Longwall Mining Panel Research Articles

An FDM-Based Dynamic Zoning Method for Disturbed Rock Masses above a Longwall Mining Panel

Underground longwall mining can seriously disturb the surrounding rock masses above the panel. A surface cracking zone, continuous deformation zone, fractured zone, and caved zone can be formed in the overlying strata (termed the “four zones”), which may further result in the spontaneous combustion of coal seams and water inrush. It is essential to study and predict the development characteristics of the four zones induced by longwall mining to guarantee mining safety. These four zones are developed during the mining process, and the mechanical properties in different regions correspondingly differ. Thus, the dynamic zoning characteristics of the disturbed rock masses should be considered in any simulations. In this paper, an FDM-based dynamic zoning method for disturbed rock masses above a longwall mining panel is proposed. This method is mainly composed of four stages: (1) establishing a simplified complete stress-strain curve; (2) determining the zoning criteria; (3) adaptively adjusting the mechanical parameters of the disturbed rock mass; and (4) numerically modeling the longwall mining based on the FDM. The proposed method was applied to a study site in the Taixi coal mine. The dynamic development process of the four zones induced by longwall mining was clearly observed in the modeling procedure. The numerical modeling results achieved in this work, including the periodical coal-seam roof caving and the dynamic development characteristics of the four zones, are consistent with the observed distribution and other studies. The heights of the caved and fractured zones are basically consistent with the empirical formula. Thus, the dynamic zoning method can analyze and predict the dynamic development characteristics of four regions.

Laboratory study on the propagation of edge cracks in rock-like material and its implication in coal mining

Rock fracture propagation is a major hazard for mining and tunnel excavation in fractured rock masses or coal seams. A longwall mining panel with a typical dimension of 200m (width)×1000m (length)×3m (height) can be considered as an open edge crack. The fracturing processes in the vicinity of the edge crack (or the longwall panel) particularly in the roof and floor are critically important for the safety of mining operation because fracturing can lead to water inrush and dynamic loading on the working face. It’s therefore important to understand and predict the pre-existing edge crack initiation and propagation in rock masses. This paper describes a study investigating the mechanisms and pathways of rock fracture under uniaxial compression. In this study, a rock-like material which consists of model gypsum, water and diatomaceous earth at a mass ratio of 165:75:2 was used. The uniaxial compression strength of the material decreased with the increase of the length of pre-existing edge crack. During the tests, wing (tensile) cracks were first observed at the tip of the pre-existing edge crack. This was followed by secondary cracks as the loading increased. The final failure of the specimens however was dominated by tensile cracks throughout the specimens. Due to the sudden crack initiations in the specimens, the loading stress in the specimen varies stepwise, and acoustic emission (AE) energy and amplitude showed abrupt changes when crack initiated. When the crack initiation occurred, the loading stress of the specimens showed a notable retreat in the stress-strain curve, and the recorded AE energy and amplitude showed a sharp spike. These findings from this experimental study have been applied to the underground longwall mining to explain the failure mechanisms in the floor of the mining panel. The fracturing process associated with the pre-existing edge crack resembles the formation of flow channels for water inrush during longwall mining.

One-Day Prognoses of Methane Concentrations for the 102 Longwall in the 325/1 Seam in the “W” Coal Mine Operating in a Continuous System

Abstract The first part of the paper concerns the natural deposition conditions of the 325/1 seam in the “W” coal mine, in the 102 longwall mining panel. It also presents the most important technical conditions regarding the exploitation at this longwall. To characterize the methane hazard in the longwall area, the parameters of ventilation and total methane concentrations as well as the volumetric flowrate of methane captured by the methane removal system, have been presented graphically. A significant part of the methane flow in the longwall area was released to the air flowing to the longwall. The most significant part of the article is the presentation and analysis of the results of prognoses of mean methane concentrations at the exhaust of the longwall area. The accuracy of the prognoses of methane concentration was verified using two methods: while not considering the release of methane to the air flowing to the longwall and while considering the total flowrate of methane to the ventilation air in the area of the 102 longwall. The method of forecast presented in the article has so far been checked for a 5-day and 6-day work day, as well as for walls operating in a non-regular mode. The article refers to the wall operating in a continuous mode, which required adaptation of the proposed method to this mode. The application of the one-day forecast proposed in the article allows for undertaking temporary methane prevention measures enabling safe use of the wall.

