Search Results (25)

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

Underground coal gasification (UCG) is a clean and automated coal technological process that has great potential. Environmental hazards such as the risk of ground surface subsidence, flooding, and water pollution are among the problems that restrict the application of UCG. Overburden rock stability above UCG cavities plays a key role in the prevention of the mentioned environmental hazards. It is necessary to optimize the safety pillar width to maintain rock stability and ensure minimal coal losses. This study focused on the investigation of the influence of pillar parameters on surface subsidence, taking into account the non-rectangular shape of the pillar and the presence of voids above the UCG reactor in the immediate roof. The main research was carried out using the finite element method in ANSYS 17.2 software. The results of the first simulation stage demonstrated that during underground gasification of a thin coal seam using the Controlled Retraction Injection Points method, with reactor cavities measuring 30 m in length and pillars ranging from 3.75 to 15 m in width, the surface subsidence and rock movement above gasification cavities remain within the pre-peak limits, provided the safety pillar’s bearing capacity is maintained. The probability of crack initiation in the rock mass and subsequent environmental hazards is low. However, in the case of the safety pillars’ destruction, there is a high risk of crack evolution in the overburden rock. In the case of crack formation above the gasification panel, the destruction of aquiferous sandstones and water breakthroughs into the gasification cavities become possible. The surface infrastructure is therefore at risk of destruction. The assessment of the pillars’ stability was carried out at the second stage using numerical simulation. The study of the stress–strain state and temperature distribution in the surrounding rocks near a UCG reactor shows that the size of the heat-affected zone of the UCG reactor is less than the thickness of the coal seam. This shows that there is no significant direct influence of the gasification process on the stability of the surrounding rocks around previously excavated cavities. The coal seam failure in the side walls of the UCG reactor, which occurs during gasification, leads to a reduction in the useful width of the safety pillar. The algorithm applied in this study enables the optimization of pillar width under any mining and geological conditions. This makes it possible to increase the safety and reliability of the UCG process. For the conditions of this research, the failure of coal at the stage of gasification led to a decrease in the useful width of the safety pillar by 0.5 m. The optimal width of the pillar was 15 m. Full article

Studying temperature evolution and distribution range during underground coal gasification is essential to optimize process efficiency, ensure safe and stable operation and reduce environmental impact. In this paper, based on the Liyan Coal Mine underground gasification project, the moving grid setting is used to simulate the moving heat transfer process of the underground coal gasification (UCG) flame working face (FWF). The results showed that the temperature distribution within the coal wall facing the flame is relatively narrow and remains concentrated within a limited range. Temperature distribution curves for T = 100 °C and T = 600 °C initially exhibit a nonlinear increase, reaching a maximum value, followed by a nonlinear decrease, ultimately trending towards a constant value. The maximum temperature influence ranges at ∆T = 10 °C (T = 30 °C) in the roof, left coal pillar, and floor are approximately 13.0 m, 9.0 m, and 10.1 m, respectively. The temperature values at the +1 m and +2 m positions on the roof exhibit a parabolic pattern, with the height and width of the temperature curve gradually increasing. By the end of the operation at t = 190 d, the length range of temperatures exceeding 600 °C at the +1 m position is 73 m, with a maximum temperature of approximately 825 °C, while at the +2 m position it is 31 m, with a maximum temperature of approximately 686 °C. Full article

This paper aims to address the issue of hydraulic support crushing accidents or support failures in the retracement roadway (RR) that frequently occurs when a fully mechanized mining face is retraced during the end-mining stage. The deformation and instability mechanism of surrounding rock in the RR during the end mining of a fully mechanized mining face at the Hanjiawan Coal Mine located in the northern Shaanxi mining area is explored through field measurement, theoretical analysis, similar simulation, and numerical simulation. The results reveal that the stability of the remaining coal pillar (RCP) and the fracture position of the main roof are the main factors contributing to large-scale dynamic load pressure in the RR during the end-mining stage. The plastic zone width limit of the RCP is identified to be 5.5 m. Furthermore, the stress distribution within the RCP during the end-mining stage is determined, and the linear relationship between the load borne by the RCP and the strength of the coal pillar is quantified. A similar simulation experiment is conducted to examine the collapse and instability characteristics of the overlying rock structure during the end-mining stage. UDEC (v.5.0) software is utilized to optimize the roof support parameters of the RR. A surrounding rock control technology that integrates the anchor net cable and hydraulic chock is proposed to ensure RR stability. Meanwhile, a method involving ceasing mining operations and waiting pressure is adopted to ensure a safe and smooth connection between the working face and the RR. This study provides a reference for the surrounding rock control of the RR during end mining in shallow, closely-spaced coal seams under similar conditions. Full article

