Interpretation of transient caving dust patterns during sequential caving operation in LTCC face

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

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.

An extraction method for deep-seated thick seam deposits by underground mining with high resource recovery has remained a great challenge for Indian mining engineers, whereas the longwall top coal caving (LTCC) method has evolved as an effective method for various geo mining conditions in China and other counties. The percentage of top coal recovery (TCR) plays a predominant role in determining the feasibility of LTCC, which relies on the First Top Coal Caving Distance (FTCD). In this paper, the critical geotechnical parameters are identified, numerically simulated, and statistically analyzed, and the FTCD for Indian geo-mining conditions is developed and validated. A financial assessment is conducted, considering 70% top coal recovery at 85% performance level, cost of production escalated by 20% and fall in coal grade by two grades. The internal rate of return (IRR) for LTCC is 30.24% as per the sensitivity analysis where it is only 18% in single pass longwall method. This study contributes to evaluating both the technical and economic feasibility of introducing LTCC in Indian geo-mining conditions.

Improved dust management at a longwall top coal caving (LTCC) face – A CFD modelling approach

Thar Coalfield in Pakistan is the largest reserve of lignite coal in the country, which is outlined by thick coal seams. The preferred mining method for these thick coal seams is the Longwall Top Coal Caving (LTCC). The connection of both the top coal caving and its compactness are still rare; therefore, the capability of top coal caving mechanism is the most significant factor in LTCC that must be adequately explained and examined. Moreover, in order to achieve the ideal coal production, a comprehensive modeling of deformation and induced stress is mandatory. In this study, a 12 m thick coal seam with cutting to caving height ratios like 1:2 and 1:3 has been modelled, and the mechanism of longwall top coal caving demonstrated and front abutment vertical stress distribution in front of face line values were computed with the help of UDEC at Block-IX, Thar Coalfield. The results reveal that a thick layer of top coal can be progressively caved behind the face at the ratio 1:3 instead of 1:2 (which explained the incompetent caving progress of top coal). Similarly, the maximum vertical abutment stress of 20 MPa was observed at 6m in front of the face when cutting to caving height ratio was 1:2 and at 3m in front of face with 1:3 (which is comparatively capable for the face advancement), respectively. Therefore, this numerical modeling study proposes the reasonable height of top coal caving at cutting to caving height ratio 1:3 for the efficient production of thick coal seams at Thar Coalfield.

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.

The optimization of top coal caving technology is an efficient method to improve the recovery ratio in longwall top coal caving (LTCC). In extrathick coal seams, the conventional single‐opening sequential caving technology (SOSCT) shows the following problems: low recovery ratio, high rock mixed ratio, and poor drawing balance. For these problems, this research verifies the applicability of multiopening caving technology (MOCT) in extrathick coal seams theoretically. However, different drawing sequences have a great effect on the drawing mechanism. Based on the progressive drawing sequence of cluster‐group‐support, this paper firstly proposes a systematic naming method for the top coal caving technology. Furthermore, an independent cluster‐group caving technology (ICGCT) is given, meaning that all supports are divided into several clusters, a cluster is divided into several groups, and clusters extract top coal in positive order while groups are in reverse order in the drawing process. By establishing an experimental model by the discrete element method PFC2D, the drawing mechanism is investigated under different caving technologies. The results show that ICGCT significantly improves the recovery ratio of the panel and mainly increases the drawing volume of top coal in the middle and upper end of the panel. The shape of the top coal boundary reflects the drawing efficiency. Due to the effect of drawing sequence in ICGCT, the generation and disappearance processes of coal ridge greatly decrease the residual top coal in the middle of the panel. The drawing body shape has a direct influence on the recovery ratio. Multiple complete drawing bodies exist in ICGCT, and the dispersion coefficient of drawing volume changes periodically in the range of 0.5–1.7, which is conducive to the management of drawing processes. In addition, discussing ICGCT and the dependent cluster‐group caving technology (DCGCT), it is found that the recovery ratio of DCGCT has a slight increase, which enlarges the maximum drawing range of top coal at both panel ends, shortening the total drawing time of the panel. In summary, ICGCT provides a new approach for improving the recovery ratio and drawing balance in LTCC with an extrathick coal seam.

