The transportation of coal-based solid waste filling slurry (CSWFS) through pipelines for underground goaf injection is essential for enhancing mine safety and promoting green, low-carbon coal mining. To address the issue of pipeline blockage caused by the suspension sensitivity of CSWFS during long-distance transportation, this study proposes the addition of the suspending agent hydroxypropyl methyl cellulose (HPMC) to transform the filling slurry into a stable suspending slurry. The mechanism by which the suspending agent modifies the rheological property of CSWFS was elucidated and verified. Firstly, an evaluation index system for the suspending state of CSWFS based on the “experimental test and theoretical calculation” was established. The values for layering degree, bleeding rate time-loss, and the corresponding average time-loss rate over 0 to 120 min of A1–A5 CSWFS were recorded as 24 mm–2 mm, 3.0–0.2%, 252.4–54.2%, and 149.6–14.6%, respectively. The concentration gradient evaluation result, C/C_A = 0.91 (≥0.8), confirmed that the suspending agent maintained a stable suspending state over time for CSWFS. Secondly, it was demonstrated that the suspending agent HPMC modified the rheological property of A1–A5 CSWFS by increasing its plastic viscosity, which strengthened the viscous resistance to particle settling, thereby transforming a semi-stable slurry into a stable one. Additionally, the formation of a spatial suspending network by the suspending agent ensures that no pipeline blockage accidents occured in practical engineering applications. Furthermore, the XRD and SEM tests were utilized to verify the microstructure of the top (T) and bottom (B) samples in A4 block. It was concluded that the type of hydration products, occurrence forms, lapping compactness, and microstructural development were consistent, ultimately forming a high-strength, dense, hardened filling block. Finally, numerical simulation confirmed that the addition of suspending agent in A4 slurry formed a comprehensive spatial suspending network and a well-structured, unified system. This is one effective approach which could contribute to addressing the technical issue of pipeline blockage during long-distance pipeline transportation.

In this paper, we investigate the evolution characteristics of floor failure during pressured mining in extra-thick coal seams. A mechanical expression relating floor failure depth to seam thickness is established based on soil mechanics and mine pressure theory. The findings reveal a linear relationship between seam thickness and floor failure depth; specifically, as the coal seam thickens, the depth of floor failure increases. To simulate the mining process of extra-thick coal seams, FLAC3D numerical simulation software is utilized. We analyze the failure process, failure depth, and the behavior of water barriers at the coal seam floor under the influence of extra-thick coal seam mining from three perspectives: rock displacement evolution in the floor, stress evolution in the floor, and plastic deformation. Based on geological characteristics observed in the Longwanggou mine field, we establish a main control index system for assessing floor water-inrush risk. This system comprises 11 primary control factors: water abundance, permeability, water pressure, complexity of geological structure, structural intersection points, thickness of both actual and equivalent water barriers, thickness ratio of brittle–plastic rocks to coal seams, as well as depths related to both coal seams and instances of floor failure. Furthermore, drawing upon grey system theory and fuzzy mathematics within uncertainty mathematics frameworks leads us to propose an innovative approach—the interval grey optimal clustering model—designed specifically for risk assessment concerning potential floor water inrush during pressured mining operations involving extra-thick coal seams. This method of mine water inrush risk assessment is applicable for popularization and implementation in mines with analogous conditions, and it holds practical significance for the prevention of mine water damage.

The collapse of thick-hard roofs after coal has been extracted is not a consequential process in all cases. Rather, it happens due to the augmentation of high stress conducted at depth, followed by a wider range of damage as the floor cracks. The extent and spread of the cracks in the floor indicate the intensity of the collapse, and the mine will be submerged by the high-pressure water of the coal ash. Therefore, it is particularly important to study the mechanism of the combined effect of high stress on the roof and confined aquifer on the deformation and failure of the coal seam mining floor. This study analyzes and compares the impact of thick-hard magmatic rocks on the destruction of thin floor rock layers in coal seams. Plastic theory calculations are used to determine the plastic zone yield length of floor destruction under hard roof conditions, and the location and height of the maximum floor destruction depth are solved. An empirical formula and BP neural network are used to establish a prediction model for floor destruction. The results of the model’s prediction of the depth of floor failure were compared with the measured values, with an absolute error of 2.13 m and a residual of 10.3%, which was closer to the true values. The accuracy of the theoretical model and prediction model is verified using numerical simulation and on-site in situ measurements. Based on this, the deformation and destruction forms of the floor under pressure and the water inrush mechanism are summarized for mining under the condition of a thick-hard roof. Thus, the floor is subjected to high vertical stress, accompanied by significant disturbances generated during coal seam mining, resulting in intense working face pressures. The floor near the working face coal wall will experience severe compression and shear deformation and slide towards the goaf. The floor in the goaf is relieved of high vertical stress, and horizontal stress compression will result in shear failure, leading to floor heave and further increasing the height of the floor destruction zone. After the mining of the working face, the goaf will undergo two stages of re-supporting and post-mining compaction. During the re-supporting stage, the floor rock undergoes a transition from high-stress to low-stress conditions, and the instantaneous stress relief will cause plastic deformation and failure in the coal seam floor. The combined action of primary floor fractures and secondary fractures formed during mining can easily create effective water channels. These can connect to the aquifer or water-conducting structures, making them highly dangerous. The main modes of floor water inrush under the condition of a thick-hard roof are as follows: the high-stress mode, inducing a floor destruction zone connected to the water riser zone; the mining damage mode, connecting to water-conducting faults; the mining damage mode, connecting to water collapse columns; and the coupled water inrush mode, between the mining damage zone and the highly pressurized water floor.

