Search Results (310)

The abandoned goaf resulting from coal resource integration in China poses a significant threat to coal mine safety. The transient electromagnetic method (TEM) has emerged as a crucial technology for detecting goafs in coal mines due to its adaptable equipment and efficient implementation. In recent years, small-loop TEM has demonstrated high resolution and adaptability in challenging terrains with vegetation, such as coal mine ponding areas, karst regions, and reservoir seepage scenarios. By considering the sedimentary characteristics of coal seams and addressing the resistivity changes encountered in single-point inversion, a joint optimization inversion process incorporating lateral weighting factors and vertical roughness constraints has been developed to enhance the connectivity between adjacent survey points and improve the continuity of inversion outcomes. Through an OCCAM inversion approach, the regularization factor is dynamically determined by evaluating the norms of the data objective function and model objective function in each iteration, thereby reducing the reliance of inversion results on the initial model. Using the Xiaolong Coal Mine as a geological context, the impact of lateral and vertical weighting factors on the inversion outcomes of high- and low-resistivity structural models is examined through a control variable method. The analysis reveals that optimal inversion results are achieved with a combination of a lateral weighting factor of 0.5 and a vertical weighting factor of 0.1, ensuring both result continuity and accurate depiction of vertical and lateral electrical interfaces. The practical application of this approach validates its effectiveness, offering theoretical support and technical assurance for old goaf detection in coal mines, thereby holding significant engineering value. Full article

The formation of acidic goaf water in abandoned coal mines poses significant environmental threats, especially in karst regions where the risk of groundwater contamination is heightened. This study investigates the geochemical processes responsible for the generation of acidic water through batch and column leaching experiments using coal mine surrounding rocks (CMSR) from Yangquan, China. The coal-bearing strata, primarily composed of sandstone, mudstone, shale, and limestone, contain high concentrations of pyrite (up to 12.26 wt%), which oxidizes to produce sulfuric acid, leading to a drastic reduction in pH (approximately 2.5) and the mobilization of toxic elements. The CMSR samples exhibit elevated levels of arsenic (11.0 mg/kg to 18.1 mg/kg), lead (69.5 mg/kg to 113.5 mg/kg), and cadmium (0.6 mg/kg to 2.6 mg/kg), all of which exceed natural crustal averages and present significant contamination risks. The fluorine content varies widely (106.1 mg/kg to 1885 mg/kg), with the highest concentrations found in sandstone. Sequential extraction analyses indicate that over 80% of fluorine is bound in residual phases, which limits its immediate release but poses long-term leaching hazards. The leaching experiments reveal a three-stage release mechanism: first, the initial oxidation of sulfides rapidly lowers the pH (to between 2.35 and 2.80), dissolving heavy metals and fluorides; second, slower weathering of aluminosilicates and adsorption by iron and aluminum hydroxides reduce the concentrations of dissolved elements; and third, concentrations stabilize as adsorption and slow silicate weathering regulate the long-term release of contaminants. The resulting acidic goaf water contains extremely high levels of metals (with aluminum at 191.4 mg/L and iron at 412.0 mg/L), which severely threaten groundwater, particularly in karst areas where rapid cross-layer contamination can occur. These findings provide crucial insights into the processes that drive the acidity of goaf water and the release of contaminants, which can aid in the development of effective mitigation strategies for abandoned mines. Targeted management is essential to safeguard water resources and ecological health in regions affected by mining activities. Full article

