Top Coal Caving Working Face Research Articles

Development Law of Water-Conducting Fracture Zones in Overburden above Fully Mechanized Top-Coal Caving Face: A Comprehensive Study

Although it is of great significance to master the height of the water-conducting fracture zone (WCFZ) to prevent coal mine disasters and ensure safe production, the most important thing is to predict the height and range of the WCFZ ahead of the working face design before coal mining. Therefore, the 150313 fully mechanized top-coal caving working face of the Yinying coal mine was taken as the engineering background. The development laws of WCFZ were studied using comprehensive research methods, including similar simulation experiments, key strata theory, the experience formula, the numerical simulation, etc. The results show that the WCFZ evolution stage is “goaf–caving zone–fracture zone” and the developing pattern is in a non-isosceles trapezoid gradually developing upward and forward. The height of the WCFZ in the 150313 working face is 89.36 m, and the fracture/mining ratio is 12.46, which is consistent with the actual production. Apparently, the set of indoor research methods in this paper is feasible to predict the height and scope of the WCFZ. The research results can provide a scientific reference for safe mining of the 15# coal seam in Shanxi Province and the prevention and control of roof water hazards.

Research on the Low-Temperature Oxygen Consumption and Low-Oxygen Gas Emission Law in Goaf of Ultrathick Coal Seams.

Low-oxygen (oxygen concentration below 18.5%) phenomena often occur in the top coal caving working face of ultrathick coal seams, posing a serious threat to the safety of workers. The characteristics of oxygen consumption and gas production at low-constant temperature and the corresponding functional group evolution of residual coal in goaf were studied by temperature-programmed and infrared spectrum experiments. The influence of different factors on the emission of low-oxygen gases was studied through numerical calculation. The results show that low-temperature oxygen consumption and gas production occurred when the coal was about 40 °C. When the temperature was constant, the oxygen consumption and gas production rate increased with the extension of time. In the early stage of coal oxidation, the aliphatic C-H components were attacked by oxygen molecules and reacted with them. The asymmetric methyl and methylene groups were more likely to oxidize and produce carbonyl compounds. With the increase of nitrogen injection, the overall width of the oxidation zone (oxygen concentration was defined as 10-18%) narrowed, and the range of the oxidation zone moved forward from the depth of the goaf. The oxygen concentration in the air return corner decreased gradually, and the low-oxygen area in the air return corner expanded gradually. The distance between the low-oxygen area of the working face and the air intake corner was gradually shortened. With the increase of air intake, the width of the oxidation zone increased and moved to the depth of goaf, and the degree of low oxygen in the air return corner increased. The research results are of great significance for the understanding and prevention of the low-oxygen phenomenon in ultrathick coal seams.

Research on overburden structural characteristics and support adaptability in cooperative mining of sectional coal pillar and bottom coal seam

In the mining process of the II1 coal seam at Zhaogu No. 2 coal mine, a method of stratified mining is employed, leaving relatively wide coal pillars in sections. To enhance the resource recovery rate, the mine carries out the cooperative mining of the sectional coal pillars and the lower layer coal seam. The 14,022 cooperative working face of fully-mechanized and fully-mechanized top-coal caving at Zhaogu No. 2 coal mine is taken as the research object. Through numerical simulation, theoretical calculations, and on-site industrial trials, a comprehensive analysis of the overburden structural characteristics and the support adaptability at the working face is conducted. It is clarified that a stress arch bearing structure can be formed above the sectional coal pillars during cooperative mining, and this structure is controlled by key strata. The formation of a stress arch bearing structure in the overburden above the sectional coal pillars provides protection for the underlying mining area. A formula for calculating the working resistance of hydraulic supports under the stress arch in sectional coal pillar is derived. Based on these results, the working resistance of hydraulic supports in the coal pillar area is calculated and selected. Field application shows that the working resistance of the support is 10,000 kN in the fully-mechanized top-coal caving working face, and is 9000 kN in fully-mechanized working face, meeting the support requirements and ensuring safe mining at the working face. This study provides a valuable engineering reference for achieving cooperative mining of abandoned sectional coal pillars and lower layer coal seam in stratified mining method.