In the central and western regions of China, coal mining operations are progressively transitioning from shallow to deep strata. This shift is accompanied by an increase in mining height and a rise in the degree of automation, leading to uneven and potentially violent settlement of overlying rock formations, as well as the formation of single or multi‐layer separation spaces above the coal seam. If the water‐conducting fracture zone intersects with the water‐bearing separation layer, it can result in catastrophic mine accidents such as water gushes and sand collapses on the working face. Based on the research background of a specific working face in the No.7 coal seam at Qianjiaying Mine, this study employs empirical formulas, BP neural networks, similarity simulations, and UDEC numerical simulations to investigate the evolution of overburden fractures and the development of separation layers under high‐intensity mining conditions. The study also analyzes the water inrush modes and mechanisms of separation layers in high‐intensity mining, leading to the following conclusions: (1) By collecting data from 42 different coal mining faces in China, empirical formulas and BP neural network prediction models for the development height of the water‐conducting fracture zone in overlying rock were established. The relative and absolute errors of the prediction results from both models were compared. For 90% of the predicted data, the error rate of the BP neural network model was less than 0.5%, with an average error of approximately 3%. (2) Through two similar simulation tests, where only the mining height of the coal seam (3 and 6 m) was varied while controlling other variables, it was concluded that higher mining heights lead to more severe damage and obvious fracture development in overlying rock. The presence of a main key layer and thick sandstone increases the expansion time and duration of the separation space. Overburden failure in high‐intensity mining exhibits a “Π” shape, with the failure height primarily influenced by the high‐intensity mining mode, overburden structural combination, and the load of the “beam‐arch structure,” as well as the overburden dilatation coefficient. (3) Numerical simulation results indicate that under different mining height conditions, larger mining heights result in more pronounced settlement of the main key layer, with step subsidence and sliding phenomena observed on both sides of the key layer. (4) Based on the aforementioned research methods, this paper explores the formation mechanisms of separation layers in both single‐seam and multi‐seam mining, focusing on the analysis of repeated disturbances in multi‐seam mining, dynamic water inrush, and structural water inrush modes. It comprehensively elucidates the mechanisms of separation layer water inrush in high‐intensity mining.

The softening and instability of the coal pillar caused by water immersion is the primary factor contributing to water inrush in the goaf of the same layer. Similarly, the softening and instability of underground reservoir dam bodies due to water immersion is a critical factor leading to reservoir water failure and dam breakage. Investigating the dynamic process of coal pillar softening and instability under water immersion conditions is of paramount importance for the prevention and control of mine water hazards in mining areas and the safety assessment of underground reservoirs. This study uses the mining of the A ore strip as an engineering background, conducts theoretical analyses and laboratory tests on coal pillars subjected to varying soaking durations, and examines the macroscopic and microscopic damage and deterioration processes of coal pillars under soaking conditions from the perspectives of mechanical properties, hydrochemical characteristics, and microstructural characterization. The experimental results indicate that within 0 to 40 days of soaking, the uniaxial compressive strength of the coal pillar decreases by 50%, the pH value of the solution shifts from weakly acidic to weakly alkaline, multiple ion concentrations undergo significant changes, mineral compositions alter, and porosity exhibits a pronounced changing trend. Based on these research findings, this paper explores the physicochemical coupling mechanism of internal structural evolution and damage deterioration in soaked coal pillars, elucidating the entire process of damage-deterioration-instability. The dynamic process of water-immersed coal pillars was studied and visualized using similar materials with water-blocking additives, revealing their spatiotemporal evolution laws. Through mechanical derivations and laboratory test results, this paper identifies the key factor influencing the instability of the overlying rock mass above the coal pillar roof, termed the “effective support width,” and investigates the mechanical deterioration mechanisms of water-immersed coal pillars. Finally, based on theoretical research and indoor experiments, considering the dual effects of stress and water pressure, the calculation formula for the effective support width of water-immersed coal pillars was derived, clarifying the evolution mechanism of “damage-deterioration-instability” in the overburden rock mass above the coal pillar roof. This study discusses the dynamic process of coal pillar softening-instability under water immersion conditions in goaf areas, which holds significant implications for the retention of waterproof coal pillars in coal mines, safe mining practices, and the protection and recycling of coal pillars. Additionally, research into the damage-deterioration-instability of coal pillars provides valuable guidance for the construction of underground reservoirs and the determination of appropriate widths for coal pillar dam bodies.