Close Distance Coal Seams Research Articles

Study on Disaster Mechanism and Prevention of Air Leakage Channel in Shallow Close Distance Coal Seam Group

Study on Disaster Mechanism and Prevention of Air Leakage Channel in Shallow Close Distance Coal Seam Group

Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam

To investigate the reasonable width of a coal pillar in the downward mining section of close-distance coal seams, the stress state of any point below the residual coal pillar in the overlying goaf and the width of a small coal pillar were studied by theoretical calculation, numerical simulation, similar simulation and field monitoring. The findings indicate that the width range of the small coal pillar is 7.92~11.42 m. The 4-1 coal seam is in the stress reduction zone when it is more than 16.6 m horizontally from the border of the residual coal pillar above it. In addition, the peak stress is situated inside the elastic zone of the coal pillar and is lower than the coal pillar’s bearing limit when a small coal pillar of 8 m is maintained. With the help of distributed optical fiber monitoring to model the coal pillars’ stress distribution, it is found that 8 m simulated coal pillars have a certain bearing capacity. The practical findings demonstrate that the 8 m small coal pillar that was left on the site satisfies the demand, and the convergence of the roadway’s floor and roof, and its two sides fall within the controllable range. The findings of the study offer a reference for the location of a return air roadway and the width of section coal pillars in the downward mining of close-distance coal seams.

Research on precise quantitative traceability of combined gas extraction in close-distance coal seam group

Research on precise quantitative traceability of combined gas extraction in close-distance coal seam group

Reasonable layout range of lower mining roadways in the close and variable interval coal seams mining: a case study.

In the context of close-distance coal seam mining, the stress transfer mechanism following the extraction of the upper coal seam is crucial for the layout of roadways in the lower coal seam. This study utilizes ground-penetrating radar (GPR) to analyze the stability and plastic zone extent of residual coal pillars after upper seam extraction. A theoretical model for the stress distribution of multiple goaves and coal pillars was established. Based on a case study with variable inter-seam spacing, the stress distribution and transfer mechanisms of the floor were analyzed. The results indicate that during the transition from the center of the coal pillar to the goaf, stress concentration initially decreases significantly and then gradually returns to the original rock stress, forming a distinct stress gradient distribution. As the depth of the floor strata increases, the weight of the overlying strata is more evidently transferred through the coal pillars to the goaf. Finally, a reasonable layout range for the lower seam mining roadways under different inter-seam spacings is proposed, providing a reference for optimizing roadway layout in similar geological conditions.

Study on the evolution of fractures in overlying strata during repeated mining of coal seams at extremely close distances

In particular, the secondary development of overlying strata fractures can easily lead to the upper goaf, resulting in gas and water gathered in the goaf entering the working face of the lower coal seam through the overlying strata fractures, threatening the safety of coal mine production. Security risks may arise. To further understand the caving and evolution law of overlying strata during repeated mining in extremely close distance coal seam down mining, 9# coal and 10# coal in the Nanyaotou Coal Industry were used as the engineering background. The caving characteristics and fracture evolution law of overlying strata during single and repeated mining were analyzed through similar material simulation tests. Based on fractal geometry theory, the relationship between the advancing distance of the working face and the fractal dimension of the overlying strata fracture is established to reflect the changing trend of fracture development. The calculation formula is derived from the tensile rate of rock strata to predict the development height of water-conducting fractures. The results show that the overlying strata failure structure is mainly a “hinged structure” and a “step structure,” which respectively promotes and inhibits the development of overlying strata fractures. Repeated mining causes mining-induced fractures in the lower coal seam to pass through the goaf of the upper coal seam and develop more vigorously in the upper coal seam, and the fractal dimension can effectively reflect the development of overlying strata fractures. The height of the water-conducting fracture zone increases in four stages: incubation, gradual increase, further gradual increase, and stability, eventually stopping development under the influence of the key layer (thick mudstone) bearing the load above. The development height of water-conducting fractures predicted by on-site water injection measurement is similar to that predicted by simulation experiments and theoretical calculations, verifying the feasibility of predicting the development height of water-conducting fractures through simulation tests and theoretical analysis. This study provides a reference for coal seam mining under similar conditions.