Optimum Chain Pillar Design at the Deepest Multi-Seam Longwall Workings in India

High-production methods like longwall mining with hydraulic powered supports are necessary to increase underground coal production in India. Longwall mining had a mixed success record for Indian geo-mining conditions. In this paper, an analysis is presented to optimally design an array of chain pillars in multi-seam longwall mining panels in the deepest coal mine in India. The study found that a chain pillar of 80-m width and 195-m length is necessary for long-term stability without any reinforcement. It was also observed that an optimum chain pillar can be designed by providing suitable side reinforcement (side bolting) which will reduce the chain pillar dimensions to 55-m width and 160-m length. Optimum design of chain pillars will reduce the coal locked up in these chain pillars by ~ 77%. This will also reduce the gestation period of longwall mining panel development. This paper also evaluates the maximum tensile subsidence strain induced at the surface and finds that the values are within the safe working limit of 3 mm/m. Thus, this paper finds direct application in the design of future multi-seam retreating longwall mining panels in Indian geo-mining conditions.

Assessment of coal mine methane (CMM) and abandoned mine methane (AMM) resource potential of longwall mine panels: Example from Northern Appalachian Basin, USA

Coal mine methane (CMM) and abandoned mine methane (AMM) are by-products of underground mining of gassy coal beds. The quantity and the emission rate of CMM and AMM may vary depending on the type of mine, gas content of the mined coal bed, and gas sourced from strata and coal beds in overlying and underlying formations affected by mining. Therefore, if a mine has the potential of accumulating gas after being abandoned and sealed properly, that methane may be produced and used as an energy source to serve local communities in the area. Producing AMM also prevents methane, which is a potent greenhouse gas, from leaking to the atmosphere through seals, shaft plugs or surface cracks.A technical barrier to economical utilization of CMM and AMM is the uncertainty about how much methane may be available in the gas emission zone (GEZ) as a resource during mining, as well as after the panels are sealed and the mine is abandoned. Another difficulty is estimating how much of the potential methane resource can be produced from gob gas ventholes (GGV) converted to capture AMM.In this study, a comparative assessment is presented to address the issues stated above. The assessment was conducted on two adjacent panels of a longwall mine that operated in Pennsylvania until 2016 in the Northern Appalachian Basin. The study is based on two approaches that might be used depending on the availability of data. The first approach uses an extensive geological data set, geostatistics, and measured shaft gas emission and GGV production values that were collected while the panel (s) were active to assess the AMM resource. The second approach uses a minimal amount of geologic data, uncertainty of the data as probabilistic distributions, and the use of publicly available software to predict during-mining emissions. Results showed that both approaches provide relatively comparable estimates of AMM resources and AMM recovery potential using wellbores. The differences in assessed quantities are mostly due to the characteristics of the two methods. In that regard, this paper can be considered as guidance to choose the assessment approach based on data availability.

Prediction of face advance rate and determination of the operation efficiency in retreat longwall mining panel using rock engineering system

A new approach for prediction of face advance rete (FAR) prior to mining operation and determination of the operation efficiency after mining operation in retreat longwall mining panel is presented based upon the concepts of rock engineering system (RES). For this purpose, six longwall panels considered in Parvadeh-I coal mine. Seven major effective parameters on FAR was selected including coal mine roof rating, gas propagation, safety factor of longwall face, ratio of joint spacing to cutting depth at longwall face, longwall face inclination, panel width, floor rock mass rating. To performance evaluation of the presented model, the relationship between the average vulnerability indexes of advance operation with FAR was determined in considered panels with coefficient of determination (R2) equal to 0.884 that indicate relatively acceptable correlation and compatibility. Investigations of the research indicated that it is possible to determine the actual operation efficiency under fair conditions by a RES-based model. The inevitable reduction of FAR for each longwall panel was determined by presented model that the difference amount between the maximum possible practical face advance rate (FARmpp) and recorded actual face advance rate (FARa) indicate the operation efficiency. Applied approach in this paper can be used to prediction of FAR in retreat longwall mining panel for same conditions that can have many benefits, including better and more accurate planning for the sales market and mine operation. Also, presented method in this paper can be applied as a useful tool to determination of actual operation efficiency for other sections and extraction methods in coal mines.

