Search

The gangue recognition technology is one of the key technologies in the intelligent construction of mines. To address the problem of low accuracy of the gangue recognition model caused by low illumination and high dust environment at the working face and the difficulty of recognizing small target gangue, a coal gangue image recognition model based on CSPNet-YOLOv7 target detection algorithm is proposed. Cross Stage Partial Network (CSPNet) is used to improve the backbone feature extraction network of YOLOv7 model, optimize the gradient information to reduce the network parameters, while Recursive Feature Pyramid (RFP) and Switchable Auto Convolution (SAC) to replace the simple up and down sampling and normal convolution modules in the neck feature extraction network, and to enhance the generalization ability of the network by using three migration training for feature learning of different widths and depths. The experimental results show that the CSPNet-YOLOv7 model has an average accuracy mean of 97.53%, an accuracy rate of 92.24%, a recall rate of 97.91%, an F1 score of 0.95, a model parametric number of 30.85×106, a floating point operation count of 42.15×109, and a frame rate of 24.37 f/s transmitted per second, Compared to the YOLOv7 model, the average mean accuracy is improved by 7.46%, and the number of parameters and floating point operations are reduced by 17.23% and 60.41%, respectively, compared to the FasterRCNN-Resnet50, YOLOv3, YOLOv4, MobileNet V2 -YOLOv4, YOLOv4-VGG, YOLOv5s models. The CSPNet-YOLOv7 model has the highest average accuracy mean for coal gangue identification, while the number of parameters and floating point operations is small, which has a good balance between identification accuracy and speed. Finally, the CSPNet-YOLOv7 model is validated through downhole field tests, providing an effective technical means for accurate coal gangue identification.

Mine gas extraction has the dilemma of low extraction rate, short effective extraction cycle, and substandard gas pre-extraction concentration. In order to realize the precise pressure relief and permeability of high gas and low gas permeability coal seam, taking the working face of high gas and low gas permeability coal seam in Shunhe Coal Mine 2404 as the research object, based on the methods of rheological theory modeling, COMSOL numerical analysis and field experiment, a multi-field coupling model of gas transport of large-diameter borehole pressure relief coal seam was constructed, and the determination index of the effective extraction radius of large-diameter borehole gas was proposed - the residual gas pressure did not exceed 0.22 MPa. The mechanism of pressure relief and permeability enhancement, as well as the law of gas migration in large diameter coal seam drilling, are unveiled: the gas pressure within a single large diameter coal seam drilling exhibits an elliptical distribution with the center of the drilling circle serving as its symmetrical center. In other words, the gas pressure gradually increases from the center towards the outer edges of the drilling circle. Furthermore, with prolonged drainage time, there is a gradual expansion of the standard drainage area. The stress transfer around the borehole is characterized by the fact that with the increase of extraction time, the vertical stress of the peripheral points of a single large-diameter borehole shows a trend of first increasing and then decreasing, and the technology of high-gas and low-gas permeability coal seam large-diameter borehole pressure relief and enhanced penetration extraction is proposed. The simulation determined the effective extraction radius of the borehole, and the drilling hole with a hole spacing of 6.0 m and a diameter of 300 mm had the best pressure relief and penetration enhancement effect, and the average permeability within 3.0 m on both sides was 1.58×10−15 m2, which was much greater than the initial value of 1.15×10−17 m2. The field application demonstrates that gas extraction in a large diameter borehole is significantly superior to that in an ordinary borehole. In a 300 mm large diameter borehole, the maximum gas volume fraction reaches 47.2%, whereas it only amounts to 10.6% in a 75 mm original borehole. Furthermore, the extraction duration of high gas volume fraction increases by 40.2% in the large diameter borehole. This approach effectively addresses the challenge of controlling small diameter gas boreholes and provides an efficient method for precise pressure relief and permeability enhancement of high gas coal seams.