Research on the mechanism and control of roof deformation in gob-side entry retaining of close-distance coal seams

Multiple mining operations in close-distance coal seams (CDCSs) can reduce the integrity and bearing capacity of the surrounding rock, leading to roof fracturing and widespread subsidence in gob-side entry retaining (GER). This highlights the critical need for research on roof control technology. For this purpose, adopting the panel of the Xinchazhuang Coal Mine (the distance between two coal seams is 7.3 m) as the engineering background, a theoretical model of GER of CDCSs was established on the basis of the surrounding rock structure and roof rock movement characteristics, and the roof failure deformation mechanism was examined. The roof failure deformation encompasses two parts: rotational sinking of roof rock blocks and roof separation deformation. To achieve roof control, a strategy involving optimized roof cutting and higher preload cables was suggested. The impacts of key roof control parameters on roadway deformation were examined and compared by numerical simulation studies. The findings showed that optimized roof cutting resulted in a 21.1 % reduction in the roof stress, whereas the use of roof cables with higher preload levels resulted in a 55.1 % decrease in roadway deformation, effectively mitigating rotational sinking and roof separation deformation. The field tests demonstrated that the optimized scheme facilitated roof integrity preservation, and the maximum deformation of the roof reached 98 mm, a decrease of 68.5 % on average from that in the original scheme. These research results could provide a reference for roof support in CDCSs roadways under similar geological conditions.

Study on Classification of Close Coal Seams in Hebao Mining Area Based on Layer Spacing

In order to understand the situation of close coal seam in Baode mining area of Hequ, it is convenient to form the surrounding rock control technology of roadway in close coal seam. Aiming at the inner staggered roadway under the goaf in Hebao mining area, a mechanical model is established, the influence of layer spacing on the inner staggered roadway is analyzed, and a short-distance coal seam classification method based on layer spacing is proposed. The results show that the close distance coal seams in Hebao mining area can be divided into three categories: 0-2m is the close distance coal seam with very small spacing, 2-6m is the close distance coal seam with small spacing, and more than 6m is the close distance coal seam with large spacing.

Stability control of goaf‐driven roadway surrounding rock under interchange remaining coal pillar in close distance coal seams

AbstractThe roadway surrounding rock stability under the influence of interchange remaining coal pillar in close distance coal seams is one of the key factors affecting the coal mines safe and efficient mining. To reduce the roadway deformation, theoretical analysis, numerical simulation, and on‐site verification were used to study the influence of geological, mining, and support factors on the stability of the floor roadway surrounding rock. A design method for strengthening the roadway surrounding rock support was proposed. The results showed that the coal seam depth, the distribution characteristics of the remaining coal pillar load, the starting point position of stress recovery in the upper coal seam goaf and the lower coal seam goaf lateral load had a significant influence on the lower coal seam stress. The peak stress in the lower coal seam increased linearly with the increase of the coal seam depth, the remaining coal pillar, and the lower coal seam goaf lateral peak stress. The remaining coal pillar influence area width increased linearly and exponentially with the increase of the remaining coal pillar width and the distance between the starting point of stress recovery in goaf and the coal pillar edge, respectively. The peak stress and shear strength of the narrow coal pillar decreased exponentially with the increase of the spacing between bolts or cables. The roadway surrounding rock deformation decreased exponentially with the increase of the distance between the strengthening support area and the remaining coal pillar edge. For the Baigou coalmine, a design scheme for bolt strengthening support parameters has been proposed. The roadway surrounding rock deformation tended to stabilize after 21 days, with deformation values of 264 and 488 mm for the roof and floor, and two sides, respectively.

Correction: Study on Roof Deformation and Failure Law of Close Distance Coal Seams Mining Based on Digital Image Correlation

Correction: Study on Roof Deformation and Failure Law of Close Distance Coal Seams Mining Based on Digital Image Correlation

Study on Roof Deformation and Failure Law of Close Distance Coal Seams Mining Based on Digital Image Correlation

Study on Roof Deformation and Failure Law of Close Distance Coal Seams Mining Based on Digital Image Correlation

Research on long-term migration behaviors of heavy metals after close-distance coal seam backfill mining