Height of fractured zone inside overlying strata under high-intensity mining in China

The height of fractured zone (HFZ) at the high-intensity longwall mining panel plays a vital role in the safety analysis of coal mining under bodies of water. This paper described definitions of the high-intensity mining. The processes of overburden failure transfer (OFT) were analyzed, which were divided into the development stage and the termination stage. Through theoretical analysis, the limited suspension-distance and the limited overhanging distance were proposed to judge the damage of each stratum. Mechanical models of strata suspended integrity and overhanging stability were established. A theoretical method to predict the HFZ at the high-intensity longwall mining panel was put forward based on the processes of OFT. Taking a high-intensity longwall mining panel (No. 11915 panel) as an example, the theoretical method proposed, the engineering analogy and the empirical formulas in the Regulation were used to predict the HFZ. The results show that the theoretical result is consistent with the engineering analogies’ result and empirical formulas’ result. The rationality and reliability of the theoretical method proposed is verified.

Numerical Estimation of Suitable Gob-Side Filling Wall Width in a Highly Gassy Longwall Mining Panel

Numerical Estimation of Suitable Gob-Side Filling Wall Width in a Highly Gassy Longwall Mining Panel

Role of multi-seam interaction on gas drainage engineering design for mining safety and environmental benefits: Linking coal damage to permeability variation

Methane gas drainage is an effective method to ensure mining-process safety and deliver greater environmental benefits through reducing greenhouse gas (GHG) emissions. As most coal seams actually coexist with one or several other coal seams, i.e. the multi-layer coal seam group. A better understanding of the interaction between the adjacent coal seams could provide better guidance for the gas drainage engineering design to enhance its performance. Extensive studies on the multi-seam interaction have focused on the effect of mining the underlying coal seam on the overlying seam from perspectives of rock deformation, stress change and permeability variation. Meanwhile, as there is few coupled permeability models taking the coal-damage effect into account, previous permeability-variation analysis seldom commonly considered the mining-induced coal damage, coal mechanics changes and gas adsorption, which results in the permeability underestimation. Therefore, in this paper, a mathematical model which incorporates the coal permeability with coal damage, coal mechanical property and the gas adsorption was developed. Then this model was implemented into a finite-element numerical simulation, which was used to investigate the impact of the overlying coal seam mining on the underlying relieved seam from the perspective of damage-based permeability variation. Meanwhile, the effect of damage on gas-emission performance from the underlying seam was analyzed. Results show that there are four permeability areas under the mine-out panel, permeability increases greatly in areas I to III (the highest as over 650 times) while it increases slightly in the area IV. These permeability results are largely consistent with the stress-analysis conclusions obtained by other researchers. By taking the damage into account, the evaluation on gas-emission condition could become more reasonable. Above research outcomes could help to determine the favorable gas-drainage areas under the longwall mining panel and guide the drainage borehole design in the relieved coal seam, to deliver better drainage outcomes for the mining safety and GHG-emission reduction.

An Innovative Approach for Gob-Side Entry Retaining in Thick Coal Seam Longwall Mining

Gob-side entry retaining (GER) is a popular non-pillar mining technique regarding how to reserve a gateroad for the use of next panel mining. When used in thick coal seams, the conventional entry retaining method requires a huge amount of filling materials and may cause entry (gateroad) accidents. Thus, an innovative non-pillar longwall mining approach is introduced. First, structural and mechanical models were built to explore the mechanism of the new approach. The modeling results indicate that effective bulking of the gob roof and reasonable support of the entry roof were key governing factors in improving entry stabilities and reducing roof deformations. Accordingly, a directional roof fracturing technique was proposed to contribute to gob roof caving, and a constant resistance and large deformation anchor (CRLDA) cable was used to stabilize the entry roof. Subsequently, the evolutionary laws of the roof structure and stresses were explored using numerical simulation. It was found that the structure of the surrounding rocks around the retained entry changed significantly after roof fracturing. The stress-bearing center was transferred to the gob area, and the entry roof was in a low stress environment after adopting the approach. Finally, the approach was tested on a thick coal seam longwall mining panel. Field monitoring indicates that the retained entry was in a stable state and the index of the retained entry met the requirement of the next mining panel. This work provides an effective and economical approach to non-pillar longwall mining in thick coal seams.

