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.

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The study of the deformation characteristics and damage evolution law of the underground water-bearing rock mass under reciprocating loads such as mine earthquake and mechanical vibration is a very crucial aspect of underground engineering. In this pursuit, the present study was envisaged to assess the deformation characteristics and damage evolution law of sandstone with different water contents under various cycles. Specifically, the uniaxial and cyclic loading and unloading tests, X-ray diffraction (XRD), and scanning electron microscope (SEM) tests of the sandstone under dry, unsaturated, and saturated conditions were carried out under laboratory conditions. Subsequently, the change laws of elastic modulus, cyclic Poisson's ratio, and irreversible strain in the loading section of sandstone under different water content conditions were analyzed. Based on the two-parameter Weibull distribution, the coupled damage evolution equations of sandstone under water content and load were established. The results showed that with an increase in the water content in the sandstone, the loading elastic modulus of the corresponding cycles exhibited a gradual decrease. Microscopic analysis revealed that kaolinite was present in the water-bearing sandstone in a lamellar structure, with flat edges and many superimposed layers, and the proportion of kaolinite gradually increased with an increase in the water content. The poor hydrophilicity and strong expansibility of kaolinite are the key factors in reducing the elastic modulus of sandstone. With the increase of the number of cycles, the cyclic Poisson's ratio of sandstone experienced three stages: an initial decrease, followed by a slow increase, and finally a rapid increase. The decrease was mainly observed in the compaction stage; the slow increase existed in the elastic deformation stage; and the rapid increase was seen in the plastic deformation stage. Furthermore, with the increase of water content, there was a gradual increase in the cyclic Poisson's ratio. The concentration degree of the distribution of the rock microelement strength (the parameter m) under the corresponding cycle of sandstone with different water content states exhibited an initial increase followed by a subsequent decrease. With the increase in the water content, the parameter m under the same cycle gradually increased, and the change rule of parameter m corresponded to the development of internal fractures in the sample. With an increase in the number of cycles, the internal damage of the rock sample gradually accumulated, and the total damage increases gradually but the growth rate decreases gradually.

The key to the construction of underground reservoirs in abandoned mines is the construction of coal pillar-artificial dams, and the choice of bonding parameters between the coal pillars and artificial dams is the deciding factor that determines the engineering stability. Based on the analysis of the force state of coal pillar-artificial dams, the influence of the interface angle was analyzed. Seven sets of coal pillar-artificial dam specimens were prepared and a PFC3D numerical model was constructed to carry out the uniaxial compression test without lateral pressure. Based on the strength, deformation, and energy evolution characteristics of the coal pillar-artificial dam, the influence of the angle of the coal pillar-artificial dam interface on the performance of the specimen was analyzed. The PFC3D model was used to investigate crack evolution, particle displacement, and spatial distribution. The research results showed that the force state of the coal pillar-artificial dam can be divided into three types: split bearing, shared bearing, and coordinated bearing, corresponding to three different constitutive models. The composite simulation curve showed obvious post-peak viscosity. The compressive strength, peak strain, and average dissipated energy curves of the coal pillar-artificial dam showed a unimodal trend that first increased and then decreased. The total energy and elastic energy of the coal pillar-artificial dam showed an increasing trend during loading. The dissipation energy curve increased obviously in the early stage, then flattened, and finally, decayed. The simulated initiation stress and damage stress of the coal pillar-artificial dam specimens were intermediate to that of the coal pillars and the artificial dams, which first increased and then decreased with the increase in inclination, reaching the peak at 70°. The failures of the single and combined models were both dominated by monoclinic splitting. As the inclination increased, the position of the main cracks gradually shifted downwards and then upwards.

The instability of underground spaces in abandoned coal mines with water-immersed rocks is one of the main hazards hindering the geothermal energy utilization 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 laws of input, elastic, dissipated, damping, and plastic energies were explored in detail, taking into account the damping effect. The normalized plastic energy was used to characterize the damage evolution of sandstone samples, which failure modes were analyzed from both macroscopic and microscopic perspectives. The X-ray diffraction technique and scanning electron microscopy were used to reveal the softening mechanism of sandstone's strength and elastic energy storage limit. The results showed that the graded cyclic loading's input, elastic, and dissipated energies increased gradually. The elastic energy share first increased and then stabilized, while dissipated energy share variation had the opposite trend. In each cycle, the input energy was primarily stored in the form of elastic energy, while the dissipated energy was mainly used to overcome the damping of sandstone. When the normalized number of cycles approached unity, the plastic energy share sharply increased, while that of the dampening energy featured an abrupt drop. Such change indicated an inevitable instability failure of the water-bearing sandstone. As the water content increased, the pore water exhibited more substantial lubrication, water-wedging, and dissolution effects on mineral particles. As a result, the latter obtained a round form, and the elastic energy storage limit of the sandstone decreased. When the water content was increased, the damage factor of sandstone after the same number of cycles increased at a relatively higher rate, and there was a transition of failure mode from brittle to ductile.