Coal pillars in soft and thick coal seam are often tall and have large exposed coal walls, and the coal quality is soft with some developed cracks, which can easily cause an instability of coal pillars and make roadway maintenance difficult. This paper takes the No. 3 coal seam in the Fuyan Coal as the engineering background, based on the shear sliding block theory, explores the mechanism of coal pillar deformation and failure under mining influence, analyzes the width of the coal pillar’s limit equilibrium zone and stress distribution rules, and reveals the movement mechanism of the coal pillar’s shear sliding block. Based on this, the cooperative control strategies for coal pillar stability are proposed and verified through engineering practice. The results show that: ① by using the limit equilibrium theory and the superposition theory, the range of shear sliding block movement of the coal pillar and the vertical stress distribution law of the coal pillar are determined, and the distribution law of the safety factor of the coal pillar shear sliding block is clarified. The safety factor of the upper part of the coal pillar is small at a depth of 0−1.26 m. In the area of 1.26−3.95 m, the central part of the coal pillar has a large area of small safety factor and is vulnerable to damage from the roof. ② A cooperative control technology for coal pillar is proposed, which is based on “reinforcement anchor cable-grouting reinforcement-advance cutting roof”, and it effectively controls the deformation and bottom bulging of the coal pillar. The side cracks of the coal pillar without supporting conditions and those with original supporting conditions are reduced by 62.89% and 46.26%, respectively. The integrity of the surrounding rock of the roadway is greatly improved, forming a strong bearing structure. ③ According to the conditions of coal pillar in the thick and soft coal seam section, the design parameters of coordinated control are reasonably determined, and the prevention and control effect of coal pillar is evaluated by test monitoring. The field test results show that the cracks in coal pillar are fully filled, and the strength of coal body is increased by more than 63% after grouting. The roadway displacement, anchor cable stress, and separation layer are all within the controllable range, indicating that the cooperative control technology significantly improves the bearing capacity of the coal pillar, and effectively controls the deformation of the surrounding rock of the mine roadway, providing space guarantee for safe and efficient mining.

Broken coal and rock (BCR) are an important component medium of the caving zone in the goaf (or gob), as well as the main filling material of fault fracture zone and collapse column. The compaction seepage characteristics of BCR directly affect the safe and efficient mining of coal mines. Thus, numerous laboratory studies have focused on the compaction seepage characteristics of BCR. This paper first outlines the engineering problems involved in the BCR during coal mining including the air leakage, the spontaneous combustion, the gas drainage, and the underground reservoirs in the goaf. Water inrush related to tectonics such as faults and collapse columns and surface subsidence related to coal gangue filling and mining also involve the compaction seepage characteristics of BCR. Based on the field problems of BCR, many attempts have been made to mimic field environments in laboratory tests. The experimental equipment (cavity size and shape, acoustic emission, CT, etc.) and experimental design for the BCR were firstly reviewed. The main objects of laboratory analysis can be divided into compression tests and seepage test. During the compaction test, the main research focuses on the bearing deformation characteristics (stress-strain curve), pore evolution characteristics, and re-crushing characteristics of BCR. The seepage test mainly uses gas or water as the main medium to study the evolution characteristics of permeability under different compaction stress conditions. In the laboratory tests, factors such as the type of coal and rock mass, particle size, particle shape, water pressure, temperature, and stress path are usually considered. The lateral compression test of BCR can be divided into three stages, including the self-adjustment stage, the broken stage, and the elastic stage or stable stage. At each stage, stress, deformation, porosity, energy, particle size and breakage rate all have their own characteristics. Seepage test regarding the water permeability experiment of BCR is actually belong to variable mass seepage. While the experimental test still focuses on the influence of stress on the pore structure of BCR in terms of gas permeability. Finally, future laboratory tests focus on the BCR related coal mining including scaling up, long term loading and water immersion, mining stress path matching were discussed.

Highlights (1) This paper reviews the water weakening on rock strength and its influencing factors. (2) Various microscopic methods for studying the pore characterization and weakening mechanism of WRI are compared and analyzed. (3) We study the weakening mechanism of WRIs using microscopic methods. (4) Future works on WRI laboratory tests were suggested.

