Longwall Coal Mining Research Articles

Analysis of Longwall Salvage Operations in JSC SUEK-Kuzbass Underground Mines

The longwall coal mining method is the most productive mining method. However, the salvage operations which include the formation of the salvage chamber and the salvage of longwall face equipment require improvement. In 2011-2017, the time spent for salvage operations in SUEK-Kuzbass mines exceeded the standard time by more than 900 days. It has been established that the longwall face width from 230 to 300 m has no effect on the duration of salvage operations.

Microseismic and electromagnetic coupling method for coal bump risk assessment based on dynamic static energy principles

Electromagnetic emission (EME) and micro-seismic (MS) monitoring are two most important and commonly used geophysical methods for coal bump monitoring in China coal mines. Research have proved during the preparation and development of coal bump the EME can represent energy accumulation in coal seam, while MS have a duality of stress concentration and energy release. And Field investigations have revealed that coal bump events occurred in longwall coal mines manly induced by static coupling dynamic loads. So it is much more valuable and useful for risk evaluate if these two methods synthesized together. Based on energy principles the precursory rules of coal bump are studied in this paper combining with the EME and MS monitoring, the coupling evaluation system was established based on the monitoring results of a specific coal bump colliery, the critical values of EME and MS are formulate based on laboratory test and a large number of site investigation that more than 100 mines in China. A typical precursory of coal bump is that the EME strength exceeds the critical value while total MS energy maintains a low lever at the same time the number of tremors appears an increase tendency. The risk will be gradually reduced when the MS events keep a steady energy release under the critical value even though the EME strength is higher. The research outcomes also provide a foundation for the control and effect inspect of coal bump.

Shield-Roof Interaction in Longwall Panels: Insights from Field Data and Their Application to Ground Control

The shield-roof interaction as mining proceeds in longwall panels remains unclear, hindering the further increase of longwall productivity. To uncover the mechanisms of shield-roof interaction, using our self-developed Status of Shield and Roof IntelliSense (SSRI) system, we investigated the effects of idle time, retreating rate, setting pressure, yielding, and shearer’s cutting, as well as neighboring shields’ advance on the spatial-temporal evolution of leg pressure and leg closure of shields. Our results show that the shield-roof interaction is not only dependent on the shield capacity, but also collectively determined by the time-related factors, the geological condition, the setting pressure, yielding characteristics, and mining method. Understanding the shield-roof interaction in longwall panels enables us to apply the SSRI system for ground control in longwall coal mines. Early warning of severe roof weighting can be achieved by establishing a warning model based on the decision tree algorithm. Apart from this, we can also assess the working condition of yield valve and diagnose fluid leakage of shield cylinder using the SSRI system. Finally, we propose the research prospects on shield-roof interaction in longwall panels to achieve a more reasonable determination of shield capacity, prediction of roof fall and coal wall spalling, and self-adaptive control of the shield.

Underground ground control monitoring and interpretation, and numerical modeling, and shield capacity design

Mine or longwall panel layout is a 3D structure with highly non-uniform stress distribution. Recognition of such fact will facilitate underground problem identification/investigation and solving by numerical modeling through proper model construction. Due to its versatility, numerical modeling is the most popular method for ground control design and problem solving. However numerical modeling results require highly experienced professionals to interpret its validity/applicability to actual mining operations due to complicated mining and geological conditions. Underground ground control monitoring is routinely performed to predict roof behavior such as weighting and weighting interval without matching observation of face mining condition while the mining pressures are being monitored, resulting in unrealistic interpretation of the obtained data on mining pressure. The importance of ground control pressure monitoring and simultaneous observation of mining and geological conditions is illustrated by an example of shield leg pressure monitoring and interpretation in an U.S. longwall coal mine: it was found that the roof strata act like a plate, not an individual block of the size of a shield dimension, as commonly assumed by all researchers and shield capacity is not a fixed property for a longwall panel or a mine or a coal seam. A new mechanism on the interaction between shield’s hydraulic leg pressure and roof strata for shield loading is proposed.