It is vital to stabilize pillar dams in underground reservoirs in coal mine goafs to protect groundwater resources and quarry safety, practice green mining, and protect the ecological environment. Considering the actual occurrence of coal pillar dams in underground reservoirs, acoustic emission (AE) mechanical tests were performed on dry, naturally absorbed, and soaked coal samples. According to the mechanical analysis, Quantitative analysis revealed that dry samples exhibited the highest mechanical parameters (peak strength: 12.3 ± 0.8 MPa; elastic modulus: 1.45 ± 0.12 GPa), followed by natural absorption (peak strength: 9.7 ± 0.6 MPa; elastic modulus: 1.02 ± 0.09 GPa), and soaked absorption showed the lowest values (peak strength: 7.2 ± 0.5 MPa; elastic modulus: 0.78 ± 0.07 GPa). The rate of mechanical deterioration increased by ~25% per 1% increase in moisture content. It was identified that the internal crack development presented a macrofracture surface initiating at the sample center and expanding radially outward, and gradually expanding to the edges by adopting AE seismic source localization and the K-means clustering algorithm. Soaked absorption was easier to produce shear cracks than natural absorption, and a higher water content increased the likelihood. The b-value of the AE damage evaluation index based on crack development was negatively correlated with the rock damage state, and the S-value was positively correlated, and both effectively characterized it. The research results can offer reference and guidance for the support design, monitoring, and warning of coal pillar dams in underground reservoirs. (The samples were tested under two moisture conditions: (1) ‘Soaked absorption’—samples fully saturated by immersion in water for 24 h, and (2) ‘Natural absorption’—samples equilibrated at 50% relative humidity and 25 °C for 7 days). Full article

In order to examine the fracture development law of overlying strata in goafs and to reasonably lay out a high gas-drainage roadway under gob-side entry retaining with roadside filling, the 91–105 working face of the Wangzhuang Coal Mine was selected as the engineering case study. The failure laws and fracture development characteristics of the overlying strata in both the strike and dip directions using gob-side entry retaining and roadside filling were studied through rock mechanic tests and PFC numerical simulations. The optimal layout of the high gas-drainage roadway was determined through theoretical analysis and coupled Fluent–PFC numerical simulations, and on-site monitoring was conducted to evaluate the extraction effects. The results indicate that the first weighting interval of the 91–105 working face was 40 m, while the periodic weighting interval was approximately 14 m. The height of the falling zone was 14.4 m, and the height of the gas-conducting fracture zone was 40.7 m. In the dip direction, compared with coal pillar retaining, gob-side entry retaining with roadside filling formed an inverted trapezoid secondary breaking zone above the retaining roadway. Using this method, the span of the separation zone increased to 30 m, and the collapse angle decreased to 52°, resulting in a shift in the separation zone—the primary space for gas migration—toward the goaf. It was determined that the optimal location of the high gas-drainage roadway was 28 m above the coal roof and 30 m horizontally from the return air roadway. Compared with the 8105 working face, this position was 10 m closer toward the goaf. On-site gas extraction monitoring data indicate that, at this optimized position, the gas concentration in the high gas-drainage roadway increased by 22%, and the net gas flow increased by 18%. Full article

Aiming at the problem that the surface subsidence caused by coal mining in mountainous areas will pose a potential threat to the safe operation of gas pipelines in goaf subsidence areas, taking the geological conditions of Mugua Coal Mine in Shanxi Province as the research background, through the combination of similar simulation and finite element simulation, the deformation and stress characteristics of gas pipelines affected by subsidence in mountainous terrain are analyzed, and the failure law of gas pipelines in different terrains of the coal mining area is revealed. The results demonstrate that topographic stress convergence creates a maximum compression zone at the valley base of the central subsidence basin, causing significant pipeline depression. Hillslope areas primarily experience tension from soil slippage, while slope–valley transition zones exhibit a high-risk shear–tension coupling. Analysis via the pipe–soil interaction model reveals concentrated mid-subsidence pipeline stresses with subsequent relaxation through redistribution. Accordingly, the following zoned protection strategy is proposed: enhanced compression monitoring in valley segments, tensile reinforcement for slope sections, and prioritized shear prevention in transition zones. The research provides a theoretical basis for the safe operation and maintenance of gas pipeline networks in mountainous areas. Full article

The mining of shallow coal seam groups triggers mine water inrush and ecological environment destruction. Effective groundwater prevention and control requires controlling the compaction and seepage characteristics (CSCs) of broken rock in goaf. In this study, the CSCs of roof lithology and goaf broken rock combinations are experimentally investigated. The results indicate that, for samples with identical gradation, the percentage of void (PV) is minimized in sandstone–mudstone combinations, while PV increases with higher coal content. Initial compaction of composite samples is primarily governed by soft rock re-crushing, whereas the stable compaction stage is determined by the initial PV. Under low axial stress, the CSCs of lithological combination samples exhibit instability, with the mudstone layer reducing flow velocity by approximately 36% under equivalent compaction and seepage conditions. Particle migration, leading to the blockage of the seepage section, is an important cause of the decrease in permeability. Based on experimental findings, a stress–void–seepage coupling model is established to describe the compaction–seepage behavior of lithologic combination broken rock in shallow goafs. Full article