The backfill mining of coal-based solid waste in goaf poses a potential risk of heavy metal pollution to the groundwater environment, and the migration behavior of heavy metals differs significantly under the disturbance of backfill mining in close-distance multi-layer coal seams and single-layer coal seams. In this study, a migration model of heavy metals after solid backfilling in the goaf of shallow-buried close-distance thick coal seams was established, and the impact of the overburden damage and the layered distribution of the filling body on the long-term migration behavior of heavy metals were analyzed. The results show that the migration of heavy metals after close-distance coal seam backfill mining exhibits a higher risk of heavy metal pollution. The peak permeability of overburden after close-distance coal seam backfill mining is about 600 × 10−19 m2 higher than that after single-layer coal seam backfill mining. The migration distance of heavy metals in the floor after backfill mining of close-distance coal seams is 7.41 m farther than that of single-layer coal seam backfill mining, and its migration time of heavy metals to the surface is 27 a earlier than that of single-layer coal seam. This research provides theoretical and empirical support for the ecological risk assessment and heavy metal pollution control in close-distance coal seam backfill mining. Environmental ImplicationThe main filling material of close-distance coal seams backfill mining is coal gangue. Heavy metal elements such as Mn and Cr will be released in the underground environment for a long time, and the migration behavior of heavy metal elements will have an impact on the groundwater environment for more than 1000 years. This research provides theoretical and empirical support for the ecological risk assessment of close-distance coal seam backfill mining and the mitigation of heavy metal pollution.

Analysis of Dynamic Evolution of Surrounding Rock Movement and Stress-Fracture in the Upward and Repeated Mining of Close-Distance Coal Seams

The distribution of mining-induced stress and the resulting rock fractures are two crucial factors affecting mineral extraction in protective layer mining. This research establishes a correlation between the vertical fracture aperture and the second derivative of the rock layer’s subsidence curve equation. The article explores the span requirement for a simply supported beam to fracture. This condition is relevant to understanding the dynamic evolution of rock movement and stress fractures during repeated mining of close-distance coal seams. Our study investigates alterations in rock stress and fractures resulting from repeated upward mining of coal seams, using the nearby coal seam cluster in Jincheng Mine as a case study. The research findings indicate that during the mining of the upper coal seam, the roof experiences significant but brief periodic loading intervals, as well as severe and moderate periodic loading. As mining progresses to the lower coal seam, pressure relief of the upper coal seam gradually increases in both degree and range. In the upper coal seam, the vertical stress distribution follows a sequence of “V,” “U,” and “W” forms. The upper coal seam undergoes five stages of expansion deformation: compression, expansion, increased expansion, decreased expansion, and stable expansion.

The evolution law of deviatoric stress and asymmetric control technology in roadways during panel mining through overlying residual coal pillars

Close-distance coal seams (CDCS) are widely distributed, and the layout of the upper and lower panels can be divided into “=” type and “+” type. The “+” superposition of upper and lower coal pillars in CDCS caused strong mine pressure, but there are few studies on the panel crossing residual coal pillars (RCP) when the upper and lower coal seams are “+” type layout. In view of the special spatial position (“+” type layout), this paper takes the typical panel 4-301 of a particular mine as the project indagation background and studies mining and crossing the overlying coal pillars by dint of field measurement, numerical simulation, indoor test, and engineering application. Compared with vertical stress or horizontal stress alone, the indexes of deviatoric stress and plastic zone can reflect the failure evolution of surrounding rock more comprehensively. Hence, this paper analyzes the expansion form of the plastic zone and the variation law of deviatoric stress before and after mining influence in the underlying mining roadway. The research results show that: (1) There is a sub-peak zone of deviatoric stress under the RCP. The deviatoric stress is bimodal in the range of 9 m below. After the peak value decays to 7.4 MPa, it changes to a single peak located in the area directly below the middle of the RCP. (2) The maximum plastic zones of the roof and two ribs of the roadway below the RCP are 3.4 m and 5 m, respectively. The crest value of deviatoric stress reaches 10 MPa. As the distance between the panel and the RCP decreases, the shape of the high deviatoric stress area presents the evolution law from the “ellipse” of the roof → the “crescent” of two ribs → the “cochlea” of the tips of the ribs. (3) When the mining of the underlying panel is 10 m, 0 m, or − 10 m away from the RCP (without passing through the RCP). The crest value of deviatoric stress within 5–10 m in advance of the roadway increases in turn. However, the peak value is significantly reduced when it is − 20 m away from the RCP (through the RCP). The crest value of deviatoric stress of two ribs decreases in turn along the panel rib → section coal pillar rib → solid coal rib. Based on this, the underlying 45 m of the RCP is divided into area I (10 m), area II (overlapping area 20 m), and area III (15 m) based on the degree of disturbance. And propose the technical scheme of asymmetric combined control in different zones by using asymmetric channel steel truss anchor cable for the top-ribs of areas I and III, and top-ribs asymmetric channel steel truss anchor cable + door-type support in area II. On-site project practice shows that the partitioned control technology successfully resisted the roadway instability and failure caused by the dynamic-static superimposed stress disturbance under the RCP and realized the primary support of the sectional coal roadway. The conclusion provides technical support and scheme design for the partitioning support of roadways under similar “+” type cross‐panels.