Investigation of factors influencing roof stability at a Western U.S. longwall coal mine

The coal mine roof rating (CMRR) was developed to bridge the gap between geological variation in underground coal mines and engineering design. The CMRR accounts for the compressive strength of the immediate roof, the shear strength and intensity of any discontinuities present, and the moisture sensitivity of the immediate roof. The CMRR has been widely used and validated in Eastern US coal mines, but it has seen limited application in the Western US. This study focuses on roof behavior at a Western coal mine (Mine A). Mine A shows significant lateral geological variation, along with localized faulting and a laterally extensive sandstone channel network. The CMRR is not used to predict roof instability at the mine. It is, therefore, hypothesized that there are other factors that are correlated with roof instability in underground coal mines that could potentially also be considered in conjunction with the CMRR. This hypothesis was tested by collecting 30 CMRR measurements at Mine A. At each measurement location, a binary record of the roof condition (stable or unstable) was made, and other parameters such as depth of cover, presence of faulting, and sandstone channels were also recorded. ANOVA tests showed that the CMRR values and the roof conditions were not strongly correlated, indicating that the CMRR input criteria are not fully predictive of roof stability at this mine. The CMRR values showed statistically significant correlations (p less than 0.05) with faulting as well as with location at an intersection. For areas that had previously experienced roof fall but were currently stable, faulting was correlated with roof condition (p less than 0.05) only when the condition was classified as unstable.

Gangue-Cemented Fill in Goaf to Control Surface Subsidence During Longwall Mining: A Case Study from the Huaheng Coal Mine, China

Surface subsidence can cause both damage to infrastructure and a number of different environmental problems. These include surface water accumulations, loss of farmland, cracks and other damage to roads and bridges, and, in the most severe cases, the collapse of house and other buildings. Surface subsidence can be especially severe during and subsequent to coal mining. Gangue-cemented fill in goaf (GCFG) has been proposed as an effective measure to control surface subsidence during longwall coal mining. In this study, GCFG was used for an industrial-scale filling test at the 61,303 working face in the Huaheng Coal mine, China. To perform the filling test, a new GCFG storage and transport system and a system for mixing the cemented gangue were designed. The test showed that GCFG can effectively reduce the load on pillars, the amount of hydraulic pillar ram descent, and roof subsidence. This means that GCFG can effectively control the movement of the strata overlying the coal seam and therefore reduce or eliminate surface subsidence. Future research on GCFG technology should include the long-term strength and the rheological behavior of the cemented gangue.

Using hydraulic fracturing to control caving of the hanging roof during the initial mining stages in a longwall coal mine: a case study

Hanging roofs can occur on a very large scale (generally can reach 15,000 m2) during the initial mining stages in longwall coal mining operations and are often a result of hard roof at the active mining face. Besides, when it suddenly fractures and caves, some risks and disasters would occur such as shock loading, wind blasts, and sudden gas extrusion from gob to working face. To control the size and competency of the hanging roof, Φ32-mm fracturing drills can create holes in the roof near the front and back coal walls, and Φ32-mm linear fracturing drills can be used to create holes in the roof at the face ends. The length of these drill holes is 3 to 5 times the height of the mining face and is based upon the principle that the gob should be fully filled with a caved roof to support vertical stresses, as well as the assumption that the broken-expansion coefficient of the rock is given as an empirical value. Considering the fracture propagation range and the support form of the cut, the drill-hole spacing is set to be between 1 and 2 times the single-hole crack propagation range. Hydraulic fracturing can generate main fractures and wing-shaped branching cracks inside the roof rock and can transform and weaken the roof structure, forming weak fracture plane favorable to roof caving. These fractures cause the roof to cave in a timely and adequate manner under in situ ground pressures. Successful field test in the no. 2 mine of Cuncaota, Shendong group indicated that the propagation range of hydraulic fractures is approximately 4–8 m. When the active face advanced 22.5 m, the roof beyond the hydraulic supports caves entirely, and gradually, the gob is fully filled with the caved roof and is absent of large voids. Thus, hazards such as shock loading, wind blasts, and sudden gas extrusion can be reduced or eliminated.

Computational Modelling of Groundwater Inflow During a Longwall Coal Mining Advance: A Case Study from the Shanxi Province, China

The paper investigates groundwater inflow into a longwall working face of the Xiegou Coal Mine located in the northwest of the Shanxi province, China. Three modelling approaches, including basic Darcy modelling, full poroelastic modelling using Biot’s classical theory, and the full Biot poroelastic modelling that takes into account permeability alterations of the coal seam and strata due to alterations in the mining-induced stress state, are used to estimate the fluid flow into the underground longwall coal mine as the excavation progresses. Based on plausible assumptions of hydrogeological structure, boundary conditions and mining conditions, a three-dimensional finite element model of the simplified geological setting is developed using the COMSOL software. The permeability changes in both the strata and the coal seam due to the mining are associated with the mining-induced stress changes and water pressure, and the correlation between them accords with the exponential relationship. The differences in the modelling results of three approaches were analyzed, and the simulation results are compared with the field measurements.

