Fully Mechanized Top-coal Caving Research Articles

Evolution of mining‐induced stress in fully mechanized top‐coal caving under high horizontal stress

AbstractIn fully mechanized top‐coal caving (FMTC), the effects of coal caving are closely related to the degree of damage to the top‐coal which is determined by the stress state therein. Therefore, research into the evolution of mining‐induced stress field is key to understanding failure mechanisms of top‐coal. By taking the 12 309 FMTC face in Wangjialing Coal Mine (China) as the engineering background and using field measurement and numerical simulation, we studied the evolution of the principal stress field in FMTC under high horizontal stress. The research results demonstrated that in situ stress in Wangjialing Coal Mine was a high horizontal stress and lateral pressure coefficients in the north‐south and east‐west directions were 1.52 and 0.45, respectively. Under the influence of mining, principal stresses were concentrated in front of the coal wall, with concentration factors of 2.26, 2.24, and 2.85, respectively. The minimum principal stress in unconstrained sides of the top‐coal in the roof‐control zone was tensile. The directions of intermediate and minimum principal stresses in the top‐coal were more sensitive to the influence of mining compared to the maximum principal stress. During rotation of the principal stress direction in top‐coal, the location (19 m in front of the coal wall) where the maximum and intermediate principal stresses were equal and the location of the coal wall are key positions.

Theoretical analysis of support stability in large dip angle coal seam mined with fully-mechanized top coal caving

1. BUZILO V., SERDYUK V., YAVORSKY A., 2010, A research of influence of support resistance of the stope in the immediate roof condition, New Techniques and Technologies in Mining-Proceedings of the School of Underground Mining, 127–130. Google Scholar

Numerical Simulation Applied in Identification of Roof Bed Separation for Mining Thick Coal Seam Under Nappe Structure

In China coalmines, economic losses and human casualties inflicted by roof bed separation water have been acutely severe in recent years. This article takes the engineering case, mining thick coal seam under nappe, in Xinji No. 1 Coalmine in Huainan mining field. Given the low tensile strength of lower roof bed under large mining heights, numerical simulation is applied to No. 1307 fully mechanized top-coal caving (FMTC) working face by coupling elastoplastic finite element model (FEM) in rock mechanics with “no-tension analysis” (NTA) to map out, in the roof bed, pattern of stress distribution and the maximum heights of caving zone (37m) and of water-conductive fractured zone (98.5m), quantitatively delineating the location of bed separation. Specifically, bed separation F is the closest to the water-conductive fractured zone (by merely 1.93m) and of a maximum width of 6.4m, flagging a major threat to mining safety. These results are further corroborated by exploratory drills. Outcomes can be of reference to other coal-mining countries when identifying bed separation and treating related water hazards during mining under complex geological conditions.

Characteristics of Strata Behavior during Thick Seam Mining by Fully-Mechanized Top Coal Caving in a Loess-Covered Gullied Region

This study systematically investigates the overburden movement and strata behavior at a fully-mechanized top coal caving (FMTCC) face in a thick coal seam in a loess-covered gullied region by field measurement and theoretical analysis. A comparative analysis was performed to examine how the attitude of surface gullies and the structure of overburden affect face support resistance. The characteristics and evolution mechanism of strata behavior during FMTCC under gully slopes in a loess-covered gullied region were discovered through a mechanical analysis of the roof structure and a comparison to the mining of a typical shallow seam under gully slopes. The study found that as the FMTCC face advanced, the strata behavior initiated in the middle part of the face and then continued to both ends, and its intensity was higher in the middle part than in the upper and lower parts. During mining under gullies, the increases in support resistance were generally less than 5% (except for the maximum increases of 7–9%), indicating that the occurrence of gullies on the surface had little influence on the strata behavior at the FMTCC face. Due to the increased burial depth and the number of key strata, the roof at the FMTCC face under gullies formed a stable load-bearing structure called a voussoir beam, which was the key difference from the strata behavior characteristics in the shallow mine under gullies.

Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face

A mechanical model to control the top-coal deformation is established in accordance with the structural characters of the gob-side entry surrounding rock for the fully-mechanic top-coal caving; the analytical solution of top coal roof-sag curve is deduced with Winkler elastic foundation beam model. By means of a calculating and analytic program, the top coal roof-sag values are calculated under the conditions of different supporting intensities, widths of narrow pillars and stiffness of top coal; meanwhile, the relationship between the roof-sag values and supporting intensity, width of narrow pillars and stiffness of top coal is analyzed as well. With the actual situation of the gob-side entry taken into consideration, the parameters of top-coal control are determined and a supporting plan is proposed for the top-coal control, which is proved to be reliable and effective by on-site verification. Some theoretical guidance and advice are put forward for the top-coal deformation control in gob-side entry for fully mechanized top-coal caving face.