There are a large number of abandoned or casing-damaged oil/gas wells in the western mining areas of China. Under the influence of mining-induced stress, the methane leaked from the oil and gas wells will be transported through fracture within the coal pillar to the longwall working face, which will seriously threaten the safe mining of coal resources. There is no mandatory standard for the practice of coal pillars in coal and gas wells in coal/gas overlapping areas, and the problems of oversized coal pillars and waste of coal resources have occurred during the implementation. In this study, through finite element numerical simulation, three different sizes of protective coal pillars are modeled in the background of Shuangma Coal Mine. The impacts of different heights and widths of protective coal pillars on the evolution of stresses and the diffusion process of leaked methane are explored, and the spatial and temporal migration law of leaked methane under multi-field coupling is revealed. The results show that under mining-induced stress, the size of the protective coal pillar has a significant effect on the stress distribution and methane transport law. Compared with the 130 m coal pillar, the peak stress of the 150 m coal pillar decreased by 6.7%, and the peak stress of the 180 m coal pillar decreased by 9%. At 150 m and 180 m widths, the permeability difference between the two sides is only 1 mD, and the diffusion ranges are similar. From the stress distribution and gas diffusion law, it is shown that the effect achieved by 150 m and 180 m coal pillars is similar. This work is of great significance for the reasonable remaining protective coal pillars for oil/gas wells penetrating longwall mining areas, as well as the prevention and control of disasters caused by leaked methane from wells. Full article

Strong mine pressure often emerges when the working face of the lower coal seam in a closely spaced coal seam system passes through the remaining coal pillar in the overlying goaf. This study investigates the law of overburden movement and the manifestation of mine pressure during mining under the remaining coal pillar. A physical model measuring 2.5 × 0.2 × 1.503 m is constructed. Fiber Bragg grating sensing technology (FBG) and Brillouin optical time domain analysis technology (BOTDA) are employed in the physical model experiment to monitor the internal strain of the overlying rock as the working face advances. This study determines the laws of overlying rock fracture and working face pressure while mining coal seams beneath the remaining coal pillar. It analyzes the relationship between the pressure at the working face and the strain characteristics of the horizontally distributed optical fiber. A fiber grating characterization method is established for the stress evolution law of overlying rock while passing the remaining coal pillar. The experimental results indicated that the fracture angle of overlying rock gradually decreases during the mining stage through and after the coal pillar. In the mining stage through the coal pillar, the cycle pressure step distance of the working face is reduced by 33.3% compared to the stage after mining through the coal pillar. Initially, the strain pattern of the horizontal optical fiber is unimodal when pressure is first applied to the working face, and it transitions from unimodal to bimodal during periodic pressure. The peak value of fiber Bragg grating compressive strain and the range of influence of advanced support pressure are 3.6 times and 4.8 times, respectively, before passing through the remaining coal pillar. Finally, the accuracy of the FBG characterization method is verified by comparing it to the monitoring curve of the coal seam floor pressure sensor. The research results contribute to applying fiber optic sensing technology in mining physical model experiments. Full article

This paper presents our study of the deformation and failure characteristics surrounding rock in roadways with super-large sections during the integrated coal pillar excavation, filling and retention process between coalfaces. Based on the theory of complex variable function, the mapping accuracy of conformal transformation is improved, and an analytical solution for surrounding rock stress in super-large sections of roadway is derived. The stress distribution law of the surrounding rock of rectangular roadways is analyzed, and numerical simulation software is used for supplementary analysis and verification. According to the research findings, the compressive stresses on two sides and the tensile stress on the roof of a rectangular roadway with super-large sections decreased with the increase in the side-pressure coefficient; however, when the side-pressure coefficient increased to a certain point, those two sides changed from a pressure-bearing status to a tensile force-bearing status, while the roof changed from a tensile force-bearing status to a pressure-bearing status. In these stress changes, all the stresses upon the surrounding rock of roadways were compressive stresses and the two critical side-pressure coefficient values were λ_up and λ_down. As the aspect ratio of the roadway increased from 1 to 9, its λup increased from 1.823 to 5.865 and its λdown increased from 0.549 to 0.888. When those side-pressure coefficients in the environment where a roadway is located exceed their critical values, tensile stress will take place on the roadway boundary and result in tensile failure, thus leading to instability in the roadway super-large section. The impact of the side-pressure coefficient upon the plastic zone range of roadway surrounding rock is greater than the impact of the roadway width. In order to secure stability in the surrounding rock of roadways with super-large sections during the excavation process, the side-pressure coefficient should remain around 1; in this situation, the plastic zone covers the smallest range and the relevant support work is the easiest. These research findings provide theoretical references for the excavation and support of roadways with super-large sections. Full article

