## 1 Introduction  
Rock joint is a kind of geological discontinuity that widely exists in rock mass. The mechanical properties of rock joints are complex and are affected by many factors such as lithology, surface morphology, compressive strength of joint wall, filling materials and load actions [1, 2, 3, 4, 5]. Because the failure of rock masses is commonly manifested as shear slip along rock joints, the overall stability of the rock mass is largely controlled by the mechanical properties, especially the shear resistance behavior of rock joints. Therefore, the shear mechanical characteristics and the failure mechanism of rock joints have always been a research hotspot in the field of rock mechanics [6, 7]. Over the years, a lot of researchers have carried out extensive research on the shear mechanism and shear strength of rock joints [2, 8, 9, 10]. [PERSON] and [PERSON] [11] systematically conducted shear tests on natural rock joints. A joint roughness coefficient (JRC) for estimating the roughness of rock joints was proposed and a JRC-JCS shear strength model of rock joints was developed, which have been of widespread interest for both researchers in laboratories and engineers in the field. Due to the limitation that JRC can only characterize joint roughness in two dimensions, [PERSON] and [PERSON] [12] introduced three-dimensional roughness-characterization parameters considering the contact area of rock joints, and developed a shear strength criterion based on three-dimensional roughness. [PERSON] et al. [13] carried out shear tests on joint specimens of three different materials and monitored the shear process using acoustic emission. The results showed that the peak value of acoustic-emission events appeared after the peak shear stress, which revealed the progressive shear failure of joints. [PERSON] et al. [14] conducted shear tests of sawtooth-shaped and wave-shaped mortar joints under different normal stresses, and adopted PFC2D to numerically study the mesoscopic shear mechanism of joints. The results showed that with the increase in joint roughness, the failure mode of the asperities changed from sliding friction failure to asperity degradation. Some researchers have conducted laboratory direct shear tests on the factors affecting the shear strength of rock joints. Taking into account the effects of the filling on the shear strength of joints, [PERSON] and [PERSON] [15] adopted the UDEC code to numerically study the shear behavior of soil infilled rock joints under constant normal stress, and a new shear strength model of soil infilled joints was proposed. In order to investigate the influence of joint materials on the shear strength of joints, [PERSON] et al. [16] used plaster and concrete with different Young's modulus to make artificial joints, and shear test results showed that the deformability of asperity significantly affected the shear mechanical behavior of joints. [PERSON] and [PERSON] [17] prepared flat and rough artificial joints made by gypsum and concrete and conducted direct shear tests at different shear rates. The results showed that the shear strength of flat and rough joints decreased as the shear rate increased. For the purpose of studying the effect of roughness on the apparent cohesion of joints, [PERSON] et al. [18] made four types of artificial mortar joints with different roughness and carried out direct shear tests under low normal stress. The results showed that the apparent cohesion was positively correlated with the roughness, and the apparent cohesion was significantly reduced when the specimen was damaged or poorly interlocked. In order to assess the effects of material strength of joints on apparent cohesion, [PERSON] et al. [19] carried out shear tests on three types of joints composed of different materials under low normal stress. The results showed that due to the conditions of normal stress, the impacts of material strength and types of joints on shear behavior, shear strength, apparent cohesion and friction angle was limited.