Monitoring the surface deformation of filling coal mines regularly and understanding their spatiotemporal evolution characteristics for mining management and disaster warning is greatly significant. However, there is a lack of research on extracting spatiotemporal evolution characteristics of large-scale and high-resolution surface deformation in backfill mining areas and on evaluating the filling effectiveness of subsidence restraint. In this study, we took the Yineng Coal Mine in the Shandong Province of China and the surrounding area of the coal mine as the study area. The advanced distributed scatterer interferometric synthetic aperture radar (DS InSAR) technique was adopted for time-series analysis. The probability integral method (PIM) model for backfilling mining and an arctangent time function were integrated with DS InSAR to overcome the sparsity of InSAR observation points due to the temporal decorrelation caused by vegetation coverage. The results show that the proposed integration strategy is helpful in improving the number of effective monitoring points and obtaining complete spatiotemporal information of the surface deformation of the working face. The whole study area has eight key deformation zones. All six working faces in the Yineng initial minery display different degrees of deformation during the study period. The comparison between the deformation results of backfill mining and the simulated deformation of nonbackfill mining in the CG1312 working face shows that backfilling mining technology effectively reduces the range (22.00%) and magnitude (61.50%) of surface subsidence and significantly reduces the potential threat to surface buildings.

In order to study the damage characteristics of superhigh-wate consolidated body under triaxial stress state, uniaxial experiments of superhigh-water consolidated body under different curing time (1, 7, 14, 21, 28 d) were carried out. Uniaxial compression models under different curing times were established by the parallel bonding model of PFC3D, and five groups of uniaxial strength of PFC3D simulation experiment were obtained, which were consistent with the uniaxial strength of the uniaxial experiment. Statistics each simulation experiment of uniaxial compression parallel bond, mesoscopic physical and mechanical parameters in the model on the basis of the parameters under different curing time, superhigh-water consolidation triaxial compression model body, the same confining pressure and axial stress is applied, record the triaxial experiments under different curing time in the process of stress-strain curve and the force when the damage distribution chain. Analyzed superhigh-water body of consolidation in the damage characteristics of three to the stress state, the results show that: ① the superhigh-water concretion body three to the stress state of the changing rule of the ultimate strength with curing time can be represented by the Bohr boltzmann equation. When curing time is 1 to 14 days, the ultimate strength increases fastest, and the maximum ultimate strength reaches 3.1 MPa when curing time is 28 days. ② The variation rule of the degree of penetration of force chains in the triaxial compression model of superhigh-water consolidated body with curing time is as follows: within curing time of 1-28 d, the number of transverse contact force chains is 4 006, 4 561, 4 891, 5 017, 5 062, respectively. The number of longitudinal contact force chains is 4 029, 4 439, 4 716, 4 917 and 5 123. The results show that the carrying capacity of the superhigh-water consolidated body increases with the increase of curing time, and increases fastest during the curing time from 0 to 14 days, and tends to be stable during 14 to 28 days. ③ The tensile chain was used to simulate the fracture development in the triaxial compression model of superhigh-water consolidated body. The results show that the cracks concentrate on the upper and lower ends of the specimen when the curing time is 1 d. With the increase of curing time, the cracks in the middle of the specimen begin to increase and finally connect with the cracks at the upper and lower ends of the specimen.

AbstractThe stability of the entries of longwall panels is the key to ensure efficient and safe production of coal mines. In order to solve the common problems of floor heave of panel entry in western China, based on a case study, this paper studies the rockburst instability mechanism of entry floor-induced mining by considering the results from a laboratory test, numerical simulations, and field practice. After testing, the coal and rock of the entry are hard and brittle. In particular under the action of impact dynamic load, its dynamic strength is higher and has a positive correlation with the impact pressure, which provides a mechanical premise for subsequent rockburst. Numerical simulation results show that with the mining of the panel, the vertical stress and the maximum principal stress of the floor are mainly concentrated in the coal pillar along the entry, and the area and degree of concentration continue to increase. The horizontal stress is mainly concentrated in the entry floor, which is distributed in the advanced range of the panel. The deformation rate of the entry roof and the ribs is stable, while the floor shows a “mutation” characteristic of not deforming when the panel is far away and suddenly rising when it is closer to the panel. The range of the plastic zone of the roof and floor remains unchanged, the ribs are further deepened, and the mechanical properties of the coal and rock mass are further weakened. The results of this study contribute to providing a reference for the control of surrounding rock of panel entry under similar geological and geotechnical circumstances.