Reasonable stopping method and retracement channel support at fully mechanized top coal caving working face of 15 m extra‐thick coal seam: A case study

AbstractExtra‐thick coal seams are widely distributed in the Datong mine area in China. The rapid stopping of mining and the support withdrawal technology for extra‐thick coal seams need to be developed urgently. The reasonable stopping top coal caving distance, main roof's fracture line position, and large section roadway during the stopping period (LSRSP) support method of the header face of 15 m extra‐thick coal seam are systematically studied by using field measurement, experiment, numerical simulation, and similar material simulation methods. With the increase in stopping coal caving distance, the range of the medium and low displacement zone of the LSRSP overburden gradually expands. In contrast, the depth of the plastic zone decreases. The scope and intensity of the high‐stress area of the overlying rock in front gradually decrease and shift from deep to the outside, while the peak stress area shifts from the top coal area to the coal rib of the retracement channel. Comparing the stopping of medium‐thick and thick coal seam working face, the interaction between “main roof−unmined top coal−supports” is analyzed. The unique characteristics of the extra‐thick coal seam working face are derived: when the stopping coal caving distance is shorter than the length of the key block, the dropped coal body cannot effectively restrict the movement of key blocks. Mutual compression and subsidence occur between key blocks, making the broken coal rock block squeeze into the supports. By the simulation study of each index, the stopping coal caving distance of the extra‐thick coal seam is optimal when it is slightly larger than the length of the key block (i.e., the periodic weighting step), simultaneously, the main roof fracture line behind the supports is most favorable for stopping mining. After ensuring the stability of the LSRSP's overburden structure, the differentiated support scheme of “controlled zoning−strength grading” was proposed, and the scheme was modified and verified by simulating the support prestress field. After field practice, the roof of the retracement channel is better controlled in deformation under the joint horizontal and vertical support, the robust control effect of the combined anchor cable is noticeable, the integrity of the polyurethane network is more substantial, and the working face supports have successfully achieved a safe and rapid evacuation.

Application of improved and optimized fuzzy neural network in classification evaluation of top coal cavability

Longwall top coal caving technology is one of the main methods of thick coal seam mining in China, and the classification evaluation of top coal cavability in longwall top coal caving working face is of great significance for improving coal recovery. However, the empirical or numerical simulation method currently used to evaluate the top coal cavability has high cost and low-efficiency problems. Therefore, in order to improve the evaluation efficiency and reduce evaluation the cost of top coal cavability, according to the characteristics of classification evaluation of top coal cavability, this paper improved and optimized the fuzzy neural network developed by Nauck and Kruse and establishes the fuzzy neural network prediction model for classification evaluation of top coal cavability. At the same time, in order to ensure that the optimized and improved fuzzy neural network has the ability of global approximation that a neural network should have, its global approximation is verified. Then use the data in the database of published papers from CNKI as sample data to train, verify and test the established fuzzy neural network model. After that, the tested model is applied to the classification evaluation of the top coal cavability in 61,107 longwall top coal caving working face in Liuwan Coal Mine. The final evaluation result is that the top coal cavability grade of the 61,107 longwall top coal caving working face in Liuwan Coal Mine is grade II, consistent with the engineering practice.

Numerical simulation of realistic top coal caving intervals under different top coal thicknesses in longwall top coal caving working face

In the process of longwall top coal caving, the selection of the top coal caving interval along the advancing direction of the working face has an important effect on the top coal recovery. To explore a realistic top coal caving interval of the longwall top coal caving working face, longwall top coal caving panel 8202 in the Tongxin Coal Mine is used as an example, and 30 numerical simulation models are established by using Continuum-based Distinct Element Method simulation software to study the top coal recovery with 4.0 m, 8.0 m, 12.0 m, 16.0 m, 20.0 m and 24.0 m top coal thicknesses and 0.8 m, 1.0 m, 1.2 m, 1.6 m and 2.4 m top coal caving intervals. The results show that with an increase in the top coal caving interval, the single top coal caving amount increases. The top coal recovery is the highest with a 0.8 m top coal caving interval when the thickness of the top coal is 4.0 m, and it is the highest with a 1.2 m top coal caving interval when the coal seam thickness is greater than 4.0 m. These results provide a reference for the selection of a realistic top coal caving interval in thick coal seam caving mining.

