## 1 Introduction  
Engineering rockmass is cut by many discontinuities of different sizes and orientations, which affect the stability of engineering rockmass to a great extent. Rock engineering disasters such as rockburst, roof fall, and pillar failure are closely related to the deformation and unstable sliding of discontinuities [1, 2, 3, 4, 5]. Therefore, many efforts have been made to study the shear failure properties and influencing factors of rock joints. The influencing factors of the shear failure properties of rock joints can be summarized as intrinsic and extrinsic influencing factors [6]. The intrinsic influencing factors mainly include \(JRC\) (determined by the grain size) and the mechanical properties of jointed rockmass (determined by the mineral composition and microstructure). [PERSON] et al. [7] conducted direct shear tests on artificial joint specimens with different inclination angles and found that the inclination angle had a positive correlation with the strength of rock joints, and the shear failure mode changes as the inclination angle increases. [PERSON] et al. [8] studied the shear failure behavior of two kinds of granite joints and found that the peak shear strength and post-peak characteristics were significantly different. Joint B with a higher peak shear strength is related to the rougher joint surface, which is associated with a larger mineral grain size [9], while joint A, with a more violent post-peak failure intensity, is due to a smaller mineral grain size, compact structure, and fewer inherent cracks [10; 11], which favor a greater energy build-up in asperities. [PERSON] et al. [12] studied the effect of joint roughness on shear strength and permeability through the numerical simulation method and found that rougher joints exhibit higher shear strength and the long-term suppression of permeability. The extrinsic influencing factors mainly include stress condition (e.g., shear rate and normal load), engineering disturbance, infill, and groundwater [13; 14; 15; 16]. Direct shear tests under various normal stresses were conducted to investigate the shear failure behavior of rock joints [17], and violent post-peak stress drops and stick-slip phenomenon occurred when normal stress was greater than 5 MPa and 10 MPa, respectively. Moreover, the shear rate also has a significant effect on the stick-slip behavior of rock joints [18; 19], and experimental results show that the stick-slip phenomenon gradually disappears with an increasing loading rate. [PERSON], [PERSON] [2] conducted direct shear tests on ice-filled rock joints to study the influence of ice-layer thicknesses on the shear failure behavior of rock joints and found that the cohesive strength of ice-filled joints evolves exponentially with increasing ice-layer thickness. In order to study the influence of engineering disturbances, shear test apparatus that can apply dynamic shear load was developed to study the influence of dynamic shear load on the shear failure behavior of rock joints [20]. Moreover, the influence of the dynamic change in the normal load on the shear failure behavior of rock joints has also been studied [21], and dynamic disturbances accelerate the fracture process of rock joints and promote the weakening effect of shear fracture. In summary, previous studies have paid more attention to the conventional shear behavior of rock joints, such as the shear strength, seepage characteristics, and wear mechanism of the rock joint surface. However, the shear instability mechanism of the rock joint and its influencing factors have not been studied extensively. Shear instability (or unstable shear failure) refers to a shear failure of rock discontinuities in a sudden and violent manner [22; 23; 24]. In addition, the influencing factors (e.g., grain size and normal load) that affect the shear instability of rock joints have been relatively unexplored.