AbstractIn order to understand the mechanism of some unconventional failures such as rockburst caused by deep rock excavation, the failure characteristics of burst-prone sandstone specimens under static and dynamic loads were studied by using the MTS816 rock mechanics testing system and the split Hopkinson pressure bar (SHPB) experimental system, as well as the effects of bedding angle and impact pressure on rock mechanical properties and failure patterns. The uniaxial compression test used a cylindrical specimen with a height of 50 mm and a diameter of 100 mm, and cylindrical specimens with height and diameter of 50 mm were adopted in the SHPB tests. The bedding angles in the tests are 0°, 45°, and 90°. In the dynamic impact test, three different impact pressures were applied to observe the magnitude of impact load on the mechanical behaviors of the burst-prone sandstone specimens. The results show that with the increase in the bedding angle, the uniaxial compressive strength firstly decreases and then increases. When the bedding angle is 45°, the uniaxial compressive strength is the lowest. The uniaxial compressive strength is highest when the bedding angle is 0°. The burst-prone sandstone specimens with different bedding angles had three different failure pattern types. Under the dynamic loading, the stress-strain curves show springback phenomenon; with the increase in impact pressure, the dynamic strength of the burst-prone sandstone specimens with each bedding angle increases; the fracture degree of the bedding sandstone specimens gradually increases. The dynamic strength of the 45° burst-prone sandstone specimen is the lowest, and it has the highest fracture degree.

Many dynamic disasters such as coal wall caving and roof falling have happened during longwall face crossing fault. Combined with the coal seam geological conditions in the 1302 longwall face AB coal mine, the paper comprehensively studies the overlying strata movement and the stress evolution laws of stope during superhigh-water material (SHWM) backfilling longwall face across fault by UDEC numerical simulation, theoretical analysis and on-site measurement. The results show that the stress during stable state is affected by the fault will increases abnormally. While the longwall face pass through the fault, the stress will abated and the vertical displacement of the overlying strata of two sides of the fault is significantly different. The fault significant blocks the propagation of stress and development of plastic zone. The superhigh-water material filling mining management roof has good protection effect on the roof when the longwall face encounters fault, which can effectively weaken the blocks effect of fault on stress propagation and plastic zone development. The numerical simulation results are consistent with the field experiments, and the research result provides theoretical basis and technical guidance for safe and efficient production during superhigh-water material backfilling longwall face through the fault.

In order to alleviate the relationship between mining and roadway, the 3204 working face and the 3206 roadway in Shanxi Taitou coal mine are taken as an example, and the width of mining and the support parameters of mining while reversing mining and excavation under dynamic pressure are optimized. The research includes field investigations, theoretical analysis, numerical simulation, and field tests. Based on the characteristics of roof fracture and the distribution of coal pillar stresses that determine the coal pillar is 18.7 m wide, the control scheme of mining while reversing mining and excavation was developed; the stress of coal pillar and the characteristics of roadway deformation and failure are summarized. By means of FLAC3D numerical simulation software, the influence of coal pillar widths and different mining positions on the stability of roadway surrounding rock are discussed. The asymmetric support structure of trapezoidal roadway is proposed as the core support, and the support scheme of dense bolts and anchor cables is proposed. The support of the 3206 return airway is composed of bolt, anchor cable, and anchor mesh, combined with M-shaped steel belt and steel beam. Through the research on the current situation of roadway support, the support scheme is optimized to make the 3206 return airway meet the production requirements, which provides a new breakthrough for roadway support under dynamic pressure in deep mines in China.

AbstractThe prevention of gas overrun in the panel is one of the key issues on green mining. In this paper, a physical goaf model was established based on the geological production conditions of a coal seam in a coal mine in Henan Province, and the combined drainage technology by burying and spilling pipeline in the upper corner was proposed. The Fluent software was used to simulate the changes of gas mass fraction and flow field distribution when the gas is extracted from the goaf without the gas pipeline, with a single buried pipeline, and with the combination of buried and spilled pipelines. Analysis and simulation showed that in the absence of drainage pipelines, the gas concentrations in both the return airway and the upper corner are up to 1.2%, which failed to meet the gas prevention and control standard on the mine. In the case of gas drainage with a single buried pipeline, the average gas concentration in the return airway was 0.7%, and only the gas concentration in the upper corner was up to 1.1%, which failed to meet the gas control standards on the mine’s panel. However, the maximum gas concentration in the combined drainage was reduced from 1.1% to 0.6%, which indicates that the technology can greatly reduce gas concentration, but there is still a gap from the mine’s gas control standard. Therefore, the impacts of the lengths of pipelines buried in different goafs on the gas drainage effect in the goaf were studied, and it was concluded that the reasonable length of pipelines buried in the goaf under the geological conditions was 20 m. This technology not only solves the problem of gas overrun in the panel but also realizes the utilization of gas resources in the goaf. The research results are of guiding significance for pipeline laying and the drainage technology in the upper corners on the U-shaped ventilation panel.