Quantitative Evaluation of Top Coal Caving Methods at the Working Face of Extra-Thick Coal Seams Based on the Random Medium Theory

Adopting an effective top coal caving method is the key to enhancing coal recovery and reducing gangue content for the fully mechanized top coal caving working face with extra-thick coal seams. In this study, the movement of coal particles generated during top coal caving is considered to follow a normal distribution. Then, the caving body and coal-rock settlement along the working face during the caving process are studied based on both the random media theory and probability theory. Accordingly, the optimal caving interval and caving sequences are determined, and a novel interval symmetrical coal caving method is proposed. The proposed method is systematically verified with results from physical similarity tests, and different caving methods are assessed by field tests. The results show the following: (1) The coal-rock settlement and the caving body demonstrate clear axial symmetrical features along the working face; the size of the caving body increases as the caving height grows and its shape turns progressively from semicircular to semielliptical with a lower foot of the coal-rock settlement. (2) The caving interval is derived using the sum of the radii of the coal-rock settlement curves formed by the two largest caving bodies. (3) The symmetrical caving approach provides a symmetrical space for the subsequent movement of the broken top coal, which enables a uniform development of the caving body. (4) Compared with the traditional sequential coal caving method with the same number of supports, the interval symmetrical caving method results in a 21.7% of coal production increase, 17% caving rate promotion, and a shortened caving time by 23.4%. (5) The interval symmetrical caving method is found to improve the controllability of the caving process at the fully mechanized top coal caving working face. In general, this work presents a theoretical approach to select the optimal caving methods for the fully mechanized caving working face in extra-thick coal seams for an improved production efficiency of the work face. The results of this study can also provide theoretical significance and referencing value for quantitative analyses of the coal caving methods for work faces with similar geological conditions.

Research on the Ground Pressure Features of Long Wall Top Coal Caving Working Face in Extra-Thick Coal Seam

Based on the raw data of shield pressure and overlying strata movement measured in the field, the ground pressure features and overlying strata movement were analyzed in 8202 extra-thick coal seam longwall top coal caving working face of Tongxin coal mine after its extraction. The research results show that: during the process of mining, the interval of the lower sub key strata caved for the first time is 120 m, the periodic weighting interval ranges from 15 m to 30 m, with an average of 25 m. When the immediate roof is relatively thick, the fractured lower sub key strata can be hinged to form a stable “Voussoir Beam” structure, which makes the ground pressure not severe and the shield pressure small in the ends of the panel. When the immediate roof is thin, the arc triangular of the lower sub key strata in the ends of the panel loses mechanical contact to the caved immediate roof and assumes a “Cantilever Beam” structure, which makes the ground pressure severe and shield pressure large in the ends of the panel. Due to the large mining space of longwall top coal caving working face in extra-thick coal seam and big rotary angle of the overlying strata, the lower sub key strata can’t connect with each other to form a stable bearing structure, therefore, the shield pressure is a given load of roof rock, which results in a constant pressure of the shield.

Strong Ground Pressure Mechanism and Control at the Longwall Top Coal Caving with a Single Key Stratum in Goaf

A strong ground pressure in the multiseam environment manifested by rib spalling and roadway deformation at the fully mechanized working face was assessed by a comprehensive combination of field measurements, physical simulations, and theoretical analysis for two coal seams in the Buertai Coal Mine in China. A structural model of overlying stratum collapse at the working face with the key stratum breaking instability was proposed, the mechanism of strong ground pressure at the longwall top coal caving working face with a single key stratum in goaf was identified, and respective control countermeasures were developed. The latter implied the directional hydraulic fracturing for supporting the key stratum-surrounding rocks, which effectively reduced the cyclic weighting intensity and weighting interval in the working face with a single key stratum in the goaf. The working face cyclic breaking interval was assessed at 30 m. After the key stratum collapse, soft rocks underwent synergistic deformation and a cutting-type failure. The goaf effect on the hydraulic support resistance in the fully mechanized working face was assessed, and cutting blocks from the overlying stratum collapse were identified as the main sources of strong ground pressure.

Strata movement and shield pressure analysis at Tongxin longwall top coal caving working face with extra-thick coal seam

