Ground surface damage caused by steeply inclined coal seam mining is widely distributed in China, but there is little research on the failure process and movement mechanism of strata induced by steeply inclined multi-seam mining. In this paper, a physical model test is carried out to study the failure process and movement mechanism of overburden in steeply inclined multi-seam stepwise mining. The results show that at the initial stage, the main failure of the rock mass is the small-scale collapse at the initial cut and the roof (stability stage of the rock mass). After the roof is exposed over a certain range, the rock mass in the downhill direction slips into the goaf and gradually destroys the interburdens of the goaf, similar to the displacement effect of dominoes (severe failure stage of the rock mass). When the structure of the goaf fails, the overburden subsides, causing extensive damage to the ground surface. The surface damage directly above the goaf is mainly caused by serious subsidence deformation, while the surface damage in the downhill direction is dominated by cracks.

The compaction characteristics of broken rock in a caving zone have a significant impact on the movement law of overburden and the prediction of surface subsidence. The mechanical properties of the broken rock were clearly affected by the original rock strength of the roof. Based on the similarity theory, the ‘quartz sand-gypsum-lime-water’ mixed material was used to make similar samples of original rocks with different strengths, and the compaction mechanical behaviour of broken loose rock masses with different original rock strengths was studied. The results show that (i) the greater the original rock strength of broken rock, the shorter the initial compaction stage, the earlier the transition and stable compaction stages and the lower the degree of compaction; (ii) the initial deformation modulus and ultimate axial strain had a linear relationship with the original strength of the broken rock; and (iii) under different axial pressures, the deformation modulus increased with the increasing original rock strength of the broken rock. The tangent modulus and axial stress change approximately linearly, the secant modulus and stress change linearly, and the tangent modulus and secant modulus exhibit an exponential/hyperbolic relationship with the strain. The research results have high engineering application value for using numerical method to predict the mechanical behaviour of roof rock mass with different strength in coal mining and analysing the surface subsidence.

The rock mass in the caving zone of a goaf is relatively broken and considered a porous medium. Additionally, it has the characteristics of irregular size and shape and sharp edges, and it is easy to break. In the process of caving zone compaction, the shape characteristic of a broken rock mass is one of the most important factors affecting the evolution of the compaction characteristics and the water permeability of the caving zone. Through discrete element numerical simulation and theoretical research, the influence of the shape characteristic on compaction characteristics and the water permeability of a broken rock mass is analyzed. The research results are as follows: (1) The number of edges on a caved broken rock mass is negatively correlated with the strain limit of compaction, the initial void ratio and the final breaking ratio. It is positively correlated with the deformation modulus and the residual dilatancy coefficient. (2) The smaller the amount of edges on the broken rock mass, the more obviously the rotation movement occurs during compaction. (3) The smaller the number of edges on the broken rock mass, the faster the decline in the rate of the water permeability, and the lower the water permeability at the final stable stage. (4) With an increasing number of broken rock mass edges, the total strain energy and the dissipative strain energy of caved broken rock masses show a decreasing trend, while the elastic strain energy shows a growing trend.

Coal mining-induced ground subsidence is a severe hazard that can damage property, infrastructure, and the environment in the vicinity when the deformation is not negligible. The boundary of a mining-induced subsidence-affected zone refers to the area beyond which the ground subsidence is less concerned. Accurately measuring mining-induced ground deformation is essential for delineating the irregular boundary of the impacted area. This study employs multitemporal interferometric synthetic aperture radar (MT-InSAR) techniques, including differential InSAR (DInSAR), InSAR stacking, and interferometric point target analysis (IPTA), to analyze coal mine subsidence and delineate the boundaries of the mining-impacted zones. DInSAR accurately reconstructs, locates, and detects the trend in mining-induced subsidence and correlates well with documented mining operations. The InSAR stacking method maps the spatial variation of the ground’s average line-of-sight (LOS) velocity over the mining area, delineating the boundary of the impacted zone. IPTA analysis combining multilook and single-pixel phases achieves millimeter-level surface measurement above tunnel alignments and measures unevenly distributed deformation fields. This study considers an average of 4 cm per year of surface deformation in the LOS direction as the subsidence threshold value for delineating the boundary of the mining-induced subsidence-affected (MISA) zone during the active coal mining stage. Interestingly, there are twin transportation tunnels near the mining area. The twin tunnels completed before the coal mining activities started were functioning well, but damage was observed after the mining began. Our study reveals the tunnels are located within the InSAR-derived MISA zone, although the tunnels approach the MISA boundary. As direct signs of subsidence, ground fissures have been identified near the tunnels via field investigations and UAV photogrammetry. Furthermore, the derived distribution of ground fissures validates and verifies InSAR measurements. The integrated approach of MT-InSAR, UVA photogrammetry, and field investigation developed in this study can be applied to delineate the irregular boundary of the MISA zone and study the accumulating effects of mining-induced subsidence on the performance of infrastructure in areas proximate to coal mining activities.

As a typical sedimentary rock, the number of beddings in the horizontal direction of sandstone is far greater than that in the vertical direction, leading to its physical and mechanical properties showing obvious anisotropy with changes in bedding angle. After high temperature exposure, bedding damage further transforms the change rule of the physical and mechanical properties of sandstone with the bedding angle. This study tested the appearance, wave velocity, uniaxial compression, and conventional triaxial compression properties of sandstone with five bedding angles before and after high temperature exposure. The results show that (1) the longitudinal wave velocity, shear wave velocity, elastic modulus, and cohesion decreased, while the internal friction angle increased slightly. At the same temperature, when the dip angle of sandstone was 30° or 60°, the mechanical properties were optimal, and when the dip angle was 45°, the mechanical properties were the worst. (2) High temperature increases the development degree of micropores and microfractures in the sandstone bedding plane and matrix, thus increasing the anisotropy degree of the physical and mechanical properties of sandstone with different bedding angles. (3) With increasing temperature, the rock samples gradually transitioned from brittle failure to ductile failure. Sandstone with a bedding angle of 0° presented splitting failure that vertically penetrated the bedding plane at different temperatures. Sandstone with dip angles of 30° and 40° presented shear failure that penetrated the matrix and bedding plane. A failure plane along the bedding plane appeared at the end. Sandstone with dip angles of 60° and 90° was more prone to failure along the bedding plane, showing shear failure along the bedding plane and tensile failure along the bedding plane, respectively.

Grouting is the most widely used technology for treating coal goafs. In this study, a numerical simulation method was used to establish a model of multi-seam goafs with different spacing conditions to investigate the subsidence reduction effects of various grouting schemes on multi-coal seam goafs. By varying the range and opportunity of grouting treatments, the effects of coal seam spacing, grouting treatment range, and grouting opportunity on subsidence reduction were analyzed. The results showed that: (1) With constant overburden (OB), the subsidence reduction ratio of the subgrade center increases linearly as the interburden (IB) decreases (1 ≤ OB/IB ≤ 2), then increases exponentially (2 < OB/IB), and eventually becomes stable. (2) When treatment is conducted based on the half-width of the subgrade, the width of the subgrade, and the range of the trapezoid, residual surface subsidence tends to adopt an inclined ‘W’-shape in open cutting. The surface residual subsidence exhibits a symmetrical ‘W’-shape when full-range grouting is adopted. (3) For a multi-coal seam goaf with longer mining stoppage time, the subsidence reduction ratio of the subgrade center is lower, and it is exponentially related to the grouting opportunity. As the grouting opportunity is extended and OB/IB decreases, the subsidence reduction ratio of the subgrade center declines exponentially.