A New Method of Predicting the Height of the Fractured Water-Conducting Zone Due to High-Intensity Longwall Coal Mining in China

Violent movement of the roof rock and severe damage to the overlying strata occur in the large mined-out space left by rapidly advancing, high-intensity longwall coal mine extraction, as the goaf forms. Knowing the height of the fractured water-conducting zone (FWCZ) above the goaf is vital in the safety analysis of coal mining, particularly under a water body. The processes of overburden failure transfer (OFT) were analyzed for such high-intensity mining, divided into two stages: transmission development, and transmission termination. Rock failure criteria were used in theoretical calculations of the maximum lengths of ‘suspended’ (i.e., unsupported) rock strata, and of the maximum ‘overhang’ (i.e., cantilever) length of each stratum. Based on this, mechanical models of the unsupported strata and the overhanging strata were established. A new theoretical method of predicting the height of the FWCZ in this form of coal mining is put forward, based on OFT processes. A high-intensity mining panel (the 8100 longwall face at the Tongxin Coal Mine, Datong Coal Mining Group) was taken as an example. The proposed theoretical method, a numerical simulation method and an engineering analogy method were used to predict the height of the FWCZ. Comparison with in situ measurements at the Tongxin mine showed that the theoretical and numerical simulation results were in close agreement with measured data, verifying the rationality of the proposed approach.

Exposure to Harmful Dusts on Fully Powered Longwall Coal Mines in Poland.

The mining production process is exposed to a series of different hazards. One of them is the accumulation of dust which can pose a serious threat to the life and health of mine workers. The analysis of dust hazard in hard coal mining should include two aspects. One is the risk of coal dust explosions, which poses a direct risk of injury or even loss of life, the second is the risk of harmful dust, associated with the possibility of negative health effects as a result of long-term exposure to dust in the worker’s body. The technologies currently applied in underground mining produce large amounts of coal and stone dust. Long-term exposure to dust and crystalline silica may cause chronic respiratory disease. The article presents the results of tests on the dust levels in the area of a fully-powered longwall. The tests were conducted for five longwalls from different hard coal mines. In each of them, the average values of inhalable and respirable dust as well as the percentage content of free silica in the dust were determined in ten selected working positions. Additionally, for the longwall with the highest dust concentration, the levels of dust were determined for the basic activities related to the phases of the technological cycle. The comparative analysis conducted and the results obtained demonstrate large variations in the dust levels in the different areas. The permissible values were significantly exceeded in a number of cases. This poses a great threat to the health of Polish miners. The results obtained indicate that it is necessary to undertake more effective measures in order to improve the working environment of the crew in hard coal mines.

Design of gas drainage modes based on gas emission rate in a gob: a simulation study

In the process of working face advance in longwall coal mining, a great deal of gas relieved by the strata adjacent to the mining coal seam and the residual coal in gob migrates to gob. If the gas drainage method in gob is unreasonable, gas will accumulate in the upper corner and overrun in the return air flow. In the paper, a CFD (computational fluid mechanics) model of gob based on the actual geological conditions and gas drainage mode of 1262 working face of Dingji Coal Mine, China, was established. The gas drainage modes that should be taken to effectively control gas accumulation in the upper corner and gas overrun in the return air flow at different gas emission rates were discussed. The simulation results show that when the gas emission rate in the working face is lower than 20 m3/min, buried pipe drainage can effectively control gas accumulation in the upper corner and gas overrun in the return air flow. When gas emission rate there is between 20 and 30 m3/min, the two problems can be solved through cross-measure borehole drainage combined with buried pipe drainage. When gas emission rate there is higher than 40 m3/min, they can be effectively controlled through a three-dimensional drainage mode including buried pipes, cross-measure boreholes, and surface wells. Arranging surface wells within the fractured zone near the return airway can increase the gas drainage rate, and the gas concentration can reach over 85%; the gas concentrations of buried pipe drainage and cross-measure borehole drainage are 15~20% and 70~80%, respectively.