Roof strata movement and shield pressure are two important factors that affect the production and efficiency of longwall top coal caving (LTCC) method in extra-thick coal seams. The performances of these two aspects can be very different from one mine to another depending on the roof strata conditions. In this paper, the geological conditions of the LTCC 8202 panel at Tongxin Coal Mine of Datong Coal Field are investigated. Low working face pressure was observed in the 8202 panel when mining the 20.6-m-thick carboniferous coal seam. This paper examines the causes of low mining–induced pressure at the LTCC 8202 working face, with both numerical simulations and in situ measurements. A finite element-discrete element software called CDEM is used for two-dimensional modelling of the #3–5 coal seam extraction at the LTCC 8202 working face. The numerical model simulates progressive collapse of the roof strata and computes shield pressure and convergence of the shield legs as the coal face advances. It is shown that the numerical model reproduces the periodical weighting phenomenon observed at the working face, which is created by the successive formation of composite cantilever beams in the lower hard rock layers, while voussoir beams form in the upper hard rock layers. As the upper roof voussoir beams interlock, they will tend to support part of the overlying strata, which ultimately lessens the pressure on the shield. A simple analytical model is developed to calculate the shield pressure under the composite cantilever beam structure. Finally, comparison of numerical simulation and analytical model results with measured shield pressure data from the field shows good agreement.

Numerical simulation on strata behaviours of TCCWF influenced by coal-rock combined body

Due to top-coal and immediate roof as cushion layer connecting with support and overlying strata, it can make significant influence on strata behaviors in fully mechanical top-coal caving working face (TCCWF). Taking Qingdong 828 working face as engineering background, FLAC3D and UDEC2D were adopted to explore the influence of top-coal thickness (TCT), immediate roof thickness (IRT), top-coal elastic modulus (TCEM) and immediate roof elastic modulus (IREM) on the vertical stress and vertical subsidence of roof, caving distance, and support resistance. The results show that the maximum roof subsidence increases with the increase of TCT and IRT as well as the decrease of TCEM and IREM, which is totally opposite to vertical stress in roof-control distance. Moreover, although the increase of TCEM and IREM leading to the increase of peak value of abutment pressure, the position and distribution range have no significant change. Under the condition of initial weighting occurrence, support resistance has negative and positive relationship with physical parameters (e.g., TCT and IRT) and mechanical properties (e.g., TCEM and IREM), respectively.

Radiation characteristics of natural gamma-ray from coal and gangue for recognition in top coal caving

Recognition of coal and gangue (roof rock) is a key technology to realize fully mechanized top coal caving automated mining. This paper proposes to detect the instantaneous refuse content of drawn coal and gangue mixture during top coal caving by using natural gamma-ray technology. The generating environment of coal and rock seams, the distribution characteristics of natural gamma ray from coal and roof-rock and the principle of coal-gangue recognition using natural gamma-ray method were analyzed. The natural gamma ray radiation characteristics of coal and roof-rock seams from seven different typical coal mine areas who has thick coal seams in China have been researched, and a connection between radiation intensity and refuse content was set up. The experiments on the mixed condition of roof-rock drawn from caving opening in the caving process of fully-mechanized top coal caving working face was taken and the radiative signals was real-time detected by using the self-developed coal-gangue recognition experimental system. The experiments results demonstrate the feasibility of using natural gamma-ray technology to perform real-time detection of refuse content of drawn coal and gangue mixture and the availability of self-developed coal-gangue recognition detector.

Optimization and Practice of Support Working Resistance in Fully-Mechanized Top Coal Caving in Shallow Thick Seam

Based on the overburdened structure of shallow coal seams in the West of China, the traditional support resistance method may not apply in such regions due to many roof shear and support crushing accidents. Four support working resistance indexes—rated working resistance, average working resistance, average-partial working resistance, and average-upper working resistance are counted as the candidate working resistance. The method of optimizing and inverting working resistance was put forward based on the roof control effect and working resistance overrun percentage. The simulation of a fully-mechanized top coal caving working face was built by using the UDEC software to analyze the top coal cavability and roof subsidence under different supporting strengths to determine the final working resistance. The project practice of 1322 working face in Jindi coal mine shows that the working resistance is 80–90% of calculated rated working resistance. The hydraulic support is worked under good condition. There is no pillar shrinkage and support crushing during coal mining.

Optimization Coal Pillar Size of Top Coal Caving Face with Dynamic Pressure Impact

AH Wilson coal pillar was used widely as it’s simply, but it’s appeared large error for field implementation as its difference assume conditions, mine depth H and mine thickness m. AH Wilson coal pillar formula was studied precisely by in-site stresses test and numerical simulation analysis for N3-5 top coal caving working face of CHANGCUN coal mine in Lu’an coal district of China, then modified AH Wilson formula was put forward as L = 0.008mH + 8.4,then the precise coal pillar size 18m was used in filed implementation, filed testing proved coal pillar size was reasonable.

