## 2 Laboratory Tests and Results  
### Experimental System  
Figure 1 shows the rock direct shear test system (RDS-200) designed by Geotechnical Consulting and Testing Systems (GCTS), which was used for conducting rock joint direct shear tests in this study. As shown in Figure 1a, RDS-200 automatically controls shear and normal load by the electro-hydraulic servo system. The shear and normal load capacities  
of this system were 10 t and 5 t, and the shear and normal strokes were 25 mm. Figure 1b illustrates the three-view drawing of shear boxes. The joint specimen was fixed at the center of shear boxes so that the joint was exposed in the middle for the direct shear test. The lower shear box was fixed, and a horizontal force induced by the movement of the shear actuator was applied to the upper shear box. The normal force induced by the movement of the normal actuator was applied on the joint surface through the upper shear box. It should be noted that the special quick-setting cement had a UCS of 80 MPa and [PERSON]'s modulus of 30 GPa, and the influence of the deformation of encapsulating material on the measurement accuracy was so slight (less than 1% in this paper) during the direct shear test that it could be ignored.  
The mechanical properties of the four groups of marble specimens are listed in Table 2. The uniaxial compressive strength (\(UCS\)) and elastic modulus (\(E_{s}\)) were obtained through uniaxial compression tests. The cohesion (\(c\)), internal friction angle (\(\varphi\)), and Poisson's ratio (\(\
u\)) were obtained through triaxial compression tests. The tensile strength (\(\sigma_{t}\)) was obtained through Brazilian-split tests. The P-wave velocity (\(V_{p}\)) was obtained by using the ultrasonic measurement apparatus.  
A self-made split tool was used to create tensile fractures (or the artificial rock joint) based on the Brazilian-split method [26], as shown in Figure 3a,b. The 3D morphology of the rock joint surface was obtained using the 3D morphology scan and analysis system, as shown in Figure 3c. The global error control module is included to control the scanning accuracy of the surface morphology, with an average accuracy of 0.02-0.03 mm. In order to reduce the error, the JRC values of four profile lines (1, 2, 3, 4), which mutually form an angle of 45 degrees between each two adjacent lines, were measured to obtain their average  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Group** & **Grain Size Classification** & **Average Grain Size (mm)** & **Dol (\%)** & **Cal (\%)** & **Ep (\%)** & **Sum (\%)** \\ \hline A & Powder-grained & 0.15 & 72 & 26 & 2 & 100 \\ B & Fine-grained & 0.375 & 65 & 33 & 2 & 100 \\ C & Medium-grained & 0.75 & 35 & 64 & 1 & 100 \\ D & Coarse-grained & 1.5 & 26 & 74 & 0 & 100 \\ \hline \end{tabular}
\end{table}
Table 1: Mineral composition of the rocks tested.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Group** & **\(UCS\)/MPa** & **\(E_{s}\)/GPa** & **\(c\)/MPa** & \(\varphi l^{\circ}\) & \(\
u\) & \(\sigma_{t}\)/MPa & \(V_{p}\)/m/s & _JRC_ \\ \hline A & 141.68 & 83.87 & 50.90 & 25.28 & 0.377 & 17.45 & 5854.56 & 9.07 \\ B & 70.93 & 47.45 & 25.79 & 25.28 & 0.316 & 17.30 & 5235.98 & 15.11 \\ C & 65.17 & 53.12 & 28.96 & 23.61 & 0.293 & 12.91 & 5270.34 & 15.66 \\ D & 48.38 & 41.88 & 15.04 & 30.02 & 0.119 & 10.30 & 5414.74 & 18.27 \\ \hline \end{tabular}
\end{table}
Table 2: Basic parameters of marble specimens.  
Figure 2: Microscopic pictures of thin sections of marble specimens under cross-polarized light.  
value with a sampling interval of 0.5 mm. Thus, the average joint roughness coefficient (\(JRC\)) of the marble joints of different groups can be calculated, as shown in Table 2. In addition, the rock joint surface obtained by the Brazilian-splitting method is random, and the larger the grain size is, the rougher the joint surface is. A previous study has shown that the grain size has a good correlation with \(JRC\)[8; 9], which was verified in Figure 3d, where \(JRC\) increases logarithmically with the increase in grain size.