Stress changes and seismicity monitoring of hard coal longwall mining in high rockburst risk areas

Rockburst represents a very dangerous phenomenon in deep underground mining as well as in underground constructions in unfavourable conditions (great depth, high horizontal stress, proximity of important tectonic structures, etc.). The rockburst problem relates to the natural and mining conditions of the rock mass. It is very difficult to decide which of the factors prevails, but we generally assume that it is the occurrence of competent rock layers in the rock mass. The destress blasting method is a very important rockburst control technique aims to competent rocks.The case study describes the control of rockburst risk by destress blasting in an example of a longwall of the Upper Silesian Coal Basin. Monitoring of stress changes and induced seismicity was implemented in the selected longwall here and a description of these techniques is given. The paper describes the natural, geomechanical, and mining conditions of the investigated locality in detail. Stress changes ahead of the advancing longwall face as well as registered induced seismicity depending on the longwall advance are analysed, including the application of the destress blasting technique. Stress release efficiency, as one goal of the destress blasting technique, is analysed according to the registered seismicity using the authors’ own methodology: seismic effect calculation.

Understanding mechanisms of destressing mining-induced stresses using hydraulic fracturing

High mining-induced stresses can cause a series of problems related to the stability of entries and pillars including roof fall, rib failure, floor heave, and even coal bursts. In extreme cases, stress relief measures have to be carried out to help control those mining-induced stress-related problems. This article presents a field experiment of utilizing hydraulic fracturing as a preconditioning measure to relieve high abutment stresses caused by longwall coal mining. Field measurements and observations demonstrate that hydraulic fracturing in the main roof above the pillar can substantially destress abutment stresses. A numerical study is performed to study the related mechanism. The result suggests that abutment stress relief is achieved by shear sliding along generated hydraulic fractures and natural fractures stimulated by fluid injection. If designed appropriately, hydraulic fracturing operation in the main roof can substantially reduce abutment stresses, rather than transfer it to deeper rock mass. Factors controlling the effectiveness of destressing abutment stresses are also discussed.

Effects of solid backfilling on overburden strata movement in shallow depth longwall coal mines in West China

Effects of solid backfilling on overburden strata movement in shallow depth longwall coal mines in West China

Numerical modelling of microseismicity associated with longwall coal mining

Microseismicity has long been a precursor for underground mining hazards such as rockbursts and coal and gas outbursts. In this research, a methodology combining deterministic stress and failure analysis and stochastic fracture slip evaluation, based upon the widely accepted fracture slip seismicity-generation mechanism, has been developed to simulate microseismic events induced by longwall mining. Using the built-in DFN facility in FLAC3D, discrete fractures following a power law size distribution are distributed throughout a 3D continuum model in a probabilistic way to account for the stochastic nature of microseismicity. The DFN-based modelling approach developed was adopted to simulate the evolution of microseismicity induced by the progressive face advance in a longwall top coal caving (LTCC) panel at Coal Mine Velenje, Slovenia. At each excavation step, global stress and failure analysis with reference to the strain-softening post-failure behaviour characteristic of coal, and fracture slip evaluation for microseismicity are conducted sequentially. The model findings are compared to the microseismic event data recorded during a long-term field monitoring campaign conducted at the same LTCC panel. It was found that the released energy and frequency-magnitude distribution of microseismicity are associated with the slipped fracture sizes and fracture size distribution. These features for recorded microseismic events were fairly constant until a xylite rich heterogeneous zone ahead of the working face was approached, which indicates that fractures within the extracted coal seam follow the same size distribution. The features obtained from modelled microseismic events were consistent over the production period, and matched well the field observations. Furthermore, the model results indicate that the power law fracture size distribution can be used to model longwall-mining-induced microseismicity. This model provides a unique prospective to understand longwall coal mining-induced microseismicity and lays a foundation to predict microseismicity, or even rockburst potential in specific geological realisations.

Forecasting the stress concentration coefficient around the mined panel using soft computing methodology

Stress analysis around the mined panel is one of the most critical topics in longwall mining for stability analysis of surrounding pillars and access tunnels. Actually, stress redistribution due to roadway extraction along with the longwall mining-induced stress must be considered in designing the panel surrounding structures. In this research, three new methods, i.e., radial basis function neural network (RBFNN), fuzzy inference system (FIS) and statistical analysis (SA) models have been developed to predict the stress concentration coefficient (SCC) around a mined panel. The transferred stress due to longwall mining has been also considered in predicted SCC from these models. Proposed models have been constructed based on the sufficient datasets gathered from the literatures. For SCC prediction, the height of destressed zone above the mined panel, rock mass unit weight, overburden depth and horizontal distance from the panel edge have been regarded as input variables. Based on the actual testing datasets, performance of the suggested models has been evaluated using the statistical indices. Accordingly, it was proved that RBFNN and FIS models have better capability compared to the SA model and their results are in a great agreement with the real values. Moreover, proposed models were practically applied in Tabas longwall coal mine of Iran and verified by comparing their results with the results of existing models. Finally, conducted sensitivity analyses of the proposed models indicate that height of destressed zone and unit weight are the most and least influencing variables on the SCC in all models.

