Longwall Top Coal Caving Mining Research Articles

Application of an Automated Top Coal Caving Control System: The Case of Wangjialing Coal Mine

China has made notable advancements in the intelligent construction of coal mines. However, for longwall top coal caving (LTCC) mining faces, a key obstacle impeding the intelligent transition of the coal-cutting process is automated control. This paper focuses on the aforementioned issue and comprehensively considers the pre-, intra-, and post-coal-caving stages. In this work, diverse detection and monitoring technologies are integrated at various stages through a computer platform, facilitating the construction of an automated coal caving control system with self-perception, self-learning, self-decision-making, and self-execution capabilities. Key technologies include ground-penetrating radar-based top coal thickness detection, inertial navigation-based shearer positioning, tail beam vibration-based identification of coal and gangue, and magnetostrictive sensor-based monitoring of the tail beam and insert plate attitude. In this study, the 12309 working face of the Wangjialing Coal Mine was experimentally validated, and the efficacy of the aforementioned key technologies was assessed. The results demonstrated that the control requirements for automated coal caving are satisfied by the maximum errors. Automatic regulation of coal caving was realized through the implementation of this system, thereby facilitating initiation and cessation and yielding promising experimental outcomes. Overall, this system offers practical insights for intelligent construction in current LTCC mining faces and the sustainable development of coal resources.

CDEM simulation of top‐coal caving characteristics under synchronous multiwindow drawing in longwall top‐coal caving mining of extra‐thick coal seam

AbstractTraditionally, longwall top coal caving (LTCC) panels adopt the single‐window drawing mode and their drawing efficiency is relatively low. The synchronous multi‐window drawing (SMWD) mode is a remarkable approach to resolve the incompatibility between shearer coal mining and top‐coal drawing. The present paper analyzes the caving characteristics of top coal under SMWD in LTCC mining of extra‐thick coal seam. The caving and migration of top‐coal fragments were simulated by using the continuum–discontinuum element method (CDEM). The results show that the SMWD mode can effectively improve the drawing smoothness with larger caving channels compared to the traditional single‐window drawing mode. The main caving channel for top‐coal fragments was directly above the assembled multi‐window (AMW), and the size of the AMW had a great influence on the distribution of the velocity field. Additionally, the simulated draw body of the initial drawing adopted an elliptical shape, which changed into a variation ellipse with the increasing size of the AMW. The boundary of the simulated draw body of the initial drawing presented an irregular shape that deviated from the results predicted by the Bergmark–Roos model. Furthermore, the drawing efficiency increased with the expansion of the AMW, whereas the recovery ratio first increased and then decreased with the expansion of the AMW. Finally, field tests revealed that the SMWD mode can greatly improve the drawing efficiency. Thus, the recovery ratio under A2 and A3 SMWD modes was higher than that of the traditional single‐window drawing mode. Based on the results of field tests and CDEM simulation, it is recommended to adopt A2 and A3 SMWD modes to enhance the top‐coal recovery ratio and drawing efficiency. These research results could constitute a technical reference for automated highly efficient top‐coal drawing and high‐yield LTCC panel extracting extra‐thick coal seam.

Bed Separation Characteristics of an LTCC Panel and Subsidence Controlling Grouting: Case Study of Longquan Coal Mine, China

The bed separation backfill grouting (BSBG) is commonly used to mitigate the surface subsidence caused by coal mining. The distribution characteristics of bed separation and its dynamic evolving process are crucial for BSBG design. This paper utilizes the continuum-discontinuum element method (CDEM) to study the distribution characteristics of bed separation for a longwall top coal caving (LTCC) panel of Longquan coal mine. Numerical results indicate that in addition to the bed separation below the primary key stratum, several small bed separations may also occurred in the strata between the primary key stratum and the subordinate key stratum. The bed separations in the overburden could be classified into three classes: the upper bed separation, the middle bed separation, and the lower bed separation. The upper bed separation has the longest duration time, and the middle bed separation has the shortest duration time. And the BSBG should be started before the closure of the middle bed separation. Based on the actual geological information, the BSBG scheme for 4203 LTCC panel is proposed to mitigate the surface subsidence by taken the results of numerical simulation into consideration. In addition, the case study of the BSBG is introduced in detail. By using gangue power slurry, BSBG could not only effectively mitigate the surface subsidence but also solve the problems of environmental pollution and land occupation caused by traditional gangue stacking. The present study could provide technical support for surface subsidence mitigation and coal gangue disposal for LTCC mining with similar conditions.

