## 2 Materials and Methods
### Shear Test of Rock with Variable Angle Mode  
Rock shear test is an experiment to determine the friction angle and cohesion of rock. The method is simple and feasible. Variable angle shear experiment usually adopts the mold change angle of \(20^{\circ}\), \(30^{\circ}\), and \(40^{\circ}\). Then, the internal friction angle and cohesion of the rock mass can be obtained by drawing shear stress and normal stress curves of the failure surface.  
The WGE-600 screen display universal experiment machine was also used in this experiment (as shown in Figure 3). Three sets of fixtures (\(20^{\circ}\), \(30^{\circ}\), \(40^{\circ}\)) were used for variable angle shear specimen, as shown in Figure 4. Among them were the first group of four samples (mudstone), the second group of four samples (coal rock), the third group of  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Serial**} & \multirow{2}{*}{**Lithology**} & \multicolumn{2}{c}{**Specimen Size**} & \multicolumn{1}{c}{**Natural**} & \multicolumn{1}{c}{**Feed Water**} & \multicolumn{1}{c}{**Drying**} & \multicolumn{1}{c}{**Soil Natural**} & \multicolumn{1}{c}{**Full Water**} & \multirow{2}{*}{**Dry Density**} & \multicolumn{1}{c}{**Bibulous**} \\  & & \multicolumn{1}{c}{**(Width \(\times\) Length)**} & \multicolumn{1}{c}{**Quality**} & \multicolumn{1}{c}{**Quality**} & \multicolumn{1}{c}{**Quality**} & \multicolumn{1}{c}{**Density**} & \multicolumn{1}{c}{**Density**} & \multicolumn{1}{c}{**Density**} & \multicolumn{1}{c}{**Bale**} \\  & & \multicolumn{1}{c}{**mm \(\times\) mm**} & \multicolumn{1}{c}{**g**} & \multicolumn{1}{c}{**g**} & \multicolumn{1}{c}{**g/cm\({}^{3}\)**} & \multicolumn{1}{c}{**g/cm\({}^{3}\)**} & \multicolumn{1}{c}{**g/cm\({}^{3}\)**} & \multicolumn{1}{c}{**g/cm\({}^{3}\)**} & \multicolumn{1}{c}{**g/cm\({}^{3}\)**} & \multicolumn{1}{c}{**\%**} \\ \hline
1-A-1-1 & \multirow{2}{*}{Coal} & 50.2 & 49.96 & 214.1 & 216.8 & 209.2 & 2.14 & 2.18 & 2.06 & 4.20\% \\
1-A-1-2 & & 50.4 & 50.51 & 218 & 217.9 & 208.4 & 2.05 & 2.04 & 2.1 & 4.87\% \\  & & mean value & & & & 2.1 & 2.11 & 2.08 & 4.54\% \\
2-B-1-1 & & 100.03 & 50.44 & 474.5 & 481.8 & 466.7 & 2.38 & 2.41 & 2.34 & 3.24\% \\
2-B-1-2 & & 99.6 & 50.5 & 495.9 & 501.9 & 490.3 & 2.49 & 2.52 & 2.46 & 2.37\% \\
2-B-1-3 & & 99.63 & 50.2 & 459.7 & 470.1 & 448.4 & 2.33 & 2.39 & 2.28 & 4.84\% \\
2-B-1-4 & & 99.31 & 50.46 & 480.3 & 490.7 & 468.7 & 2.42 & 2.47 & 2.36 & 4.69\% \\  & & mean value & & & 2.4 & 2.45 & 2.36 & 3.78\% \\
1-C-1-1 & \multirow{2}{*}{Fine} & 99.39 & 50.42 & 493.7 & 505.6 & 491.6 & 2.49 & 2.55 & 2.48 & 2.8\% \\
1-C-1-2-2 & & 100.17 & 50.5 & 487.9 & 502.3 & 486.6 & 2.43 & 2.5 & 2.43 & 3.22\% \\
1-C-1-3 & & \multirow{2}{*}{sand-} & 101.25 & 50.61 & 467.5 & 480.3 & 465 & 2.3 & 2.36 & 2.28 & 3.29\% \\
1-C-2-4 & & & 98.1 & 48.84 & 454.66 & 473.58 & 452.68 & 2.36 & 2.55 & 2.37 & 3.14\% \\  & & mean value & & & 2.39 & 2.49 & 2.39 & 3.22\% \\
1-D-1-2 & & \multirow{2}{*}{Medium} & 99.36 & 48.7 & 455.32 & 467.1 & 447.9 & 2.71 & 2.74 & 2.65 & 4.61\% \\
1-D-1-2 & & & 98.92 & 48.74 & 457.86 & 469.6 & 450.7 & 2.55 & 2.56 & 2.44 & 4.19\% \\
1-D-1-3 & & \multirow{2}{*}{sand-} & 98.9 & 48.9 & 444.78 & 456.2 & 436.5 & 2.39 & 2.46 & 2.35 & 4.50\% \\
1-D-1-4 & & & 100.1 & 48.72 & 451.48 & 463.56 & 442.68 & 2.42 & 2.43 & 2.37 & 4.70\% \\  & & & mean value & & & 2.42 & 2.43 & 2.37 & 4.70\% \\ \hline \end{tabular}
\end{table}
Table 1: Density and water absorption calculation table.  
Figure 3: WGE-600 Experiment and investigation equipment with microcomputer control displaying of WGE-600.  
three samples (fine sandstone). For the four groups of four samples (medium sandstone), shear strength calculation formula is as follows in Equation (1):  
\[\sigma=P\sin\alpha/A\ \ \ \tau=P\cos\alpha/A \tag{1}\]