Coal has remained the primary component of China’s energy structure, and high-intensity extraction has continued in the central and western coal-producing regions of China. In contrast to the abundant coal resources, water resources have become extremely scarce in these regions, creating a conflict between coal resource extraction and water resource conservation. The coal mine underground reservoir (CMUR), as a typical technology for combined coal and water extraction and water-preserving coal mining, has been applied in numerous mines in central and western China. This effectively alleviates water resource shortages and achieves the goal of water resource conservation. The CMURs utilizes the goaf created by longwall mining as the water storage space. The reservoir dam structure comprises coal pillars, which serve as protective coal pillars in the mining area, and artificial dam structures that filled the gaps between these coal pillars. The stability of the dam structure under the complex stress effects of hydraulic coupling has been identified as the key to maintaining the safe operation of the CMUR. The mechanical properties, stress field, fracture field, and seepage field (“three fields”) change mechanisms, as well as the research results on size optimization of coal pillar dams and artificial dams in CMURs, were systematically reviewed. The core content included the instability and failure mechanisms of dam structures under the comprehensive coupling effects of factors such as dry–wet cycles of mine water, long-term immersion, chemical effects of high-salinity water, dynamic and static loads, and cyclic loads. This paper is considered to have certain reference value for the study of the stability of dam structures in CMURs and to provide some guidance for the safe operation of CMURs. Full article

A considerable number of coal mines employed room and pillar mining in the last century in northern China, where the goaf remained stable for a period of time; however, with the increased exposure of coal pillars, their collapse may gradually increase. The stability assessment of these old rooms and pillar goafs is challenging due to their concealment, irregular mining patterns, and the long passage of time. The methodology developed in this study, based on “space-sky-earth” remote sensing such as InSAR to trace historical deformation, the UAV observation of current surface damage, and comparison of mining spaces, can rapidly detect on a large scale the collapse of old goafs and the trend of damage. This study is conducted with an example of a coal mine in Yulin, Northern China, where obtained quantitative surface deformation values were integrated with qualitative surface damage interpretation results, followed by a yearly analysis of the overlying rock movement in accordance with the underground coal mining process. The results show that from 2007 to 2021, corresponding surface deformation and damage occurred following mining progress. However, the room and pillar goaf areas had not undergone any surface deformation, nor had there been incidents of landslides or ground fissures; therefore, it was speculated that no roof collapse had occurred in this region. The surface deformation and damage associated with underground coal mining are complex and influenced by the coal seam occurrence, mining methods, strata lithology, terrain slope, temporal evolution, and anthropogenic modifications. These phenomena are representative of the coal mining area, and this methodology can provide a reference for similar endeavors. Full article

In response to the severe occurrence of mining pressure in the fully mechanized top coal caving face of the extra-thick coal seam and the problem of strong rock pressure caused by the remaining coal pillars in the mining area on the isolated island fully mechanized top coal caving face, taking the 8102 isolated island working face of Tongxin Coal Mine as the background and by using methods such as on-site measurement and numerical simulation experiments, the characteristics of roof mining in the island longwall panel of extra-thick coal seams were analyzed. Establishing a mechanical model for the mining stress and overlying rock stress arch of an isolated working face, the mechanical characteristics of the isolated working face under special conditions were obtained. The results show that the longwall panel no. 8102 has an asymmetric long-arm T-shaped covering layer structure before mining and a C-shaped covering rock structure during mining, which will exacerbate the degree of mining pressure manifestation in the working face. Directional high-pressure hydraulic fracturing was implemented in the gob of longwall panel no. 8102, and the pressure reduction effect of the advance support section of the gob was obvious, ensuring the safety of the working face. Full article

Remaining coal pillars in remining areas exhibit clear creep characteristics, and dynamic pressure accelerates their instability and failure. The creep and hardening characteristics of coal under dynamic pressure are of great engineering significance for the stability of remaining coal pillars in remining areas and their control. To investigate the creep and hardening characteristics of anthracite under static–dynamic coupled loading, graded loading creep tests with different loading rates were conducted. In this research, the creep strain, instantaneous elastic modulus, and creep rate of anthracite were studied under different graded loading rates. The results showed that the hardening effect of the samples manifested as an increasing instantaneous elastic modulus at the loading stage and a decreasing strain rate at the creep stage. When the graded loading rate increases from 0.01 to 0.1 mm/s, the instantaneous elastic modulus increases by 0.16–2.32 times. The sudden increase in the instantaneous elastic modulus at the failure stress level explains the instantaneous failure of the samples well. The actual yield levels corresponding to the peak instantaneous elastic modulus of the samples linearly decreased with increasing graded loading rate. The functional relationship between the graded loading rate and the elastic modulus hardening coefficient, the actual yield stress, and the strain rate decay coefficient were established, which could quantitatively describe the influence of different graded loading rates on the creep and hardening characteristics of anthracite and predict its creep damage. Full article