The green depressing cropping system (GDCS) has existed in the farming-pastoral ecotone of northern China (FPENC) for hundreds of years. This ancient practice aims to promote an agricultural system that is both green and sustainable at the same time. However, there is a knowledge gap regarding the extent and implementation of the GDCS in China. Here we present a study that analyses GDCS fields and their related farmland use patterns in Ulanqab, China. This study takes into consideration satellite imagery, ground truth data, phenological records, and other ancillary data from 2015 to 2019 at the Ulanqab region. The integration of the former datasets, combined with visual interpretation and a random forest classification approach, successfully mapped the distribution of GDCS fields, GDCS frequencies, GDCS-crop rotation, and the determinants (terrain, irrigation) of the distribution of the GDCS practice. Our results suggest that GDCS is still prevailing and relatively widespread in Ulanqab, especially in the Houshan area, where a total of 10.29% of farmlands in the study region have adopted the practice. Cereal crops, potato, and other drought-resistant crops were the preferred previous and subsequent crops of GDCS, rather than maize and sunflowers. According to our results, 91.5% of the GDCS fields were rainfed in Ulanqab, suggesting that the practice is associated with dry farming. We also found that the practice is particularly prevalent in the fields with a steeper slope. Our findings provide empirical evidence for the importance of remote sensing towards the implementation of pilot programs that involve crop rotation and fallow systems.

When a working face is crossing the abandoned roadways, problems such as roof subsidence, rock fracture, and instability will occur, resulting in widespread roof fall and rib spalling, which seriously affect safe and efficient mining on the working face. In this paper, the no. 23 coal pillar working face of Juji coal mine is taken as the engineering background, a mechanical model of crossing the abandoned roadways is constructed aimed at the problem of the working face crossing the abandoned roadway group, the collapse of the abandoned roadway roof is analyzed, a scheme of crossing the abandoned roadways is designed, and the development law of the stress and plastic zone after the reinforcement scheme is stimulated and analyzed. The results show that when the working face advances to the abandoned roadway, key block B crosses the abandoned roadway and the solid coal to form a “cross-roadway long key block.” It is calculated that the minimum support resistance required for the abandoned roadway is 6700 kN. Based on the results of numerical comparison, it is concluded that filling wood pile when the working face passes through the roof abandoned roadway and adding anchor cables for reinforcement support when the working face crosses the coal seam abandoned roadway effectively reduce the stress concentration of surrounding rocks, decrease the development of the plastic zone, and achieve safe and efficient mining when the working face crosses the abandoned roadways.

While the fully-mechanized longwall mining technology was employed in a shallow seam under a room mining goaf and overlained by thin bedrock and thick loose sands, the roadway pillars in the abandoned room mining goaf were in a stress-concentrated state, which may cause abnormal roof weighting, violent ground pressure behaviours, even roof fall and hydraulic support crushed (HSC) accidents. In this case, longwall mining safety and efficiency were seriously challenged. Based on the HSC accidents occurred during the longwall mining of 3-1-2 seam, which locates under the intersection zone of roadway pillars in the room mining goaf of 3-1-1 seam, this paper employed ground rock mechanics to analyse the overlying strata structure movement rules and presented the main influence factors and determination methods for the hydraulic support working resistance. The FLAC3D software was used to simulate the overlying strata stress and plastic zone distribution characteristics. Field observation was implemented to contrastively analyse the hydraulic support working resistance distribution rules under the roadway pillars in strike direction, normal room mining goaf, roadway pillars in dip direction and intersection zone of roadway pillars. The results indicate that the key strata break along with rotations and reactions of the coal pillars deliver a larger concentrated load to the hydraulic support under intersection zone of roadway pillars than other conditions. The “overburden strata-key strata-roadway pillars-immediate roof” integrated load has exceeded the yield load that leads to HSC accidents. Findings in HSC mechanism provide a reasonable basis for shallow seam mining, and have important significance for the implementation of safe and efficient mining. Keywords: Shallow seam, Room mining goaf, Longwall mining, Hydraulic support crushed mechanism, Safe and efficient mining

