## 3 Triaxial Cyclic Loading and Unloading Experiment on High-Strength Sandstone
After the surrounding pressure reaches a certain value, the modulus of elasticity of the sandstone sample is gradually stabilised by continuing to increase the surrounding pressure. This is due to the fact that the internal structure of the sample may gradually deteriorate after undergoing repeated stresses. This includes phenomena such as crack extension and penetration, which lead to the elastic decay of the sample and a decrease in the changes in the elastic modulus. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Loading Method** & \begin{tabular}{c} **Surrounding** \\ **Pressure/MPa** \\ \end{tabular} & \begin{tabular}{c} **Average Peak** \\ **Strength/MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Average Modulus** \\ **Strain/10\({}^{-3}\)** \\ \end{tabular} &
\begin{tabular}{c} **Average Modulus** \\ **of Elasticity/GPa** \\ \end{tabular} \\ \hline S1–S3 & & 0 & 94.2 & 5.9 & 18.5 \\ S4–S6 & Conventional & 5 & 133.5 & 8.2 & 22.4 \\ S7–S9 & triaxial & 10 & 158.1 & 8.9 & 22.7 \\ S10–S12 & compression & 15 & 183.7 & 10.2 & 23.5 \\ S13–S15 & & 20 & 204.9 & 11.3 & 23.4 \\ SC1–SC3 & & 0 & 81.6 & 6.4 & 15.0 \\ SC4–SC6 & Triaxial cyclic & 5 & 125.3 & 8.5 & 20.7 \\ SC7–SC9 & loading and & 10 & 151.1 & 9.4 & 22.8 \\ SC10–SC12 & unloading & 15 & 177.5 & 10.0 & 23.6 \\ SC13–SC15 & & 20 & 198.1 & 11.3 & 23.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Strength parameters of sandstone samples under conventional triaxial compression and triaxial cyclic loading at different surrounding pressures. Strain Distribution Pattern of Sandstone under Triaxial Cyclic Loading with Different Surrounding Pressures  
The strain curves of sandstone samples under triaxial cyclic loading and unloading with different surrounding pressures are shown in Figure 9. The volumetric strain of the sample is taken to characterise the overall deformation of the sample. When the surrounding pressure is low (0 MPa or 5 MPa), the sample volumetric strain before cyclic loading and unloading is mainly concentrated in the tiny cracks and pore structures of the sample. At this time, there is obvious plastic deformation of the sample, which is manifested in the beginning of the volumetric strain curve as an upward concave and downward convex. With cyclic loading, some of the micro cracks and pores close and the volumetric strain of the sample is distributed uniformly throughout the post-peak stage. When the surrounding pressure is high (15 MPa or 20 MPa), most of the micro cracks have closed. Under the action of cyclic loading micro cracks slide, the sample volumetric strain is concentrated in the peak after the stage of the occurrence of obvious macroscopic damage. It is manifested by the rapid change in the volumetric strain curve along the horizontal direction. From the figure, it is also found that the stress-volume-strain curve can be divided into a volume-decreasing phase and a volume-increasing phase. When the external load is small, the gradual increase in the load will lead to the gradual closure of the internal micro cracks in the sandstone sample, resulting in a gradual decrease in the volume of the sample. When the external load reaches the critical load, the external load will cause the internal micro cracks of the sample to turn from closed to open, which will lead to an increase in the volume of the sample and destabilisation. The point of maximum triaxial cyclic loading and unloading volumetric strain is taken for analysis. The maximum values of volumetric strain were 0.37%, 0.47%, 0.56%, 0.60%, and 0.65% when the surrounding  
Figure 8: Effect of conventional triaxial compression and triaxial cyclic loading on strength parameters of samples at different surrounding pressures. (**a**) Peak strength; (**b**) peak strain; (**c**) modulus of elasticity. pressure was 0 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa, respectively.