## 2 Laboratory Tests and Results
### Experimental Scheme and Results  
The influencing factors of the shear failure properties of rock joints can be summarized as intrinsic influencing factors (e.g., grain size) and extrinsic influencing factors (e.g., CNL). The grain size is associated with \(JRC\)[9], which in turn affects the shear failure properties of rock joints, and the CNL mainly affects the peak shear strength and shear instability characteristics of rock joints. Therefore, the experimental scheme was designed based on the above two influencing factors, as shown in Table 3. According to the longitudinal arrangement in Table 3, direct shear tests were conducted on artificial rock joints with various average grain sizes (0.15 mm, 0.375 mm, 0.75 mm, and 1.5 mm). According to the transverse arrangement in Table 3, direct shear tests were conducted on artificial rock joints under different _CNLs_ (5 kN, 10 kN, 15 kN, 20 kN, 25 kN, and 30 kN). The shear rate and final shear displacement were 1 mm/min and 10 mm, respectively. It should be clarified that the chosen ranges for grain size and normal load were selected according to the division of common rock engineering and mining depth of underground mines in real-world situations.  
Figure 3: Artificial rock joint and its relationship between grain size and \(JRC\). (**a**) Split tool, (**b**) Tensile fractures, (**c**) 3D morphology scan and analysis system, (**d**) Relationship between \(JRC\) and grain size.  
The experimental results are shown in Figures 4 and 5 according to the influencing factors (e.g., grain size and CNL). Figure 4 shows the shear stress-displacement curves of marble joints under different _CNLs_, while Figure 5 shows the shear stress-displacement curves of marble joints with various grain sizes. The shear stress-displacement curves of the marble joints can be summarized as the model curve in Figure 6, where \(\tau_{p}\) is the peak shear strength, \(u_{p}\) is the peak shear displacement, \(S\) is the post-peak modulus, \(\Delta\tau\) is the stress drop, \(\Delta u\) is the displacement jump, \(\tau_{r}\) is the residual shear strength, \(u_{p}\) is the residual shear displacement, \(E\) is the shear modulus, and \(u\) is the shear displacement. In addition, it can be seen from the experimental results that both the normal load (_CNL_) and the grain size (\(d\)) have an important influence on the shear mechanical properties of rock joints, such as peak shear strength (\(\tau_{p}\)), peak shear displacement (\(u_{p}\)), post-peak modulus (\(S\)), stress drop (\(\Delta\tau\)), and residual shear strength (\(\tau_{r}\)). However, the comparison between Figures 4 and 5 shows that the influence of _CNL_ on the shear mechanical properties of joints is more obvious than that of grain size. In summary, (1) the pre-peak yield stage becomes more obvious with the increase in CNL and grain size. (2) Marble joints of different groups generally experience a violent post-peak stress drop when _CNL_ is higher than 15 kN. (3) Group B and C were more prone to experiencing violent post-peak stress drop. (4) The yield stage of group A was not obvious, and the post-peak curves of groups A and D decreased more gently than those of groups B and C.