## 3 Triaxial Cyclic Loading and Unloading Experiment on High-Strength Sandstone
The hysteresis loop curves for different cyclic loading stages at various surrounding pressures are shown in Figure 7. Under cyclic loading, there is a stiffness degradation in high-strength sandstone samples where the peak point displacement per cycle increases with the number of cycles. The stiffness degradation is progressively more pronounced with the increase in the surrounding pressure. The hysteresis loops are more dispersed in the first 3-4 cycles and the distribution of hysteresis loops generated by the rest of the loading cycles is more concentrated, with some hysteresis effect of hysteresis loops. From the variation in the hysteresis loop area of sandstone samples under the same surrounding pressure, it can be seen that the energy dissipation in the same level of cyclic loading decreases slightly with the increase in the number of cycles. This indicates that the first stage of cyclic loading caused some damage to the sandstone samples and micro cracks formed in the samples. The later stages of cyclic loading caused the samples to consume less energy to produce the same strain. Sandstone samples are subjected to strain accumulation under cyclic loading, as evidenced by the stress-strain curve where the end point of a single cycle is higher than the start point. The cyclic loading creates a permanent deformation of the samples. This strain accumulation increases with the number of cyclic loads applied. Figure 5: Conventional triaxial compressive stress–strain curves under different surrounding pressures. Figure 6: (a) Stress–strain curves of triaxial cyclic loading and unloading under different surrounding pressures; (b) cyclic loading and unloading hysteresis loop. Figure 7: Hysteresis loop curves for different cyclic loading stages at various surrounding pressures. Strength Characteristics of Sandstones under Triaxial Cyclic Loading at Different Surrounding Pressures  
The peak strength, peak strain, and modulus of elasticity of the samples under conventional triaxial compression with different surrounding pressures and triaxial cyclic loading with different surrounding pressures are shown in Table 2. The sample undergoes predominantly elastic deformation in unloading and can be considered as having no damage rupture generated within the rock. Its deformation is the elastic recovery caused by the release of elastic energy saved by the rock and the unloaded stress-strain curve is approximated as a straight line. The slope of the unloading curve is taken as the modulus of elasticity of the rock at this time. As shown in Figure 8, the peak strength of sandstone samples in both the conventional triaxial compression test and the triaxial cyclic loading test increased with the increase in the surrounding pressure. The increase in surrounding pressure restricts and closes the tiny cracks and pores within the sandstone, thus increasing its load-bearing capacity. When the cycle reaches a certain number of cycles or a certain intensity, it will lead to internal structural damage to the sample and cause its peak strength to be reduced compared to the peak strength in conventional triaxial compression. This deteriorating effect on peak strength is more pronounced at lower surrounding pressures. The peak strain of sandstone samples under both conventional triaxial compression and triaxial cyclic loading increases with increasing surrounding pressure. When the surrounding pressure reaches a certain value, the peak strain of the sample has a tendency to reach a certain value and remain stable. The modulus of elasticity of the sample also increases with increasing surrounding pressure. The closure of micro cracks under surrounding pressure resulted in a denser interior of the sample and a consequent increase in the modulus of elasticity. The modulus of elasticity of sandstone samples under cyclic loading increases with the increase in surrounding pressure.