under constant normal stress is mainly the mixed failure of tension and shear,while the failure mode under unloading normal stress is mainly the mixed failure of tension and shear.(4)Based on the analysis of the strength characteristics under unloading normal stress conditions,and considering the influence of the reduction of cohesion and roughness of rock bridge,the shear strength criterion under unloading normal stress condition is proposed.Compared with the experimental results,the error of the predicted results of the strength criterion under unloading normal stress conditions is within 10%,which is in good agreement.