Periodic action mechanism of soaking on the evolution of lignite pore structure and microactive groups

As the mining of the overlying coal seam advances, the risk of spontaneous combustion (SC) of water-immersed and air-dried coal in the overlying goaf increases. The structure of functional groups in coal is the fundamental factor for the occurrence of oxidation and heat production, and the surface pores are the main sites of oxidation reactions. In order to explore the mechanism of action of water immersion on the macrostructure and micro-group of lignite, the water-immersed coals (WIC) of 30 days (I30), 60 days (I60), 90 days (I90), 120 days (I120) and 150 days (I150) were prepared. The surface pore characteristics of coal and the evolution of active functional group structure were studied by nitrogen adsorption and infrared spectroscopy. The adsorption capacity of pores and surface structure of coal were quantitatively characterized by pore adsorption kinetic equations and fractal methods. The results showed that the fine-necked bottle-shaped pore structure of water-immersed lignite decreased, and the wedge-shaped pores and parallel plate slits increased. After water-soaking, the specific surface area of lignite increased first and then decreased, the pore volume and pore diameter increased gradually, D1 decreased gradually, D2 increased gradually, and the pore adsorption potential increased gradually, which characterized the gradual pore structure complexity and enhanced adsorption capacity. Water-soaking triggered the conversion of active carbon-hydrogen bonds at the α-position of lignite molecules into active carbon–oxygen single bonds with alcohol-oxygen bonds. I90 and I120 have the highest content of active functional groups, indicating that the oxidation activity of lignite immersed in water for 90–120 days is the highest. This study provides a new method for the quantitative characterization of coal adsorption and has reference value for the study of SC mechanism of water-immersed, air-dried coal in the overlying goaf of close distance coal seam.

Non-uniform failure and differential pressure relief technology of roadway under irregular goafs in deep close-distance coal seams

In response to the control problems of large-deformation roadways in close-distance coal seams, taking a typical roadway under irregular goafs and residual coal pillar in deep close-distance coal seams as the background, the characteristics of mine pressure and key difficulties in surrounding rock control of roadway are explored and an improvement strategy for controlling surrounding rock is proposed. The stress expression in roadway floor under the influence of residual coal pillar is derived by theoretical calculation. The peak lines of deviatoric stress and vertical stress in roadway after the mining of the upper coal seam are obtained by numerical simulation. The roadway is divided into two key zones: ordinary zone and disturbance zone by residual coal pillar, and the disturbance range of roadway below residual coal pillar is determined to be 44.60 m. It reveals the differential, asymmetric, and non-uniform distortion and failure laws of roadway at different positions under irregular goafs and residual coal pillar. The differential control technology named asymmetric support in ordinary zone of roadway and combined support and drilling pressure relief in disturbance zone below residual coal pillar is proposed. The feasibility of differential pressure relief and control technology has been verified through on-site engineering test, which ensures the safety and stability of roadway and provides technical references for surrounding rock control in similar deep and complex roadways.

Research on the evolution and height prediction of WCFZ in shallow close coal seams mining

Based on the mining conditions of shallow close coal seams in the Jurassic coalfield of northern Shaanxi, in order to explore the evolution and development height prediction of water-conducting fracture zone (WCFZ), combining with measured statistical analysis, physical simulation, numerical calculation, and theoretical analysis, the evolution of mining-induced fractures and the development characteristics of WCFZ are analyzed, and the development height predicting method of WCFZ in shallow close coal seams mining is proposed. The results show that in shallow single coal seam mining, during the stage of incritical mining, the height of the WCFZ increases in a “jumping” pattern with the main roof periodic caving. After critical mining, it is basically stable. Its development height is positively correlated with the mining height. The ratio of development height to mining height is generally 20–30, and the change is not significant with the mining height, overall, the distribution of the WCFZ is “saddle shaped”. Based on close distance coal seams repeated mining, the primary fractures act as activation development when the interburden broken completely, the development of WCFZ at the mining boundary is significantly intensified, and ratio of the height to mining height in repeated mining is obviously lower than that in single seam mining, which is generally 18–25. The traditional empirical formula cannot be applied to the development prediction in shallow close coal seams mining, with an error of 10–50%. Considering the mining height, interburden thickness, and roof caving characteristics, the development height predicting method of WCFZ in shallow close coal seams mining is proposed. Compared with measured results, the calculation error is basically within 15%, which has good prediction accuracy, The research results can provide a theoretical basis for predicting the height of WCFZ in shallow close coal seams mining in the Jurassic coalfield of northern Shaanxi.