Repeated mining under insufficiently collapsed gobs is a complex process in underground mining and is associated with safety hazards such as ground collapse and subsidence. The effect of layer spacing on the fracture network evolution and fluid transport mechanisms in rock strata during this process has not been systematically studied. In this work, the discrete element method was employed to analyze the fracture development and seepage evolution of surrounding rocks in the Nanliang coal mine across varying layer spacings (5, 20, 35, 50, and 65 m). A systematic evaluation of the rock mass integrity was conducted through damage coefficient quantification. The key findings revealed that an increase in the layer spacing progressively reduced the damage coefficients in both the overburden strata above the goaf and in the interlayer formations ahead of the working face, accompanied by reduced fracture propagation intensity. Shear failure mechanisms dominated throughout the mining process. Fractal characteristics of the fractures intensified with the advance of the working face, while the hydraulic conductivity and interstitial pressure in the interlayer strata exhibited declining trends with reduced attenuation rates. Our findings provide critical insights for ensuring the safety and improving the efficiency of repeated mining under insufficiently collapsed gobs. Full article

The layout of directional high-level long boreholes in fracture zones for extracting pressure-relieved gas from goaf is a key technology to address the gas concentration exceedance in upper corners. To solve the gas exceedance issue at Fengtai Mine’s upper corner, this study established a gas–solid coupling model using COMSOL Multiphysics 6.3 based on actual mine parameters. The research investigated the influence patterns of different extraction parameters (negative pressure, borehole diameter, and extraction duration) on coal seam gas pressure and effective extraction radius (critical gas pressure 0.74 MPa). The results demonstrate the following. The effective extraction radius shows positive correlations with extraction negative pressure, borehole diameter, and extraction time while exhibiting a negative correlation with gas pressure. When borehole diameter exceeds 203 mm, extraction negative pressure surpasses 25 kPa, and extraction duration extends beyond 90 days; the effective extraction radius stabilizes, with the gas pressure influence range ceasing to decrease significantly with further parameter increases. Field validation results showed that, during the observation period, both pure gas extraction volume and mixed gas volume exceeded those of high-level drainage galleries. The gas concentrations in both the upper corner and return airflow remained within safe limits, effectively resolving the gas exceedance issue. This achievement not only established an efficient gas control system for the Fengtai Mine but also provides valuable parameter optimization methods and engineering experience for similar coal mines. The implementation has significantly enhanced gas control efficiency and safety production levels in the mining area. Full article

Hidden mining-induced fissures connected to a goaf may induce spontaneous combustion of abandoned coal, threatening safe coal mining operation and ecological and environmental protection. To identify hidden mining-induced fissures rapidly, accurately and in a timely manner, a novel method involving infrared remote sensing via an unmanned aerial vehicle (UAV) was proposed. Hidden mining-induced fissures above working face No. 52605 of the Daliuta coal mine were continuously monitored using this method. Field experiments revealed that hidden mining-induced fissures could be effectively identified via infrared technology. The diurnal variation in the hidden mining-induced fissure temperature was cosinusoidal. The temperature of the hidden mining-induced fissures was highly correlated with burial depth, and the burial depths of the identified hidden mining-induced fissures differed at various times. The temperature differences among hidden mining-induced fissures, aeolian sands and vegetation varied with time and burial depth. The temperature difference variation between in situ hidden mining-induced fissures and aeolian sand matches that between hidden mining-induced fissures at a 20 cm burial depth and sand. In situ hidden mining-induced fissures could be identified from 1:00 to 5:00 a.m. and from 11:00 a.m. to 7:00 p.m. under the studied conditions. Full article