The first caving of top coal and the first weighting of roof strata in Longwall Top Coal Caving (LTCC) mining were analyzed in a parametric study by using a discontinuum modeling technique...

Stress analysis of longwall top coal caving

Investigation of the effect of caving height on the efficiency of the longwall top coal caving production method applied in inclined and thick coal seams by physical modeling

This review details the state of the art in research on top coal drawing mechanisms in Longwall top coal caving (LTCC) by examining the relevant literature over the last two decades. It starts with an introduction of the brief history and basic procedures of LTCC. The framework of research on the drawing mechanism, basic concepts, and some theoretical models of LTCC are detailed in sect. research framework of top coal drawing mechanism. The authors note that the Top coal drawbody (TCD), Top coal boundary (TCB) and Top coal recovery ratio (TCRR) are key factors in the drawing mechanism. The Body–boundary–ratio (BBR) research system has been the classic framework for research over the last 20 years. The modified Bergmark–Roos model, which considers the effects of the supporting rear canopy, flowing velocity of top coal, and its shape factor, is optimal for characterizing the TCD. A 3D model to describe the TCB that considers the thicknesses of the coal seam and roof strata is reviewed. In sect. physical testing and numerical simulation, the physical tests and numerical simulations in the literature are classified for ease of bibliographical review, and classic conclusions regarding the drawing mechanism of top coal are presented and discussed with elaborate illustrations and descriptions. The deflection of the TCD is noted, and is caused by the shape of the rear canopy. The inclined coal seam always induces a larger TCD, and a deflection in the TCD has also been observed in it. The effects of the drawing sequence and drawing interval on the TCRR are reviewed, where a long drawing interval is found to lead to significant loss of top coal. Its flowing behavior and velocity distribution are also presented. Sect. practical applications of drawing mechanisms for LTCC mines 4 summarizes over 10 cases where the TCRR of LTCC mines improved due to the guidance of the drawing mechanism. The final section provides a summary of the work here and some open questions. Prospective investigations are highlighted to give researchers guidance on promising issues in future research on LTCC.

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.

The authors review the international experience in thick coal seam mining. Advantages and disadvantages of multi-slice and full-seam longwall mining technologies are compared. The low-efficiency multi-slice longwall mining is little used and is being replaced by the full-seam longwall top coal caving technology with undercut and with top coal disintegration under the physical effect of overburden pressure. In the recent decade, the longwall mining systems used in top coal caving have been given additional functions connected with control of coal extraction above and behind the powered roof support. The authors have examined the patent documentation and invention activity in the field of the LTCC methods and equipment in the top coal mining countries. This paper describes the revealed trends in engineering facilities capable to enhance efficiency of longwall top coal caving in mining of thick coal seams. Different type feeders are described, and recommendations on their application are given.

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

Ground hydraulic fracturing has emerged as an effective technique for mitigating strong mining pressure manifestations in longwall top coal caving (LTCC). However, the influence of different hydraulic fracture types on the strength characteristics of hard roofs (HR) remains unclear, as does their impact on the fracture process and stress redistribution characteristics of HR. In this study, a numerical simulation tool based on the material point method (MPM) and a strain-softening model was employed to construct a model for LTCC involving overburdened multi-layer HR panels. Furthermore, LTCC mining simulation research was conducted, encompassing prefabricated horizontal hydraulic fracturing, vertical fracturing, and non-fracturing models. The results revealed the following: 1) The fundamental mechanism of HR fracture involves tensile failure induced by the gravity load of the overburdened rock layer when suspended. Vertical cracks resulting from surface hydraulic fracturing significantly diminished the tensile strength of HR, thereby greatly reducing its collapse step distance. 2) In LTCC, the stress transfer dynamics within rock layers were characterized by the following: horizontal stress concentrated in the middle through bending deformation of the rock layer upon suspension. Furthermore, upon reaching its peak, the rock layer fractured and collapsed, thereby releasing horizontal stress. Hydraulic fracturing-induced reduction in HR tensile strength effectively mitigated horizontal stress concentration. 3) Vertical stress concentration occurred through the collapse of lower rock layers and the pressure exerted by suspended upper rock layers. The appearance of its peak represents the collapse of multiple rock layers, and through hydraulic fracturing, the collapse step distance was effectively shortened, weakening the concentration of vertical stress.