Abstract: In order to study the tensile-shear behavior of rock discontinuities, the self-developed tension-shear device was used to carry out the direct shear test of the natural limestone bedding planes under normal tensile stresses, and the shear stress-displacement curves, the bedding plane fracture morphologies and strength characteristics were analyzed. The fracture surfaces have no friction crushing zone and local split under a tensile-shear stress. The shear strength decreases nonlinearly with the increase of normal tensile stress. Further, PFC simulation was used to study the influence of saw-tooth bedding plane undulation characteristics on its tensile-shear rupture and strength characteristics. As the normal tensile stress increases, the shear cracks decrease and the tensile cracks increase. When the undulation angle is small, cracks are generated along the saw-tooth bedding planes; when the undulation angle is large, cracks are generated along the saw-tooth bedding planes and saw-tooth. The saw-tooth bedding plane failure can be divided into three modes: tensile and shear ruptures along the saw-tooth planes, tensile and tensile-shear ruptures along the saw-tooth planes, and mixed saw-tooth plane and saw-tooth ruptures. And the damage evolution of each failure mode was analyzed. The shear strength of the saw-tooth bedding planes decreases first and then increases with the increase of the undulation angle, and the minimum is at 30º. It decreases almost linearly with the increase of the normal tensile stress, which can be described by the Mohr-Coulomb criterion. The friction angle and cohesive force decrease with the increase of the undulation angle. The variation of shear strength, friction angle and cohesion force with undulation angle is mainly controlled by the failure mode. With increasing levels of adhesive strength of bedding plane, the saw-tooth plane rupture is gradually converted into a mixed saw-tooth plane and saw-tooth ruptures. When the adhesive strength reaches a certain level, tensile-shear strength no longer increases, mainly controlled by the strength of the saw-tooth rock.

Key words: rock mechanics, bedding plane, tensile-shear, undulation angle, particle flow simulation