In coal seam mining operations, the presence of overlying water bodies presents persistent challenges, particularly during multi-seam extraction, where water accumulation in upper seam goafs requires careful management. This study examined the Lingzhida Coal Mine, focusing on the geological conditions of the 3# seam (upper) and the 15# seam (lower), as well as the distribution of water accumulation in the corresponding goafs. The mechanism of water inrush from the upper goaf was studied, and the role of the water-resisting belt (WRB) is suggested. By utilizing empirical equations and field measurements, a method for calculating the floor fracture depth of the 3# seam and the roof fracture height of the 15# seam was derived through multi-linear regression analysis. Based on the relationship between the thickness of the WRB (H_w) and the protective layer (H_p), a classification criterion for the water-inrush risk (the likelihood of water entering the lower seam from the upper goaf) is proposed. The mining area was divided into four risk zones: high-risk (H_w < 0), medium-risk (0 ≤ H_w < 0.5H_p), low-risk (0.5H_p ≤ H_w < H_p), and safe (H_w ≥ H_p). Then, an adaptive zoning approach for water-preserved mining was introduced, considering the spatial distribution of goaf water. This approach incorporates water-preserved mining technologies, including the staggered layout of working faces, reduction in mining height, and the transfer–storage of water resources. These research findings provide crucial insights for ensuring the safe and efficient extraction of the multi-seam. Full article

Based on Brillouin optical time-domain reflectometry (BOTDR) technology, this study integrates laboratory tensile tests and similarity simulation experiments to systematically investigate the relationship between overlying strata collapse and fiber strain during coal seam mining. An analytical expression was established to describe the correlation between overlying strata displacement and fiber strain. The horizontal fiber monitoring results indicate that fiber strain accurately captures the evolution of overlying strata collapse and exhibits strong agreement with actual displacement height. When the working face advanced to 115 m and 155 m, the rock strata primarily underwent stress adjustment with minimal failure. At 195 m, the collapse zone expanded significantly, resulting in a notable increase in fiber strain. By 240 m, severe roof failure occurred, forming a complete caving zone in the goaf. The fiber strain curve exhibited a characteristic “double convex peak” pattern, with peak positions closely corresponding to rock fracture locations, further validating the feasibility of fiber monitoring in coal seam mining. Vertical fiber monitoring clearly delineated the evolution of the “three-zone” structure (caving zone, fracture zone, and bending subsidence zone) in the overlying strata. The fiber strain underwent a staged transformation from compressive strain to tensile strain, followed by stable compaction. The “stepped” characteristics of the strain curve effectively represented the heights of the three zones, highlighting the progressive and synchronized nature of rock failure. These findings demonstrate that fiber strain effectively characterizes the collapse height and evolution of overlying strata, enabling precise identification of rock fracture locations. This research provides scientific insights and technical support for roof stability assessment and mine safety management in coal seam mining. Full article

To address the challenges in the collaborative control of strong mine pressure and surface damage during fully mechanized shallow soft coal seam top-coal caving mining, this study takes the 22,031 working face of Xindeng (Zhengzhou, China) Coal Mine as the research background. By combining analytical modeling and discrete-element granular flow simulation, this research elucidates how overburden fractures evolve and how the ground surface responds throughout the mining of shallow, soft coal seams. This research shows that the mechanical model analysis based on plate theory indicates that the first fracture of the immediate roof occurs 0.5 m from the goaf side of the mined-out area. Numerical simulations demonstrate that when the working face advances 80 m, the mining-induced influence extends to the surface. The displacement field of the overburden undergoes a dynamic temporal evolution law following the sequence of “rectangle–trapezoid” → “hyperbola-like” → “trapezoid”. During the advancement of the working face, the fracture pattern of the overburden evolves from “rectangle–trapezoid” to “trapezoid”, and the affected range on the surface transforms from an “inverted trapezoid” to a “trapezoid”. This study ultimately clarifies the dynamic law of collaborative deformation between the overburden and the surface, providing a theoretical basis for the safe mining of shallow coal seams, the prevention of roof accidents, and the optimization of mining technology. Full article