Application of the Vibration to the Top Coal Caving

The article discussed the advantages and disadvantages of the traditional long wall top coal caving (LTCC) first, then proposed a new technique of top coal caving with vibration. The theoretical basis of the technique was elaborated.The top coal vibration system and its control system have been designed as such when the system works, the emulsion pump station provides power for whole system, the current pulse generator provides current pulse. This technique has been applied to the 5209 top coal caving working face in Wang Zhuang coal mine (Shanxi Province). Based on the experiment, the vibration can easily destroy the arch structure which is formed during the top coal caving process. The top coal caving with vibration technique has a higher coal recovery ratio. This technique improves the top coal broken effect and lowers the waste content rate. The significant economic and social benefits proved it to be a promising technique.

Research on the Catapulted Dynamic Mechanism of Broken Cable-bolt and the Defense Measures

Abstract During the extraction process of the top coal caving working face, the fierce looseness, deformation as well as fracture of the surrounding rocks would usually occur because the slots will get strong disturbance. Meanwhile, anchored bolt and cable-bolt shall deform accordingly. When the deformation of the anchored bolt or cable-bolt comes to the extreme extent, they will crack. Then the catapulting and wounding incident will easily happen if the fractural part of the anchored bolt and cable-bolt is not fully griped by the rocks. In the paper, based on the dynamic mechanism analysis on fracture phenomenon about the anchored cable-bolt, the formulas of the kinetic energy and speed of the fractural catapult of the horizontally anchored cables in the ribs and the vertically anchored cables in the roof have been established. And a bolt support theory “small pretightening force & large expected anchorage strength” has been put forward, which is useful for seriously deformed roadways to prevent anchored cable-bolts from breaking. In addition, a technological method to stop anchored cable-bolt ejecting by means of pipe cap has been put forward.

Technical parameters of drawing and coal-gangue field movements of a fully mechanized large mining height top coal caving working face

Under fully mechanized, large mining height top coal caving conditions, the shield beam slope angle of the support increases due to the enlargement of the top coal breaking and caving space. This results in a change of the caving window location and dimensions and, therefore, the granular coal-gangue movement and flows provide new characteristics during top coal caving. The main inferences we draw are as follows. Firstly, after shifting the supports, the caved top coal layer line and the coal gangue boundary line become steeper and are clearly larger than those under common mining heights. Secondly, during the top coal caving procedure, the speed of the coal-gangue flow increases and at the same drawing interval, the distance between the coal-gangue boundary line and the top beam end is reduced. Thirdly, affected by the drawing ratio, the slope angle of the shield beam and the dimensions of the caving window, it is easy to mix the gangue. A rational drawing interval will cause the coal-gangue boundary line to be slightly behind the down tail boom lower boundary. This rational drawing interval under conditions of large mining heights has been analyzed and determined.

Technique of top coal caving with vibration

How to understand the top coal caving process and increase the top coal recovery ratio is one of the urgent problems to be resolved for the longwall top coal caving technology. The paper mainly studies a new theory and technique of top coal caving with vibration. The vibration device is mounted on the shield beam of the top coal caving support. The laboratory experiment and the numerical simulation were adopted in the study. The Distinct Element Method (DEM) numerical software (PFC2D) is used for the numerical simulation. Finally, the technique is applied to the 5209 top coal caving working face. Based on the experiment, the numerical simulation and the practice, the vibration can easily destroy the arch structure which is formed during the top coal caving process. The parameters about when the arch structure would be formed during the top coal caving process are obtained. The movement of the top coal caving process under the vibration condition makes a more stable layer uniform caving performance. The aim of the top coal caving with vibration technique, to increase the top coal recovery ratio and to lower the waste content rate, has been achieved.

Control effect of fracture on hard coal cracking in a fully mechanized longwall top coal caving face

Through theoretical analysis, simulation test and practice, the law of a fracture’s influence on hard top coal press cracking was studied. The study focused on the relation between fracture and coal strength, top coal caving ability and work face layout. Based on the investigation of the fracture system, the control of press cracking was achieved by matching working face to fracture orientation to improve top-coal caving ability and recovery. The matching principle was pointed out: The top-coal caving working face should be perpendicular to or obliquely cross the primary fracture at a large angle, and cross the secondary fracture at a small angle. The rational match can increase the recovery ratio of top-coal and avoid rib spalling. The application of control technology on hard top coal press cracking was introduced at the longwall top-coal caving face.