Highwall mining (HWM) technology is an efficient method for exploiting residual coal resources in Chinese open-pit coal mines. However, on-site personnel and equipment can be damaged by the instability of the highwall mining residual coal pillars and subsidence of final end-walls. This paper considers the geological conditions of an open-pit mine in Shendong Coal Field (China) in order to prevent overlying rock fall accidents; the Mark-Bieniawski formula and the FLAC3D numerical simulation are used to analyse reasonable coal pillar widths outside and under the road, which were determined to be 1.7 m and 1.3 m, respectively. Using the EBH132 cantilever excavator for remote control mining, the field experiment shows that the recovery ratio of highwall residual coal resources was over 67%; hence, safety, efficiency and high recovery ratio of highwall mining were achieved for the residual coal resources of an open-pit mine.

In coal mines, underground reservoir systems can increase the availability of water and are an effective technical approach for the protection and utilization of water resources. The stability of coal pillar dams is the key factor in the safety and stability of these underground water storage systems. However, coal pillar dams must operate in complex environments that combine dynamic-static superimposed stress fields and water immersion; moreover, coal pillar dams subjected to both stress and seepage are more susceptible to damage and even collapse. In this study, a seepage-stress coupling model of a coal pillar dam was constructed using the Universal Distinct Element Code (UDEC) simulation software. This model provides a platform for analyzing the characteristics of fracture development in surrounding rock in active mines and the coupled development of crack fields and seepage fields in coal pillar dams. Methods were developed for (1) calculating the water content for the coal pillar dam numerical simulation model and (2) reducing water immersion weakening. The maximum seepage width of a coal pillar dam subjected to water immersion was obtained, and a damage and failure evolution mechanism for coal pillar dams experiencing flooding was developed. The results provide a scientific basis for enhancing the stability control of coal pillar dams and are of great significance for realizing water conservation in coal mines.

Rock and coal dynamic failures have a higher chance to occur when thick and competent roof strata are presented, which can build up a high level of energy. This paper studied the fracturing characteristics of the competent roof strata during longwall face retreat and their effects on roadways’ stability to gain an improved understanding of the rock dynamic failure occurred in roadways as well as the associated ground controls. The results indicated that the main reason of dynamic failure in roadways was the structural change of lateral main roof over a chain pillar, which transformed from the cantilever beam in the period of primary mining impact (PMI) to the triangular arch in the period of secondary mining impact (SMI). Based on corresponding mechanical models developed for the lateral main roof over the chain pillar, it was calculated that the force on roadways’ roof in the period of SMI was 3.7 times larger than that in the period of PMI. Then, the dynamic failure control method of increasing the chain pillar width was proposed. The stress distribution and deformation of roadways with different chain pillar widths were simulated using FLAC3D. The results indicate that as the pillar width increases, the influence of secondary mining on roadways’ stability decreases, and a threshold of not less than 30 m pillar width was suggested. The outcomes of this study would provide guidance for the design of chain pillars where thick and competent roof strata are presented.

The aim of this paper is to study the particle size effect of fly ash (FLA) on pore structure and strength of Fly Ash Foamed Geopolymer (FAFG). Information on the macro-pores such as macro-pore size and distribution of FAFG is captured through binarization processing. Porosity and compressive strength of FAFG are respectively tested by Archimedes density test method and uniaxial compressive strength test method. It can be concluded that the FLA particle size has an effect on the pore structure and strength of FAFG. More specifically, the effect of FLA particle size shows itself macroscopically on the quantity of middle and large macro-pores and the uniformity of macro-pores distribution, and microscopically on the quantity of micro-holes and cracks and calcium silicate (C-S-H) quantity at the early stage of FAFG mixture. All of the properties of FAFG follow some kind of changing rule except at the turning point when FLA particle is of 0.125~0.25 mm in size. To explain clearly the root cause of FLA particle size effect on FAFG, SEM, and XRD are employed to explore the microstructure of FAFG and the component of FLA. It turns out to be the amorphous phase SiO2 content in FLA of different particle sizes which could determine the reaction extent of FAFG mixture.

