Large-scale goafs are left after coal seam mining. Due to the low-lying terrain, the goaf will be filled and soaked by groundwater, which may lead to instability of the remaining coal pillars in the goaf and cause uneven settlement of the overlying rock. Consequently, there may be overlying rock movement and surface subsidence, which endangers the safety of the building (structure) above the goaf. Considering the strip goaf of Dai Zhuang coal pillar as an example, this study investigated the evolution of instability and deformation of surrounding rocks affected by water immersion using the similar material simulation test method. The results of the study reveal that under the effect of prolonged water immersion in the goaf, the damage to the coal pillar in the strip underwent a stagewise evolution process of several instances of creep damage at the edge of coal pillar followed by overall destabilization damage, and the overburden movement revealed stage characteristics of small step subsidence several times followed by sudden large subsidence. Furthermore, based on Wilson’s coal pillar instability theory, the instability mechanism of the strip coal pillar under the action of water immersion was found to be triggered by the reduced strength of the coal pillar from the effect of water immersion, the continuous creep damage to the strip coal pillar from outside to inside, and the continuous shortening of the elastic zone of the coal pillar until its bearing capacity was lower than the load it was carrying. The research results are expected to serve as theoretical guidance for the study of coal pillar stability and the development and utilization of surface construction above goafs.

The development pattern of fractured water-conducting zones (FWCZs) is closely related to mining safety and water conservation in mining areas. However, neither the development pattern nor the empirical formula in the prediction of the height of FWCZs are clear for fully mechanized caving mining conditions in wet collapsible loess areas. We took Lu’an Coal Mine as a case study, employing field measurements and numerical simulation to investigate surface subsidence and FWCZs. Under the conditions of fully mechanized top coal caving in thick coal seams, surface movement is more intense, and surface cracking is more frequent. Field measurements using boreholes indicated the ratio of fracture height to coal seam thickness was 19.1–23.4, and the ratio stays stable both by sliced fully mechanized mining and fully mechanized mining. The height of the FWCZ with fully mechanized mining is 1.37 times and 2.31 times larger than the height with sliced fully mechanized mining and ordinary mining, respectively. The numerical simulation by RFPA illustrated that the height of the FWCZ remained stable when it reached a certain rock level with the working face moving forward. Furthermore, we developed an empirical formula for predicating the maximum height of an FWCZ under fully mechanized mining conditions in wet collapsible loess areas. Finally, the research findings were applied in several coal mines to achieve safe production and water conservation. This work provides guidance for effectively mining safely and water conservation in similar geographic conditions.

Studying mining fracture development is vital for geotechnical and mining engineering and geological disaster prevention. This research assesses crack effects on rock mass stress equilibrium during coal mining, potentially causing geological disasters such as land subsidence and landslides. Using fractal geometry theory, the present study investigates the development of horizontal and vertical mining cracks, revealing their propagation patterns. The fractal dimension generally increases as the propulsion distance increases; however, fluctuations vary from 250 to 287.5 m, forming a wavering line chart. The proportion of mining fracture area relative to mining space area increases with greater propulsion distance, indicating expanded upward mining space due to separation layers. The horizontal distribution of mining cracks persists, while the vertical distribution decreases, suggesting ground subsidence results from upward transmission. The fastest increase in fractal dimension occurs at 87.5–100 m. At 250 m, it peaks at 1.4136, indicating complex crack structures. During propulsion, the fractal dimension decreases due to upward mining space expansion through overlying rock layer collapse, forming new cracks. The proportion of mining crack area to mining space area increases gradually throughout the mining process. The present study presents a simulation model for crack identification, noting limitations in identifying tiny cracks.

Similar material simulation experiments on grouting in the goaf, aiming to investigate the damage characteristics of the overburden and the movement behavior both before and after grouting were conducted. The results demonstrate that after grouting the goaf, large fissures, fractures, holes, and other cavities are effectively sealed by the slurry. Compared to untreated goaf, grouting significantly reduces subsidence, with a reduction rate of up to 92.3%. The slurry grouting compacts the residual voids in the goaf, slowing the further subsidence of the collapsed area. It also provides lateral support and a barrier to water infiltration in the residual coal pillars. Additionally, the slurry improves the stress conditions of these coal pillars, transforming the initial unidirectional (or bidirectional) stress state into a bidirectional (or triaxial) stress state. This significantly enhances the support strength and stability of the coal pillars.

The long-term stability of water-immersed coal pillars in proximity to urban areas is critical for ensuring the safety of urban development. This study investigates the creep damage evolution in strip coal pillars through a comprehensive approach integrating in-situ investigations, laboratory experiments, and numerical modeling based on a real-world case study. The analysis focuses on the evolution of stress distribution, creep damage propagation, and the mechanisms driving continuous damage in coal pillars. Findings indicate that in the studied case, concentrated stress around the elastic-plastic boundary progresses toward the central region of the coal pillar as creep damage expands. Initially, the propagation of creep damage is rapid but decelerates due to the residual strength of damaged coal, which bears the overburden pressure and provides lateral constraint. The state of creep damage expansion in a coal pillar depends on the evolution of triaxial creep strength of coal induced by the stress conditions of the intact coal body, which will be changed by the transferred overburden pressure and increased confining pressure.

Leveraging the complementary advantages of InSAR and GNSS, this study proposes a refined method for monitoring mining-induced surface subsidence by integrating both technologies. The method begins with calculating the time-series cumulative subsidence basin from InSAR. Subsequently, a constraint condition is established to identify large-gradient deformations, thereby distinguishing the subsidence edge from the subsidence center. For the subsidence edge with minor deformation, the InSAR results are retained. For the large-gradient subsidence center, the subsidence basin around the mining panel is reconstructed by integrating InSAR and GNSS models. Continuous surface deformation information in a geographic coordinate system is then obtained through spatial interpolation, ultimately yielding comprehensive surface subsidence results across the mining area. Taking a mining area in Shanxi Province as the study region, the feasibility and accuracy of the proposed method were validated using 35 SAR images acquired between April 2016 and September 2017, along with leveling measurement data from the mining panel. The maximum surface subsidence rate of the settlement basin obtained from the solution is −186.68 mm/year, and the maximum surface subsidence amount is 248 mm. Compared with the InSAR monitoring results, the root mean square error of the data collaborative monitoring is reduced by 96.8%, and it is reduced by 64.4% compared with the GNSS probability integral method. The results demonstrate that the proposed method can achieve subsidence results consistent with the actual situation. Its monitoring capability is significantly superior to that of using either InSAR or GNSS alone, effectively compensating for the limitations inherent in each individual technology when applied to mining subsidence monitoring. Consequently, this integrated approach provides more accurate and reliable information on surface subsidence in mining areas.