Fully Mechanized Longwall Mining in an Underground Bauxite Deposit: A Case Study

Based on the boundary surfaces of bauxite deposit modelled by the two-dimensional biharmonic spline interpolation, it was ascertained that the fully mechanized longwall mining of bauxite is feasible in terms of lithological characteristics, orebed occurrence, physical–mechanical properties of ore–rock, faults, Karst caves, permeable fissures, etc. Furthermore, a comprehensive target function consisting of waste-rock interfused ratio and ore loss ratio was used to obtain the optimal cutting height of shearer by a virtual mining procedure that was proposed to numerically perform the fully mechanized longwall mining. For the 1102 longwall mining panel in Wachangping Bauxite Mine, the optimal cutting height of shearer ranges from 1.16 to 2.32 m supplied from two 1.16-m-diameter cutting drums, which meets the requirement of low-waste and high-recovery mining to achieve high-grade and low-loss exploitations of underground bauxite. These efforts can improve the application and popularization of fully mechanized longwall mining in non-coal underground mines.

Separation and fracturing in overlying strata disturbed by longwall mining in a mineral deposit seam

Mining-induced fractures influence the stability of overburden and may provide pathways for transferring heat and mass in underground environment. A novel approach was proposed to quantify the separation and fracture evolution in the undermined overburden, consisting of (1) theoretical distribution models of the void ratios of fractures (VRFs), based on analytical evaluation of key strata subsidence, and (2) numerical modeling using the universal distinct element code and a sequence of image post-processing procedures. For a longwall mining panel, both theoretical calculations and numerical simulations indicated that VRF first increased rapidly, then gradually decreased, and finally stabilized to a minimum from the surrounding edges of disturbed strata to their centers, and gradually decreased from deep to shallow strata. The fractures exhibited a “fracture-rich arch” distribution type along a vertical section. Numerous fractures occurred around the arch feet near the perimeters of mined-out area and/or arch crown near the center of disturbed strata. The distributions of VRFs show the heights, shapes and damage intensities of fractured zones, and can also be used as porosity parameters to determine the permeabilities of mining-disturbed overburdens. Therefore, the VRF models can be used as a quantitative parameter to assess the extent of possible risk zones, e.g. for water inflow into the mine or escape of hazardous fluids to the surface.

Grade Distribution Modeling within the Bauxite Seams of the Wachangping Mine, China, Using a Multi-Step Interpolation Algorithm

Mineral reserve estimation and mining design depend on a precise modeling of the mineralized deposit. A multi-step interpolation algorithm, including 1D biharmonic spline estimator for interpolating floor altitudes, 2D nearest neighbor, linear, natural neighbor, cubic, biharmonic spline, inverse distance weighted, simple kriging, and ordinary kriging interpolations for grade distribution on the two vertical sections at roadways, and 3D linear interpolation for grade distribution between sections, was proposed to build a 3D grade distribution model of the mineralized seam in a longwall mining panel with a U-shaped layout having two roadways at both sides. Compared to field data from exploratory boreholes, this multi-step interpolation using a natural neighbor method shows an optimal stability and a minimal difference between interpolation and field data. Using this method, the 97,576 m3 of bauxite, in which the mass fraction of Al2O3 (Wa) and the mass ratio of Al2O3 to SiO2 (Wa/s) are 61.68% and 27.72, respectively, was delimited from the 189,260 m3 mineralized deposit in the 1102 longwall mining panel in the Wachangping mine, Southwest China. The mean absolute errors, the root mean squared errors and the relative standard deviations of errors between interpolated data and exploratory grade data at six boreholes are 2.544, 2.674, and 32.37% of Wa; and 1.761, 1.974, and 67.37% of Wa/s, respectively. The proposed method can be used for characterizing the grade distribution in a mineralized seam between two roadways at both sides of a longwall mining panel.

