Comparative study on shear mechanical properties and failure mechanisms of rough joints under CNL and DNL boundary conditions

The dynamic loads induced by excavation disturbance and earthquake significantly influence the deformation and instability of jointed rock masses. To investigate the shear mechanical properties of rock joints under dynamic normal load，a series of shear tests were conducted on red sandstones under both dynamic normal load(DNL) and constant normal load(CNL) boundary conditions. The test results，i.e.，shear strengths，failure modes，acoustic emission characteristics and failure pattern obtained under the two boundary conditions were comparatively analyzed. The analysis shows that the shear stress reflects was lagged the dynamic normal stress and the phase offset was positively correlated with the initial normal stress and joints roughness. The influence of dynamic normal load on peak shear strength depended on the failure mode. When the primary failure mode was climbing and dilation，the peak shear strength was increased by the dynamic normal load；conversely，when the failure mode was dominated by shearing and fracturing of asperities，the peak shear strength was decreased. The dilation was significantly confined by the dynamic normal load，which exacerbated the wear on joints surface and the degradation of asperities. The acoustic emission monitoring results indicated that the dynamic normal load intensified the damage degree at the initial shear stage. It increased the proportion of shear fractures whereas decreased the proportion of tensile fractures. Based on obtained test results and revealed shear mechanism，a dilatancy model was deduced considering the influence of the dynamic normal load. Finally，a model for predicting the peak shear strength of joints under DNL conditions was proposed and it was validated to have a satisfactory prediction accuracy.