Study on Distribution Laws of Abutment Pressure for Fully-Mechanized Top-Coal Caving Face in Deep and Large Mining Height

Based on the geologic and technology condition of 2301 fully-mechanized top-coal caving and large mining height face in Longgu colliery, the paper have comprehensive studied distribution laws of abutment pressure by theoretical analysis, FLAC3D numerical simulation and field measurement. The study draws the trend and inclination distribution range and value and concentration factor of the peak, which of these were impacted by mining height and depth and advance distance, and reveals distribution laws of abutment pressure with face advancing, and also pointed out that laws was in favor of top-coal caving, but not conductive to the prevention and treatment of the rib fall of coal wall; field measurement and numerical simulation and theoretical analysis results can be achieved mainly unified.

Study on Fully-Mechanized Top Coal Caving Mining under Water Body with Shallow Overburden and Thin Bedrock

Based on the geological condition of MaHuangliang coal in the Yushen mining area, the paper systematically studied the overburden's destructiverules of fully-mechanized top coal caving, and established hydro-geological model of research area by using GMS numerical simulation. Based on the model, the paper predicted the water inflow of different aquifer, analyzed the influence of aquifer to coal mining, and introduced the technical plans about fully-mechanized top coal caving mining under water body with shallow overburden and thin bedrock.

Characteristic of Development of the Fractured Zone in Mining under Medium Hard Overburden Using Fully-Mechanized Top-Coal Caving Method

For the prediction of the height of the fractured zone developed in mining with fully-mechanized top-coal caving system, the traditional calculation method, if used, generally yield a value with a comparatively large error. Based on mining condition of 6206 face at Wangzhuang Coal Mine in Luan mining area, a comprehensive study is made in the paper of the characteristics of the development of the fractured zone in the medium hard overlying strata under such full-seam mining conditions by taking various means, including field measurement and numerical and physical simulation. Through comparison with the field measurement data available and regression analysis, it can be seen that the height of the fractured zone developed in the medium hard overlying strata in mining with fully-mechanized top-coal caving system is directly proportional to the total mining thickness. In comparison with the result obtained with the use of empirical formula, the method used in the paper can better bring to light the occurrence of the intensive failure of overlying strata under such a mining condition. The research-derived achievement can serve as a basis for reference for the prediction of the height of fractured zone in mining under medium hard cover by using fully-mechanized top-coal caving method, and for decision making in mining under a water body.

Distribution laws of abutment pressure around fully mechanized top-coal caving face by in-situ measurement

In order to obtain the distribution rules of abutment pressure around the 1151(3) fully mechanized top-coal caving (FMTC) face of Xieqiao Colliery, the KSE-II-1 type bore-hole stress gauges were installed in the tailentry and headentry to measure the mining-induced stress. The distribution rules of the front and side abutment pressure were demonstrated. The results show that distribution rules of stress are obviously different in the vicinity of the face and entries. The peak value of abutment pressure in the protective coal pillar and face are located commonly in front of the working face along the strike, and they are located at the stress-decreased zone near the face. There is no stress peak value in the lateral coal mass beside the headentry in front of the face on the strike, and the peak value of abutment pressure appears at the rear area of the face. There are stress peak values both in the protective coal pillar and in the lateral coal mass beside the headentry to the dip.

Top coal flows in an excavation disturbed zone of high section top coal caving of an extremely steep and thick seam

Compared with gentle dip long-wall caving, the length of a working face in fully-mechanized top-coal caving for extremely steep and thick seams is short, while its horizontal section is high with increasing production. But the caving ratio is low, which might result in some disasters, such as roof falls, induced by local and large area collapse of the top coal in a working face and dangers induced by gas accumulation. After the development of cracks and weakening of the coal body, the tall, broken section of the top coal (a granular medium) of an extremely steep seam (over 60°) shows clear characteristics of nonlinear movement. We have thoroughly analyzed the geological environment and mining conditions of an excavation disturbed zone. Based on the results from a physical experiment of large-scale 3D modeling and coupling simulation of top coal–water–gas, we conclude that the weakened top coal can be regarded as a non-continuous medium. We used a particle flow code program to compare and analyze migration processes and the movements of a 30 m high section top coal over time before and after weakening of an extremely steep seam in the Weihuliang coal mine. The results of our simulation, experiment and monitoring show that pre-injection of water and pre-splitting blasting improve caving ability and symmetrical caving, relieve space for large area dynamic collapse of top coal, prolong migration time of noxious gases and release them from the mined out area and so achieve safety in mining.