The mining confrontation caused by a super-long working face is an important factor that leads to difficulties in the control of surrounding rock in facing-mining roadways. To address this issue, this study takes the 18106 working face of Xiegou Coal Mine as the engineering background. First, deformation characteristics of the surrounding rock in two stages of roadway mining are explored, and the principle of determining the time for fracturing construction is presented. Additionally, the influence of fracturing space position on the control effect of the roadway’s surrounding rock is examined. Results show that the roadway which is influenced by adjacent working face mining has obvious asymmetric deformation, and the order of deformation is as follows: pillar side > roof > coal side > floor. The roof activity in goaf is the key factor of roadway deformation, and the proportion of deformation is 60%, followed by the influence of advance stress, accounting for 38%. After fracturing, with the increase in fracturing height, the deformation of the surrounding rock decreases, and the decrease in deformation of the roof and two sides also reduces, while the decrease in deformation of the floor remains relatively stable. The reasonable fracturing height is 10 m, and the reasonable fracturing construction time is 17 days before the confrontation between the working face and the roadway. Field practice suggests that there are effective cracks in the roof after fracturing, and the deformation of the surrounding rock in facing-mining roadways is reduced by more than 60% compared to that without fracturing. Hydraulic fracturing is significant for controlling the deformation of the surrounding rock in facing-mining roadways. Full article

In China, soft rock roof makes up the majority of coal mine roof, yet it is easy to break due to low strength and poor integrity. As such, it is difficult for the traditional pillar-based roadway protection means and pillarless mining techniques to effectively control the roadway roof. In order to solve the problems with pillarless mining under soft rock roof conditions, using the 1510 working face of Xinyi Coal Mine as an example, a gob-side caving roadway forming (GSCRF) technique for broken immediate roof is developed. This paper discusses the adaptability and feasibility of this technology through theoretical modeling and on-site engineering testing. A roadway rock surrounding control scheme based on “cable + steel beams + yielding prop” is further designed, and field tests and monitoring are carried out. Field tests show that, during GSCRF of the 1510 working face, the maximum subsidence of the roof remains within 200 mm. The convergence of the two sides causes stabilization. The tension on the anchor cables is gradually becoming stable. The monitoring results show that the roadway has a good stress environment and the surrounding rock is effectively controlled. Compared with the traditional pillarless mining mode, this technology has the technical advantages of achieving complete elimination of coal pillars, reduced pressure on the roadway roof, and interference-free mining of the working face. The research outcome can provide useful reference for pillarless mining by GSCRF and a solution for pillarless mining under soft rock roof conditions. Full article

On the working face below shallow and close coal seams, there are residual pillars. The mine’s ability to operate safely is constrained by the coal pillars’ vulnerability to sudden instability and powerful ground pressure disasters during withdrawal. This paper uses the 31,106 working face of the Huoluowan coal mine as its research backdrop and employs field observation, theoretical analysis, and numerical simulation to examine the strong dynamic load mechanism of the overlying coal pillars. According to the analysis, the residual pillar’s stress diffusion angle is 29 degrees after mining the working face above it, which has an impact on the main roof’s stability above the working face’s retracement roadway. The main roof is impacted by the excavation disturbance and the remaining pillars during the working face’s final mining phase, displaying a complex stress superposition state. The retracement roadway is significantly deformed as a result of the plastic zone of the surrounding rock changing from small-scale damage to extensive damage. The proposed “hydraulic roof cutting + reinforcement support” prevention technology is based on the prevention idea of weakening important rock strata, changing the stress transmission path, and strengthening adjacent rock. Field testing shows how hydraulic fracturing reinforces the roof structure, lessens the heavy dynamic load on the supporting pillars of overlying residual coal, reduces rock deformation in the retracement roadway, and ensures the stability of the working face during withdrawal. The study’s findings are significant for the secure removal of a working face under similar circumstances. Full article

Coal-dependent industries are the economic pillar of many countries; however, their contribution to air pollution also restricts long-term economic development. While the negative effects of environmental regulations on coal-dependent industries has attracted much attention, the health effects of such regulations remains barely quantitatively studied. Our study is based on a quasi-natural experiment created by a command-and-control policy in China’s cement industry, whereby cement enterprises are forced to stop production for a specific period of time every year. This paper adopted DID methods and found that direct pollution control measures for coal-dependent industries could significantly improve residents’ health levels and that the resultant reduction in medical expenditures could save 456.8 RMB yuan per capita per year. Additionally, our mechanism analysis found that the COPP is beneficial to residents’ health in that it reduces air pollution and sewage treatment rates. This means that pollution from coal-dependent industries has a large and underestimated impact on residents’ health. Environmental regulation of the coal-dependent industry could greatly improve the health of residents. Full article