Stress distribution ahead of mechanized longwall top coal caving face with great cutting height

Longwall Top Coal Caving (LTCC) technology with great cutting height is a new development trend in mining thick coal seam. The cutting height of LTCC face typically ranges from 2.8 m to 3.2 m in many coal mining countvies, but it recently reaches up to 4.2 m in many coal mines in China. Because the cutting height increases, the caving height accordingly decreases that changes the stress distribution around coal face and law of roof rock caving. Based on the geological condition of Longwall 4108 at Ping Shou coal mine, ShanXi province, China, this paper presents a modelling of LTCC mining process with a cutting height of 4.2 m by using the numerical program FLAC3D. From the modelling, the paper presents an analysis of stress distribution ahead of LTCC face with great cutting height. The results show that as the coal face advances, the stress magnitude ahead of coal face increases. The peak front abutment stress moves further away from coal face. The stress concentration ratio increases, and stress concentration zone expands correspondingly. These changes of stress facilitate the failure of top coal, increasing the efficiency of top coal recovery and improving longwall face stability.

An acoustic wave method for evaluating the effectiveness of blast to enhance fragmentation in longwall top coal caving

AbstractThe advanced deep hole blasting (ADHB) is commonly used to improve the fragmentation of thick hard top coal in longwall top coal caving (LTCC) mining. This paper presents an acoustic method to quantitatively evaluate the effectiveness of the ADHB. First, the variations of P‐wave velocity of hard coal samples under uniaxial and triaxial compression are obtained from the laboratory acoustic test. Then, an acoustic wave method based on seismic tomography is proposed to assess the effectiveness of the ADHB by introducing the P‐wave velocity attenuation model into the data processing. Finally, a field seismic tomography test is conducted to obtain the distribution of the P‐wave velocity of top coal before and after the ADHB implementation. And the P‐wave velocity attenuation is calculated to estimate the fragmentation effectiveness of ADHB. The results indicate that the P‐wave velocity attenuation model could well represent the broken degree of hard coal, suggesting that the proposed acoustic wave method could quantitatively evaluate the fragmentation effectiveness by exporting the nephogram of P‐wave velocity attenuation. The present study may provide technical support for improving the recovery ratio of thick hard top coal for LTCC mining.

Analysis of the Geomechanical Phenomena That Led to the Appearance of Sinkholes at the Lupeni Mine, Romania, in the Conditions of Thick Coal Seams Mining with Longwall Top Coal Caving

Thick coal seam no. 3, block V, Lupeni mine was mined by longwall top coal caving (LTCC). After the coal mining, the ground surface underwent continuous subsidence, but since 2008, three sinkholes have appeared on the surface with important dimensions, atypical for the geo-mining conditions in this coal basin. This article is a synthesis of the study meant to decipher the geo-mechanical phenomenon that led to the emergence of these sinkholes and highlighting the main factors that contributed to the development of this phenomenon. For this purpose, measurements were made on the terrain deformations using photogrammetric methods and aerial laser scanning, when modeling with 3D finite elements, in elasto-plasticity and with the help of the Knothe–Budrik influence function. The factors that contributed to the occurrence of discontinuous subsidence phenomena are shallow mining depth, the LTCC mining method, and the presence of faults in the vicinity of the mining panels. Additionally, the geo-mechanical phenomena of subsidence terrace development and sinkholes in the mining subsidence troughs at the Lupeni mine were described.

Caving mechanisms of loose top-coal in longwall top-coal caving mining based on stochastic medium theory

A good understanding of the caving mechanisms of loose top-coal is significant for longwall top-coal caving (LTCC) mining. In this paper, the equations of the particle motion, top-coal boundary, and drawing body were derived based on the stochastic medium theory. Then, the particles moving boundary and the shape of the top-coal loss were discussed, and the dynamic evolvement of the top-coal boundary and drawing body was analyzed. A theoretical model for calculating the top-coal recovery ratio was established, and the relationship between the top-coal recovery ratio and rock mixed ratio was studied and then the criterion for terminating the drawing cycle was proposed. Finally, model experiments were conducted to verify the theoretical analysis. It is concluded that the starting top-coal boundary keeps moving to the lower left with increasing drawing volume. The top-coal loss is zonal distribution and tilts toward the goaf and is composed of the top-coal surrounded by the floor, the starting top-coal boundary and the ending top-coal boundary between the drawing window and the lowest moving point in the starting top-coal boundary. The drawing body tilts toward the goaf and its deflection angle decreases with increasing drawing volume. The top-coal recovery ratio increases non-linearly with increasing rock mixed ratio, and the rock mixed ratio of 10–15% can be used as the criterion for terminating the drawing cycle. This work is the first attempt at demonstrating the caving mechanisms of the loose top-coal using the stochastic medium theory in LTCC mining.

