## 3 Test Results
### Colesion and Internal Friction Angle  
According to the Mohr Coulomb criterion and the stress-strain curve of sandstone under triaxial compression, the cohesion and internal friction angle of sandstone can be calculated from the following formula:  
\[\sigma_{1}=\frac{1+\sin\varphi}{1-\sin\varphi}\sigma_{3}+\frac{2\mathrm{ccos} \,\varphi}{1-\sin\varphi} \tag{5}\]  
where \(\sigma_{1}\) and \(\sigma_{3}\) are the maximum principal stress and the minimum principal stress, respectively, and \(c\) and \(\varphi\) are the cohesion angle and internal friction angle, respectively.  
Figures 10 and 11 show the change rule of sandstone cohesion and internal friction angle with temperature at different bedding angles. It can be seen from the figure that the sandstone cohesion generally increases first and then decreases with temperature. When \(T\) = 200 \({}^{\circ}\)C, except for \(\beta\) = 30\({}^{\circ}\), the cohesion of sandstone decreases slightly, and that of sandstone at the other four dip angles increases slightly compared with that at \(T\) = 25 \({}^{\circ}\)C. When \(T\) = 400 \({}^{\circ}\)C, the cohesion of \(\beta\) = 30\({}^{\circ}\) sandstone increases slightly, and the cohesion of sandstone in the other four groups of dip angles decreases to different extents. When \(T\) = 600 \({}^{\circ}\)C, the cohesion of \(\beta\) = 60\({}^{\circ}\) sandstone is increased and that of the other four groups of dip angle is decreased slightly. When \(T\) = 800 \({}^{\circ}\)C, the cohesion phases of sandstone in the five sets of dip angles decrease significantly compared with that at low temperature. When \(T\) = 1000 \({}^{\circ}\)C, the cohesion of all samples decreases to approximately 33% of the original state, indicating that thermal damage has a serious deteriorative effect on the bond strength of sandstone at this time.  
Figure 10: **(a)** Change trend of \(c_{t}/c_{0}\) with temperature for rock samples with different bedding dip angles; **(b)** change trend of \(c\) with dip angle for rock samples with different temperature treatments.  
The internal friction angle of sandstone decreases first, then increases, and then decreases with temperature. When \(T=200\)\({}^{\circ}\)C, the internal friction angle of 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) sandstone decreases, and the internal friction angle of 0\({}^{\circ}\) and 30\({}^{\circ}\) sandstone increases slightly. After heating to 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, the internal friction angle of sandstone at each bedding dip angle continuously increases and reaches a maximum at \(T=800\)\({}^{\circ}\)C. After heating to 1000 \({}^{\circ}\)C, the internal friction angle of sandstone decreases slightly, but its size is still larger than that of the original state.  
When \(T<600\)\({}^{\circ}\)C, the change trend of sandstone cohesion with bedding dip angle is relatively close; the cohesion is minimal at \(\beta=45^{\circ}\), except for the sandstone at \(T=200\)\({}^{\circ}\)C, the cohesion is maximal at 60\({}^{\circ}\), and the others are maximal at \(\beta=30^{\circ}\). When \(T>600\)\({}^{\circ}\)C, the cohesion does not change obviously with the increasing bedding dip angle.