Analysis on characteristics of damaged field of surrounding rock and face of FMTC

Numerical simulation-FLAC3D and equivalent material simulation were carried out to analyze the damaged patterns and lows, distribution of plastic width of face, and surrounding rock of FMTC during the advance of work face with different thick coal seams based on engineering geology and exploitation technology of 1151(3) fully mechanized top-coal caving (FMTC) face in Xieqiao Colliery. The results show that there is damage, and the destruction characteristics of surrounding rock and coal mass are different obviously in asymmetric exploitation layout. The damaged zone in surrounding rock and the coal of the return airway is larger than that of the intake airway. Moreover, the retained coal pillars are all damaged by tension and shear fracture, and plastic zone in coal mass in the dip direction ahead of Face is nonuniform. There are large damage zones in roof and floor strata, surrounding rock, and coal of return and intake airways near work face. The damaged zone in the upper part of Face is larger than that in middle and lower parts. The fruits of this paper are of guiding significance for engineering practices, such as support design and choice, roadway supporting and maintaining, rock pressure control of FMTC face, etc.

Caving thickness effects of surrounding rocks macro stress shell evolving characteristics

In order to explore the influence of different caving thicknesses on the MSS distribution and evolving characteristics of surrounding rocks in unsymmetrical disposal and fully mechanized top-coal caving (FMTC), based on unsymmetrical disposal characteristics, the analyses of numerical simulation, material simulation and in-situ observation were synthetically applied according to the geological and technical conditions of the 1151(3) working face in Xieqiao Mine. The results show that the stress peak value of the MSS-base and the ratio of MSS-body height to caving thickness are nonlinear and inversely proportional to the caving thickness. The MSS-base width, the MSS-body height, the MSS-base distance to working face wall and the rise distance of MSS-base beside coal pillar are nonlinear and directly proportional to the caving thickness. The characteristics of MSS distribution and its evolving rules of surrounding rocks and the integrated caving thickness effects are obtained. The investigations will provide lots of theoretic references to the surrounding rocks’ stability control of the working face and roadway, roadway layout, gas extraction and exploitation, and efficiency of caving, etc.

Numerical simulation and analysis of underground pressure in the 101 fully-mechanized top coal caving face of the Tingnan Coalmine

The forms of roof break, roof fall, and effects of the region scope in a fully mechanized top-coal caving (FMTC) face of the 101 fully-mechanized top coal caving face of the Tingnan Coalmine were analyzed by UDEC and FLAC3D software. The analysis result confirms the phenomenon of roof falling, roof-off-strata, roof breaking, first weighting, periodic weighting and stress concentration, redistributions of surrounding rock and so on. It provides the gist to analyze the law of roof movement, characteristic of confining pressure, and to determine the formative structure forms of the immediate roof and main roof during the caving process. These results and the underground pressure observation results are in agreement.

Investigations into stress shell characteristics of surrounding rock in fully mechanized top-coal caving face

A key issue in underground mining is to understand and master the evolving patterns of stress induced by mining, and to control and utilize the action of rock pressure. Numerical and physical modeling tests have been carried out to investigate the distribution patterns of stress in the rock surrounding a fully mechanized top-coal caving (FMTC) face. The results showed that a macro-stress shell composed of high stress exists in the rock surrounding an FMTC face. The stress of the shell is higher than its internal and external stress and the stresses at its skewback producing abutment pressure for the surrounding rock. The stress shell lies in the virgin coal and rock mass in the vicinity of the face and its sagging zone. The stress shell, which bears and transfers the loads of overlying strata, acts as the primary supporting system of forces, and is the corpus of characterizing three-dimensional and macro-rock pressure distribution of mining face. Its external and internal shape changes with the variations in the working face structure as the face advances. Within the low-stress zone inside the stress shell, another structure, i.e. voussoir beam, which only bears parts of the load from the lower-lying strata, will produce periodic pressures on the face instead of great dynamic pressure even if the beam ruptures and loses stability. The results show that the FMTC face is situated within the lower-stress zone, which is protected by the stress shell of the overlying surrounding rock. We give an explanation of lower occurrence of rock pressure on FMTC faces, and reveal the mechanical nature of the top coal of an FMTC face acting as a “cushion”. The strata behaviors of the face and its neighboring gates are under control of the stress shell. Drastic rock pressure in mine may occur when the balance of the stress shell is destruction or the forces system of the stress shell transfers.

Copper mine roof–pillar–floor behavioral assessment based on in situ measurements and numerical modeling

A mechanical model to control the top-coal deformation is established in accordance with the structural characters of the gob-side entry surrounding rock for the fully-mechanic top-coal caving; the analytical solution of top coal roof-sag curve is deduced with Winkler elastic foundation beam model. By means of a calculating and analytic program, the top coal roof-sag values are calculated under the conditions of different supporting intensities, widths of narrow pillars and stiffness of top coal; meanwhile, the relationship between the roof-sag values and supporting intensity, width of narrow pillars and stiffness of top coal is analyzed as well. With the actual situation of the gob-side entry taken into consideration, the parameters of top-coal control are determined and a supporting plan is proposed for the top-coal control, which is proved to be reliable and effective by on-site verification. Some theoretical guidance and advice are put forward for the top-coal deformation control in gob-side entry for fully mechanized top-coal caving face.