To address the issues of roadway instability and severe air leakage in goaf areas during overlapping coal pillar mining in shallow multi-seam coalfields, this study takes the 22,209 working face of Huojitu Shaft in the Shendong Daliuta Mine as the research object. Using the discrete element method (DEM), the optimal layout of roadways in the lower coal seam and the corresponding evolution of overburden fractures were simulated. In addition, the effectiveness of goaf backfilling in controlling overburden air leakage channels was analyzed and verified. The results indicate that the width of coal pillars in the upper seam should be greater than approximately 23 m to ensure that roadways remain in a stress-stable zone. Roadways in the lower seam should be horizontally arranged within a range of 35–55 m from the center of the overlying coal pillar. This layout effectively avoids placing the roadway beneath the high-stress concentration zone or the pressure-relief area of the goaf. After mining the upper coal seam, the overburden collapse zone takes on a “trapezoidal” shape, and mining-induced fractures develop upward to the surface, forming vertical and inclined fracture channels that penetrate to the surface, resulting in severe air leakage in the goaf. Following the mining of the lower seam, the interlayer strata are completely fractured, leading to secondary development of fractures in the overlying old goaf. This results in the formation of a connected fracture network spanning from the surface through the seam goaf linkage. Implementing goaf backfilling measures significantly reduces the vertical settlement of the overburden, prevents the formation of through-layer air leakage channels, and effectively mitigates interlayer air leakage problems during lower-seam mining. Full article

To address the issues of severe surrounding-rock failure and ground support component failure in advancing working-face driving roadways (AWFDRs) in ultra-close coal seams, this study used the 5202 air-return roadway in Huaye Coal Mine as a case study and for engineering background. Numerical simulation, theoretical analysis, and industrial application methods were adopted to analyze the laws of the dynamic evolution of vertical stress in such roadways. The mine pressure behaviors of AWFDRs in ultra-close coal seams were also clarified, thereby enabling the proposal of a solution; namely, zoned support technology. The results show that the 5202 air-return roadway, as an AWFDR in an ultra-close coal seam, exhibits five different characteristic behaviors of mine pressure zones during excavation. Zone 1 is influenced by the adjacent working-face mining under goaf; Zone 2 is influenced by the adjacent goaf lateral abutment stress under goaf; Zone 3 is influenced by the stress of the overlying solid coal; Zone 4 is influenced by the adjacent goaf lateral abutment stress under the overlying solid coal; and Zone 5 is influenced by stabilized stress under the overlying solid coal. The mine pressure behaviors of these zones were ranked, from most intense to weakest, as follows: Zone 3 > Zone 1 > Zone 4 > Zone 2 > Zone 5. Based on this, a basic support scheme was proposed, which involves using bolt–mesh–beam supports combined with shed supports under the goaf and bolt–mesh–beam supports combined with roof anchor cables under the overlying solid coal. Additionally, in Zones 1 and 3, roof anchor cables or rib anchor cables were supplemented as reinforcing supports, which were combined with the basic support scheme described above to form a zoned support scheme for the AWFDR. The analysis of mine pressure behavior and implementation of a zoned support scheme for AWFDRs in ultra-close coal seams provides technical and engineering references for roadway supports under similar mining conditions. Full article

Based on the engineering conditions of the 1303 working face in Zhaoxian Coal Mine, this study investigates the characteristics of mine pressure behavior and the stress-relief mechanism of advanced presplit blasting in a working face with a thick and hard roof in an extra-thick coal seam. Through a combination of numerical simulations and field experiments, the effects of advanced presplit blasting on stress distribution, roadway stability, and microseismic activity are analyzed. Corresponding mitigation measures and optimization strategies are proposed. The results indicate that the primary cause of deformation in the gob-side roadway is the superposition of lateral abutment pressure from the goaf and the front abutment pressure of the advancing working face. Advanced presplit blasting effectively reduces the magnitude of front abutment stress, inhibits its transmission, decreases the hanging area of the goaf roof, and alleviates vertical stress on the roadway side adjacent to the goaf. Furthermore, both the daily average and peak microseismic energy levels decrease as the working face approaches the advanced blasting zone. The implementation of advanced presplit blasting technology in working faces with thick and hard roofs within extra-thick coal seams significantly mitigates rockburst hazards, enhances roadway stability, and improves overall mining safety. Full article