In longwall mining, the risk of water inrushes from the floors of deeply buried coal seams is closely related to the degree and depth of the destruction for the mining floor. To analyze the main factors affecting floor failure and the evolution of such failures, this study considered the LW2703 working face of the Chengjiao Coal Mine in China, which is characterized by a large buried depth, complex fault structure, and high pressure from a confined aquifer. The characteristics affecting floor crack development depth were analyzed by considering friction angle, cohesion force, floor pressure, stress increase coefficient, and peak position. A FLAC3D simulation was performed to compare the degrees of floor damage that occurred for caving and backfilling methods during the mining process. High-density electrical detection was performed on-site and used to (1) determine the maximum depth range of the floor damage, (2) reveal the laws governing the evolution of damage in a mining floor, and (3) provide a reasonable basis for evaluating and preventing floor water inrush accidents.

Mine gas overflow is a serious threat to the safe and efficient longwall mining of gassy coal seams. Based on the field mining conditions and gas extraction of the fully mechanized top-coal caving face of a gassy coal mine, the space volume fraction distribution and emission (extraction rate) of gas in the face were tested by an arrangement of measuring points in the stereo grid. The isograms of gas volume fraction distribution for each measurement section and air direction in the face are drawn. The research shows that each measurement section gas volume fraction distribution is presented for an asymmetric concave curve along the vertical direction of the coal wall in the air-inlet side and the air-return side of the face; on the working face air-return side, the determination of gas volume fraction distribution of the section appears as falling straight line along the vertical direction of the coal wall. Before the first weighting, the absolute quantity of gas emission in the working face increased with the advancing of the working face, reached the maximum at the time of the first weighting, and then remained stable.

The “Three Under Mining” (mining under the buildings, the railways, and the waters) coal resources are stored in the central and eastern China. Many large-scale mine disasters occurred due to overburden strata movement and surface subsidence. Longwall mining with superhigh-water material backfilling technology has been improved efficiently to prevent the underground disasters and protect the surface ecological environment. Since underground mine pressure behavior and overlying strata movement are influenced by the backfilling strength and backfilling rates, rational design of backfilling parameters is key to realize the green mining of deep buried coal seams. Based on the combination of geological and production conditions of a deep buried coal seam with composite beam theory, the roof fracture distance was analyzed. The software of UDEC was used to simulate the overlying strata movement laws affected by the different backfilling strength and backfilling rates. With the comparative analysis of the vertical displacement movements and the vertical stress distributions, the reasonable filling rate and water volume fraction were determined to be 90% and 95%, respectively. According to the field experiments, the underground dynamic load was low enough for the safe mining, and the village building can be kept in a stable state with the application of the backfilling technology. The research results in a scientific basis for the coordinated development between the safe and efficient mining of deep buried coal resources and protection of the surface ecological environment.

Aiming at the shallow depth seam proximity beneath a room mining goaf, due to that the shallow depth seam is exploited using the longwall mining and overlain by thin bedrock and thick loose sands, many accidents are likely to occur, including roof structure instability, roof step subsidence, damages of shield supports, and the face bumps triggered by the large area roof weighting, resulting in serious threats to the safety of underground miners and equipment. This paper analyses the overlying strata movement rules for the shallow seams using the physical simulation, the 3DEC numerical simulation and the field measurements. The results show that, in shallow seam mining, the overburden movement forms caved zone and fractured zone, the cracks develop continuously and reach the surface with the face advancing, and the development of surface cracks generally goes through four stages. With the application of loose blasting of residual pillars, reasonable mining height, and roof support and management, the safe, efficient and high recovery rate mining has been achieved in the shallow depth seam proximity beneath a room mining goaf. Keywords: Shallow depth seam, Room mining goaf, Closely-separated coal seams, Longwall mining, Overlying strata movement rules

Marine gas hydrates (MGHs), which have great potential for exploitation and utilization, account for around 99% of all global natural gas hydrate resources under current prospecting technique. However, there are several potential hazards associated with their production and development. These are classified into four categories by this paper: marine geohazards, greenhouse gas emissions, marine ecological hazards, and marine engineering hazards. In order to prevent these risks from occurring, the concept of “lifecycle management of hazards prevention” during the development and production from MGHs is proposed and divided into three stages: preparation, production control, and post-production protection. Of these stages, economic evaluation of the resource is the foundation; gas production methods are the key; with monitoring, assessment, and early warning as the guarantee. A production test in the Shenhu area of the South China Sea shows that MGH exploration and development can be planned using the “three-steps” methodology: commercializing and developing research ideas in the short term, maintaining economic levels of production in the medium term, and forming a global forum to discuss effective MGH development in the long term. When increasing MGH development is combined with the lifecycle management of hazards prevention system, and technological innovations are combined with global cooperation to solve the risks associated with MGH development, then safe access to a new source of clean energy may be obtained.