Grade Distribution and Orebody Demarcation of Bauxite Seam Based on Coupled Interpolation

A three-dimensional coupled interpolation method with three steps was proposed to establish a smooth and precise grade distribution model of bauxite seam. Compared with and supported by the field grade data measured by some exploratory boreholes, the stability of interpolation model established by natural neighbor method is the best to depict a thin bedded deposit to be more physically realistic in data interpolation. Based on this model, the total volume of bauxite seam in the 1102 longwall mining panel is \(189{,}270 \, \hbox {m}^{3}\), in which the average values of mass fraction of \(\hbox {Al}_{2} \hbox {O}_{3}\) and mass ratio of \(\hbox {Al}_{2} \hbox {O}_{3}\) to \(\hbox {SiO}_{2}\) are 55.09% and 17.72, respectively. Demarcated by industrial standards, the volume, the average mass fraction of \(\hbox {Al}_{2} \hbox {O}_{3}\) and the average mass ratio of \(\hbox {Al}_{2} \hbox {O}_{3}\) to \(\hbox {SiO}_{2}\) of bauxite orebody are \(96,654 \, \hbox {m}^{3}\), 61.58% and 27.10, respectively. These mean that the bauxite orebody has abundant high-grade ore.

Three-dimensional orebody modelling and intellectualized longwall mining for stratiform bauxite deposits

A coupled biharmonic spline and linear interpolation algorithm was proposed to create a three-dimensional smooth deposit model with minimal curvature containing grade and position data. To obtain the optimal technical parameters, such as cutting height and drum diameter, a virtual longwall mining procedure was modelled by simulating the actual fully mechanized longwall mining process. Based on the above work, a bauxite deposit in a longwall mining panel was modelled by scattered grade data from ores sampled on the entry wall. The deposit was then demarcated by industrial indexes and sliced according to the virtual longwall mining procedure. The results show that the proposed interpolation algorithm can depict the stratiform structure of bauxite deposits and that the uncovered bauxite deposit has high proportions of high-grade and rich ore. The ranges of optimal cutting height and drum diameters are 1.72–2.84 m and 1.42–1.72 m, respectively. Finally, an intellectualized longwall mining procedure was designed to guide the mining process with the lowest dilution and loss rates.

Strengthening borehole configuration from the retaining roadway for greenhouse gas reduction: a case study.

A monitoring trial was carried out to investigate the effect of boreholes configuration on the stability and gas production rate. These boreholes were drilled from the retaining roadway at longwall mining panel 1111(1) of the Zhuji Coalmine, in China. A borehole camera exploration device and multiple gas parameter measuring device were adopted to monitor the stability and gas production rate. Research results show that boreholes 1~8 with low intensity and thin casing thickness were broken at the depth of 5~10 m along the casing and with a distance of 2~14 m behind the coal face, while boreholes 9~11 with a special thick-walled high-strength oil casing did not fracture during the whole extraction period. The gas extraction volume is closely related to the boreholes stability. After the stability of boreholes 9~11 being improved, the average gas flow rate increased dramatically 16-fold from 0.13 to 2.21 m3/min, and the maximum gas flow rate reached 4.9 m3/min. Strengthening boreholes configuration is demonstrated to be a good option to improve gas extraction effect. These findings can make a significant contribution to the reduction of greenhouse gas emissions from the coal mining industry.

Application of seismic velocity tomography in underground coal mines: A case study of Yima mining area, Henan, China

A better understanding of geological structures , stress regimes, and rock burst risks around longwall mining panels can allow for higher extraction efficiency with reduced safety concerns. In this paper, the stress change of rock mass was first examined by using ultrasonic technique into laboratory-scale rock samples. Subsequently, the active and passive seismic velocity tomograms were simultaneously applied into two study cases with field-scale. Similar characteristics can be found between the active and passive tomography results. More specifically, in the first case, a geological discontinuity was clearly indicated by a linear image in both active and passive seismic tomography results. The results of the second case suggest that seismic tomography can be used to infer stress redistribution, and assess rock burst hazard or locate high-seismicity zones. Ultimately, comparisons have been made between the results of active and passive seismic tomography. Active tomography is found to be better applied in accurately detecting stress distribution and geological structures prior to the extraction of longwall panels, while passive tomography has advantages in continuously monitoring the stress changes and assessing rock burst potential during the mining of longwall panels. This study is expected to increase the safety and efficiency of the underground mining. • Ultrasonic technology was conducted at the laboratory-scale. • Active and passive seismic velocity tomograms were applied at the field-scale. • Geological structures, stress regimes, and rock burst risks were illustrated. • Comparisons have been made between the active and passive seismic tomography.

