The instability of underground spaces in abandoned coal mines with water-immersed rocks is one of the main hazards hindering the geothermal energy use and ecological restoration of post-mining areas. This study conducted graded cyclic loading–unloading tests of five groups of sandstone samples with different water contents. The evolution of input, elastic, dissipated, damping, and plastic energies were explored, considering the damping effect. The normalized plastic energy serves to characterize the damage evolution of sandstone samples, whose failure characteristics were analyzed from both the macroscopic and microscopic perspectives. X-ray diffraction technique and scanning electron microscopy were used to reveal the softening mechanism of sandstone. The results show that under graded cyclic loading, input energy, elastic energy, and dissipated energy all increase gradually, and the fraction of elastic energy increases gradually at first and then tends to stabilize. The variation in the fraction of dissipated energy is opposite to that of elastic energy. In each cycle, the input energy is stored primarily in the form of elastic energy, whereas the dissipated energy is used primarily to overcome the damping of sandstone. When the normalized number of cycles approached unity, the plastic energy fraction sharply increases, while that of the dampening energy drops abruptly. With increasing water content, the effect of pore water on the lubrication, the water wedge, and dissolution of mineral particles becomes more obvious, reducing the elastic-storage limit of sandstone, meanwhile the sandstone damage factor increases significantly under the same cycle and the failure mode changes from brittle to ductile.

水浸岩废弃煤矿井下空间的不稳定性是阻碍地热能利用和采后区生态恢复的主要危害之一。本研究对五组不同含水量的砂岩样品进行分级循环加卸载试验。考虑到阻尼效应，探索了输入、弹性、耗散、阻尼和塑性能量的演变。归一化塑性能用于表征砂岩样品的损伤演变，从宏观和微观两个角度分析其破坏特征。X射线衍射技术和扫描电子显微镜被用来揭示砂岩的软化机制。结果表明，在分级循环载荷下，输入能量、弹性能量、和耗散能均逐渐增加，弹性能分数先逐渐增加，然后趋于稳定。耗散能量分数的变化与弹性能量的变化相反。在每个循环中，输入的能量主要以弹性能的形式存储，而耗散的能量主要用于克服砂岩的阻尼。当归一化循环数接近1时，塑性能量分数急剧增加，而阻尼能量急剧下降。随着含水量的增加，孔隙水对矿物颗粒的润滑、水楔作用和溶解作用的影响越来越明显，降低了砂岩的弹性贮藏极限，