Abstract: In response to the damage and failure of surrounding rock caused by different amplitudes of loading and unloading, red sandstone from a certain water conveyance tunnel was selected for pre-peak and post-peak loading and unloading tests with different stress amplitudes, and the stress-strain curve, hysteresis loop evolution, and macroscopic failure characteristics during the loading and unloading process were analyzed. Based on the shear damage mechanics process that takes into account compaction deformation, the four-stage mechanical evolution characteristics of red sandstone were revealed, and the theoretical calculation formulas for loading and unloading moduli were derived to explore their evolution laws. The results show that the larger the amplitude of loading and unloading stress prior to the peak of red sandstone, the larger the hysteresis loop area. Under the same stress amplitude, the hysteresis loop area before the peak is less than that after the peak. The hysteresis loop before the peak shows a development trend of “sparse-dense-sparse” with axial strain, while the hysteresis loop after the peak shows a development trend of “dense-sparse”. Red sandstone exhibits overall single slope shear failure under loading and unloading conditions, with considerable irreversible deformation occurring throughout the process before and beyond the peak. The mechanical evolution characteristics of red sandstone are manifested in four stages: pore crack compaction, stable development of microcracks, unstable development of cracks, and rock failure. The strength and deformation characteristics of red sandstone may be better described using the shear damage mechanical mechanism that takes into account compaction deformation. The analysis of the loading and unloading moduli shows that the pre-peak loading modulus generally increases with the increase in stress amplitude, while the unloading modulus tends to stabilize. The research results have certain reference values for analyzing tunnel surrounding rock damage and stability.

Key words: pre-peak and post-peak loading and unloading, compaction deformation, loading and unloading moduli, shear damage, mechanical characteristics