Evaluation of coal longwall caving characteristics using an innovative UDEC Trigon approach

The paper presents an innovative numerical approach to simulate progressive caving of strata above a longwall coal mining panel. A proposed Trigon logic is incorporated within UDEC to successfully capture the progressive caving of strata which is characterized by fracture generation and subsequent propagation. A new damage index, D, is proposed that can quantify regions of both compressive shear and tensile failure within the modelled longwall. Many features of progressive caving are reproduced in the model and found to fit reasonably well with field observations taken from a case study in the Ruhr coalfield. The modelling study reveals that compressive shear failure, rather than tensile failure, is the dominant failure mechanism in the caved strata above the mined-out area. The immediate roof beds act like beams and can collapse in beam bending when vertical stress is dominant or in beam shear fracture when horizontal stress is dominant. The proposed numerical approach can be used to guide the design of longwall panel layout and rock support mechanisms.

Numerical Investigation of the Dynamic Mechanical State of a Coal Pillar During Longwall Mining Panel Extraction

This study presents a numerical investigation on the dynamic mechanical state of a coal pillar and the assessment of the coal bump risk during extraction using the longwall mining method. The present research indicates that there is an intact core, even when the peak pillar strength has been exceeded under uniaxial compression. This central portion of the coal pillar plays a significant role in its loading capacity. In this study, the intact core of the coal pillar is defined as an elastic core. Based on the geological conditions of a typical longwall panel from the Tangshan coal mine in the City of Tangshan, China, a numerical fast Lagrangian analysis of continua in three dimensions (FLAC3D) model was created to understand the relationship between the volume of the elastic core in a coal pillar and the vertical stress, which is considered to be an important precursor to the development of a coal bump. The numerical results suggest that, the wider the coal pillar, the greater the volume of the elastic core. Therefore, a coal pillar with large width may form a large elastic core as the panel is mined, and the vertical stress is expected to be greater in magnitude. Because of the high stresses and the associated stored elastic energy, the risk of coal bumps in a coal pillar with large width is greater than for a coal pillar with small width. The results of the model also predict that the peak abutment stress occurs near the intersection between the mining face and the roadways at a distance of 7.5 m from the mining face. It is revealed that the bump-prone zones around the longwall panel are within 7–10 m ahead of the mining face and near the edge of the roadway during panel extraction.

Passive seismic velocity tomography on longwall mining panel based on simultaneous iterative reconstructive technique (SIRT)

Mining operation, especially underground coal mining, always has the remarkable risks of ground control. Passive seismic velocity tomography based on simultaneous iterative reconstructive technique (SIRT) inversion is used to deduce the stress redistribution around the longwall mining panel. The mining-induced microseismic events were recorded by mounting an array of receivers on the surface, above the active panel. After processing and filtering the seismic data, the three-dimensional tomography images of the p-wave velocity variations by SIRT passive seismic velocity tomography were provided. To display the velocity changes on coal seam level and subsequently to infer the stress redistribution, these three-dimensional tomograms into the coal seam level were sliced. In addition, the boundary element method (BEM) was used to simulate the stress redistribution. The results show that the inferred stresses from the passive seismic tomograms are conformed to numerical models and theoretical concept of the stress redistribution around the longwall panel. In velocity tomograms, the main zones of the stress redistribution around the panel, including front and side abutment pressures, and gob stress are obvious and also the movement of stress zones along the face advancement is evident. Moreover, the effect of the advance rate of the face on the stress redistribution is demonstrated in tomography images. The research result proves that the SIRT passive seismic velocity tomography has an ultimate potential for monitoring the changes of stress redistribution around the longwall mining panel continuously and subsequently to improve safety of mining operations.