Numerical investigation on the effect of cyclic loading on macro⁃meso shear characteristics of rock joints

Rock joints will continuously slip and close under cyclic loading，leading to continuous deterioration of their bearing properties，which in turn affects the stability of the rock engineerings.In order to investigate the shear characteristics of rock joints under cyclic loading，firstly，joint shear tests were carried out to calibrate the physical and me⁃ chanical parameters in the simulation and to determine the characteristic parameters of cyclic loading.Then，a cyclic shear loading implementation method based on FISH language was used to simulate the shear of rough joints under cy⁃ clic shear loading with different loading amplitudes ( 1.5，1.7，1.9，2.1 and 2.3 MPa ) and loading frequencies ( 0.1， 0.5，1.0，1.5 and 2.0 Hz ).The macro⁃meso shear characteristics of rock joints and the effect of cyclic loading charac⁃ teristic parameters on the shear properties of joints were investigated.The results show that with the cyclic shear loading and unloading，damage to the surface of the joints can lead to the hysteresis effect in the shear stress⁃shear displace⁃ ment curve of the joints.The hysteresis loops are small and densely distributed at the early stage of shear and large and loosely distributed at the late stage.The crack distribution and contact force distribution of the joints during shearing are related to the surface roughness of the joints.In the early stage of shear，the cracks and high contact force are most⁃ ly concentrated on the second⁃order bulge，and with the destruction of the second⁃order bulge，the shear force gradually acts on the first⁃order bulge， and after the destruction of the first⁃order bulge， the joints are destabilized. The growth trend of the nodal shear displacement is generally consistent with the growth trend of the accumulated crack number.Only a small number of cracks were generated near the peak stress at the early stage of shear，while the stress range and the number of cracks corresponding to the later stage of crack generation increased significantly.The larger the loading amplitude，the more joint damage is generated within each cycle，and the greater the joint shear dis⁃ placement.The number of cycles required to achieve the same target displacement is inversely proportional to the load⁃ ing amplitude.The greater the loading frequency，the less damage produced by the joint in each cycle，and the smal⁃ ler the joint shear displacement.The number of cycles experienced to reach the target shear displacement is proportion⁃ al to the loading frequency. The lower the loading frequency， the shorter the acceleration phase， and the more abrupt the destabilization of the joint.