A discontinuum modelling approach for investigation of Longwall Top Coal Caving mechanisms

This paper presents a discontinuum modelling approach to investigate Longwall Top Coal Caving (LTCC) behaviour including stress distribution, coal and rock failures, top coal caving and roof strata rupture, and to analyse the impact of overburden movement on top coal caving. The current model ia successful in using plastic material in a discontinuum code for intact rocks. The model scale is large enough to capture the critical features of LTCC, including steady-state caving of top coal and repeatable periodic weighting of roof strata. The applicability of the numerical model was assessed by calibration with field measurements obtained from a longwall mine site. The numerical study found that the stress distribution caused by LTCC mining is in general similar to that caused by conventional longwall mining; top coal predominantly fails in shear whereas roof rock mostly fails in tension; top coal starts to cave in stress caving while main roof strata first rupture in crushing mode; and roof strata weightings periodically increase and decrease top coal cavability. The findings of this study should assist engineers in better understanding fundamental rock mechanics associated with LTCC, identifying key geotechnical parameters dominating caving behaviour, and managing top coal productivity and mine safety involved in LTCC operation.

Theoretical equation of initial top-coal boundary in longwall top-coal caving mining

The theoretical equation of initial top-coal boundary (ITCB) is one of fundamental problems of top-coal drawing law in longwall top-coal caving (LTCC) mining. The granular mechanical model of ITCB is established based on loose medium flow field theory. By analysing stressed top-coal on ITCB along section of coal seam thickness and face advancement directions, theoretical equation of ITCB is deduced respectively in two conditions of first gangue drawing out and excessive gangue drawing out. The theoretical equation is further modified by a correction coefficient k, which is given by a large number of experimental data. Meanwhile, it is found that ITCB at excessive gangue drawing out condition is in special shape that the top opening is elliptical and bottom opening is rectangle. The theoretical equation of ITCB can provide meaningful guidance to improvement of top-coal recovery ratio in LTCC.

Monitoring and modelling of gas dynamics in multi-level longwall top coal caving of ultra-thick coal seams, Part II: Numerical modelling

The longwall top coal caving method, which enables the most productive exploitation of thick/ultra-thick coal seams, may result in a distinct geomechanical response of strata and associated gas emission patterns around longwall layouts. A two-way sequential coupling of a geomechanical and a reservoir simulator for the modelling of gas emissions around a longwall top coal caving (LTCC) panels was developed building on the understanding established from the analysis of in-situ gas pressure and concentration measurements carried out at Coal Mine Velenje in Slovenia. Model findings have shown that the modelling method implemented can reproduce the dynamic changes of stresses and gas pressure around a LTCC face and predict the total gas emissions and mixed gas concentrations accurately. It was found that, in LTCC panels, although the rate of gas emission from mined coal depends highly on the coal face advance, floor coal and roof goaf act as a constant and steady gas source accounting for a considerable part of the overall gas emission. Research has shown that, at first and/or second mining levels of multi-level LTCC mining, a notable stress relief and pore pressure drop induced by fracturing of the mined and roof coal can be experienced within 40m ahead of the face-line. In the floor coal, on the other hand, the pore pressure change was found to extend to 20m below the mining horizon. Model results have clearly shown the permeability enhancement and gas mobilisation zones around the LTCC panel, which can be the target zones for gas drainage boreholes.

Three-dimensional experimental study of loose top-coal drawing law for longwall top-coal caving mining technology

Based on the loose medium flow field theory, the loose top-coal drawing law of longwall top-coal caving (LTCC) mining technology is studied by using self-developed three-dimensional (3D) test device. The loose top-coal drawing test with shields and the controlled test without shields are performed in the condition without any boundary effect. Test results show that shields will cause reduction in drawing volume of coal in the LTCC mining. The deflection phenomenon of drawing body is also observed in the controlled test, which is verified that the deflection of drawing body is caused by shield. It is found that the deflection angle decreases with increasing caving height, with the maximum value of αtail and the minimum value of 0. In addition, the formula to calculate the drawing volume is proposed subsequently. The deflection of drawing body is numerically simulated using particle flow code PFC3D and the proposed formula to calculate drawing volume in LTCC is also verified.

Prefeasibility study—Geotechnical studies for introducing longwall top coal caving in Indian mines

Longwall Top Coal Caving (LTCC), a relatively newer technology in India, is an underground mining method developed for thick seam extraction. Various factors and systematic approaches are to be considered and properly evaluated in order to qualify and gain confidence in the assessment of introducing LTCC mining method. The proposed paper presents the various processes involved in designing a LTCC technology in one of the mine sites at Singareni Coal Company Limited (SCCL), India. The project undertook a comprehensive analysis of geological and geophysical data of the mine site and developed detailed geotechnical frameworks for the assessment of LTCC technology.