Cooling pathways for deep Australian longwall coal mines of the future

Cooling of coal mines in the Bowen Basin, characterized by steep geothermal gradient, is presently achieved mostly through rental surface bulk air cooling in summer months. This paper argues that future mines will be required to focus their cooling resources more intensively to manage a challenging thermal environment where virgin coal temperatures over 50 °C at a depth of 500 m are expected. Currently, mine cooling systems are employed to maintain the wet bulb temperatures (WBT) to below 27 °C at which point the risks of heat stroke or other heat related issues are manageable. The capacities of these systems are in the range of 6–10 MW refrigeration power. The relationship between high working temperature environment and injury frequency rates is well established. Therefore, provision of appropriate cooling strategies and understanding their optimum performance and suitability are important to Australian coal mines of the future. This paper evaluates the underground temperature profiles of deep, gassy coal mines with propensity for spontaneous combustion and proposes the long term cooling pathways to effectively manage the thermal hazards. Thermodynamic modeling is performed on a longwall face and includes air leakage effects from goaf streams at various locations along the longwall face. The strategy summarizes the application of underground bulk air cooling, chilled water sprays on the shearer and the resulting temperature profiles. Considering the new mining projects planned for the Bowen Basin region, a review of implementable cooling strategies such as mid-gate mobile bulk air coolers (BACs), spot coolers, underground bulk air cooling and the use of chilled water to enhance the positional efficiency of cooling plants, are discussed in this paper. Finally, the comparison of ‘rental’ versus ‘ownership’ of cooling plants is analysed as part of long-term cooling strategies.

Analysis of surface subsidence due to longwall mining under weak geological conditions: Turgut basin of Yatağan-Muğla (Turkey) case study

Accurate prediction of surface subsidence due to the extraction of underground coal seams becomes a significant challenge in engineering. For a projected longwall coal mining operation, conditions of weak cover rock and thick coal seams (6–12 m) in Turgut basin in Yatagan-Turkey are described and discussed based on the field data, using surface deformation prediction system software. This paper aims at predicting the subsidence values only numerically by evaluating the influence of subsidence on irrigation pipeline structure to be built on the ground for the cases with pillars left in the coal seam.

Assessment of the Importance of Parameters Affecting Roof Strata Cavability in Mechanized Longwall Mining

This paper was assesses the cavability of roof strata in mechanized longwall coal mining in terms of their principle parameters. For this purpose, the fuzzy decision-making trial and evaluation laboratory (DEMATEL) method was employed to study and analysis the influencing parameters and interrelationships among them. Since this method is based on the survey of experts to indicate the influences and involves uncertainty, therefore, the fuzzy coding was used. Given that the behavior of roof strata in the caving process is controlled by many factors, seventeen effective parameters in five categories were recognized. As a result of analysis, the parameters with the highest exerted and received influence were introduced. Besides, the impact and relation of factors in the cavability system were determined. After that, an approach was proposed for weighting the parameters in the fuzzy DEMATEL framework and then the importance of significant parameters were assessed. The conducted method provides a systematic approach for holistic analysis the principles parameters affecting the cavability of immediate roof strata in mechanized longwall coal mining and hence, is worthwhile for studying this complex and dynamic problem under uncertainty.

A physical and numerical investigation of sudden massive roof collapse during longwall coal retreat mining

One of the most challenging safety problems in underground longwall coal mining is massive roof collapse in mined-out area during retreat mining. Understanding the mechanisms and geotechnical precursors of massive roof collapse is of great importance for risk assessment and ground control. In this study, a large-scale physical model was created based on a real case to simulate massive roof collapse during longwall coal retreating mining. A good agreement was achieved between field observation and physical result in terms of monitored working resistance, indicating that the physical model is able to capture the realistic response of the rock strata above mined-out area. A numerical model was created based on the model configuration of the physical model. The physical result and the numerical result were in good agreement with each other in terms of many features including roof failure mechanism, abutment pressure distribution and collapsed roof pattern. The physical and numerical result suggested that massive roof collapse is a typical snap-through failure where bedded roof crack through cross joint at midspan. Substantial increase in horizontal stress at the center of the potential failure zone is a reliable geotechnical precursor of massive roof collapse. It is suggested that horizontal stress increasing up to five times pre-mining can be considered as a threshold of the precursor of massive roof collapse.