A thin coal seam mined as a protective coal seam above a gas outburst coal seam plays a central role in decreasing the degree of stress placed on a protected seam, thus increasing gas permeability levels and desorption capacities to dramatically eliminate gas outburst risk for the protected seam. However, when multiple layers of coal seams are present, stress-relieved gas from adjacent coal seams can cause a gas explosion. Thus, the post-drainage of gas from fractured and de-stressed strata should be applied. Comprehensive studies of gas permeability evolution mechanisms and gas seepage rules of protected seams close to protective seams that occur during protective seam mining must be carried out. Based on the case of the LongWall (LW) 23209 working face in the Hancheng coal mine, Shaanxi Province, this paper presents a seepage model developed through the FLAC3D software program (version 5.0, Itasca Consulting Group, Inc., Minneapolis, MI, USA) from which gas flow characteristics can be reflected by changes in rock mass permeability. A method involving theoretical analysis and numerical simulation was used to analyze stress relief and gas permeability evolution mechanisms present during broken rock mass compaction in a goaf. This process occurs over a reasonable amount of extraction time and in appropriate locations for comprehensive gas extraction technologies. In using this comprehensive gas drainage technological tool, the safe and efficient co-extraction of thin coal seams and gas resources can be realized, thus creating a favorable environment for the safe mining of coal and gas outburst seams.

Abundant steeply-dipping thick coal seams (SDTCS) have been found in Xinjiang, China, and they are mined largely by the horizontal section top-coal caving (HSTCC) method. The hanging wall of the HSTCC face is nearly vertical and does not fracture easily after the underlying coal is extracted. As a result, stress tends to concentrate in the hanging wall of the lower-section working face (LSWF) and then induce dynamic disasters. In this study, a mechanical model of a HSTCC face’s hanging wall in steeply-dipping seams was constructed to study the characteristics of hanging wall deformation. The mechanism of hanging wall pressure relief by deep-hole blasting (DHB) was analyzed and the effectiveness of DHB was investigated by simulation using the LS-DYNA software. Based on these studies, parameters relevant to pressure relief by DHB were determined and then DHB was applied to the 4301 working face in the Jiangou coal mine. The results show that the average pressure of measured at the 4301 working face decreased about 34% from those at the 4501 face where the hanging wall was not blasted. Accidents related to dynamic rock pressure, such as support crushing and large-scale rib fall, did not occur at the 4301 working face throughout the mining process. Additionally, in order to constrain the surface “V”-shaped collapsed grooves induced by repeated mining of HSTCC faces and prevent the subsequent failure of the surrounding rock on the sides of the collapsed grooves, loess was used to fill in the grooves to provide constraint and dynamic control on the surrounding rock. The two complementary technologies proposed in this study provide a guide on how to control hanging wall of SDTCS in similar conditions.

Taking the sand-inrushing accident of the Selian No. 1 coal mine in the ordos of inner Mongolia as the research background, four main factors of sand-inrushing, including sand source, channel, sand-breaking power, and flowing space, were analysed. The disaster formation process (SCPS) illustrated that sand--inrushing disasters in shallowly buried coal seams with soft surrounding rock have the characteristics of being significantly influenced by mining, the development of vertical overburden channels, and sufficient space for water-sand mixed particles to flow. universal Distinct Element Code (UDEC) software has been used to reveal that the vertical cracks in the overburden between the coal wall and support undergo a process of development and expansion along with the cumulative stress of mining. This showed that the vertical fissure through the overburden is the main pathway for the disaster. Combined with the site conditions, disaster occurrence mechanism, and numerical simulation results, a comprehensive prevention and control technology based on the working face and roadway grouting to block the channel was proposed. it contains reasonable mining height and optimisation of advancing speed, so that safe and efficient mining of coal seams in shallow-buried soft surrounding rocks could be achieved. [ABSTRACT FROM AUTHOR]