Evaluating the Safety of Using the Combined Mining Method in Close-Distance Coal Seams

Evaluating the Safety of Using the Combined Mining Method in Close-Distance Coal Seams

Failure mechanism and stability control of surrounding rock in mining roadway with gentle slope and close distance

In this study, theoretical analysis, numerical simulation and field measurement were adopted to analyze the difficult problem of controlling the stability of surrounding rock in gently inclined and close-distance coal seam mining roadways. The results showed that: (1) According to the properties and stress environment of the roadway roof, to determine the specific spatial range of the original roof stage, transition stage and regeneration roof stage of the roadway, and corresponding to the distribution of plastic zones in the spatial stages, was an important basis for analyzing the deformation mechanism of the roadway and proposing a phased collaborative control plan. (2) Under the influence of mining, the plastic zone of the transportation roadway expanded violently by about 10 m ahead of the working face. Within the range of roadway passing the overlying goaf, the plastic zone of the surrounding rock between the layers runed through, especially extending to the roof and two shoulder corners of the roadway. The roof and two shoulder corners of the roadway became the key parts for the stability control of the surrounding rock. (3) The mechanical mechanism of asymmetric expansion of the plastic zone was obtained as follows: the range was determined by the magnitude of the deviatoric stress and the orientation was dominated by the direction of the principal stress. The combined effect of the magnitude of deviatoric stress and the angle of principal stress caused asymmetric expansion of the plastic zone. (4) A phased collaborative control scheme was proposed, and numerical simulation and industrial verification were carried out on the proposed scheme. The deformation of the surrounding rock of the roadway at the engineering site was less than 150 mm, and the control effect of the surrounding rock was good, ensuring the safe and efficient recovery of the fully mechanized mining face. The present study revealed the mechanical mechanism of failure and instability of the surrounding rock of the gently inclined close distance coal seam mining roadway and proposed a phased collaborative control scheme, which could provide valuable information for mining research.

Study on deflection of principal stress and deformation characteristics of roadway surrounding rock under mining action in close-distance coal seams

The traditional index to determine the width of the stopping coal pillar (WSCP) is generally only the front abutment pressure, but the direction of the principal stress will deflect due to the mining action, which brings significant challenges to controlling the surrounding rock. Based on theoretical calculation and numerical simulation, we studied the deflection law of principal stress under mining action, quantified the bearing capacity of roadway surrounding rock, and determined WSCP. The results show that: (1) With the deflection of the principal stress, the distribution of the surrounding rock plastic zone also rotates; (2) λ and repeated mining dominate and aggravate the deflection angle and range of the principal stress in front of the working face, respectively. When λ is close to 1, the deflection of principal stress should be paid attention to in determining WSCP; (3) The anchorage composite bearing structure (ACBS) provides a scientific basis for quantifying the bearing capacity of surrounding rock. WSCP of the lower coal seam is determined to be 60–75 m, and the corresponding principal stress deflection is about 135°. The control of the surrounding rock in the diagonal direction where the maximum principal stress is concentrated should be paid attention to.

Failure analysis and prediction of roof instability in end face under repeated mining using early warning system

The overlying strata of the lower coal seam is easy to be collapsed causing the roof caving accident at the end face of the mining working face under repeated mining in close-distance coal seams. In order to predict the roof instability of the end face, the mechanical model of the granular arch structure is established in this study to further analyze its main influencing factors. The results show that the mining height of the working face, the advancing speed, the distance of coal seams, the tip-to-face distance, the strength of the surrounding rock and the support setting the load of the support are the main influencing factors on the roof caving of the end face. Subsequently, the prediction model of roof instability in the end face under repeated mining is constructed through the radial basis function neural network (RBFNN) and the above main influencing factors are regarded as input layer indexes. Meanwhile, the roof subsidence, coal wall deformation and support load are determined as the output layer indexes. The predicted results are closer to the results of sample tests. Finally, the early warning system, including monitoring and early warning, data query, emergency management, user management, and system settings, is designed to monitor roof conditions of the end face and timely warn the roof accidents. The field application proves that the system has good practical value, which is of great significance to intelligent prediction of coal mine stope disaster and prevent the end face roof disaster under repeated mining and. This will promote the safe and efficient construction of coal mine production.