Abstract: The instability of rock mass usually starts from the fractures,and different fracture types and angles have a significant impact on its failure characteristics. This article simulates the structural effects of actual rock masses by prefabricating different fracture dip angles in the original rock,and intends to conduct triaxial compression and numerical simulation experiments on fractured sandstone to study the influence of open / closed fracture dip angles and confining pressure on the mechanical properties and failure modes of sandstone. The results show that:① With the increase of the dip angle of the closed fracture,the peak stress and elastic modulus of the rock sample first decrease and then increase. With the increase of confining pressure,both the elastic modulus and peak stress of the closed fracture rock mass increase. ② The peak stress of red sandstone with open and closed fractures is positively correlated with the proportion of microcracks generated during the failure process. That is, the stronger the bearing capacity of red sandstone, the more microcracks are generated during the failure process,and the more severe the degree of rupture. This indicates that the proportion of microcracks can be used as an important indicator to quantify the degree of internal damage in the model specimen. ③ The closed crack in the vertical loading direction closes at the tip after compression,and the stress concentration degree at the tip is smaller than that caused by the nonuniformity of sandstone particles,resulting in a failure mode and peak stress that is close to that of the intact rock sample;After the opening crack in the vertical loading direction is compressed,severe tensile stress concentration occurs at the tip,and the tip first generates a tensile crack that extends along the compression direction,resulting in a smaller peak stress. The above research results reveal the deformation and fracture mechanisms of open / closed fractured rock masses under different dip angles, which can provide a theoretical basis for the prevention and control of engineering disasters and safety evaluation of fractured rock masses. 

Key words: fracture type,dip angle,mechanical properties,destructive mechanism,discrete element simulation