Longwall Coal Mining Research Articles

Numerical modelling techniques for studying longwall geotechnical problems under realistic geological structures

Longwall - associated geotechnical problems have been extensively studied by using numerical modelling methods. However, proper representation of its geological structures remains a challenging task. This paper presents a systematic understanding of numerical modelling techniques for studying longwall coal mining with geological structures. The modelling techniques derived from conventional and advanced continuum and discontinuum methods were reviewed in detail with emphasiz on their mechanic's formulation and applications. This study suggests that the successful selection of a proper modelling technique should be based on the physical principle of longwall problem, texture and shape of materials, and mechanics formulation of the numerical program used for modelling. The paper’s conclusions assist numerical modellers in quickly and properly selecting modelling technique for investigating a site - specific longwall problem.

Modeling Microgravity Anomalies That Accounts of the Pore Water Drainage Inferred from The Dc-Resistivity Sounding Data Over a Coal Panel

A microgravity data set presented in a previous study exhibits distinct short-wavelength anomalies over a longwall coal mine panel at Soma-Darkale Coalfield. Nevertheless, our preliminary models suggest that the wavelength after the coal removal from a panel at a moderate depth and fracturing alone should be incomparably greater than that of the measured anomalies. Understanding the mechanism that causes these anomalies usually becomes critical to provide credible evidence for longwall mining-related legal cases. This study improves the model by including the post-subsidence drainage as it occurs because of fracturing that causes an increase in water storage and local density change. Since no water-level information was available at the site, we attempted to infer the drained zone from the dc-resistivity sounding measurements acquired shortly before the gravity field survey. The wavelengths of the model anomalies became reasonably comparable with that of the residual anomalies after the inclusion of the inferred drainage information. This presented approach that does not require water level measurements shows that the inclusion of the inferred drained zone to the model became an amplifying indicator of a coal panel at a moderate depth. Therefore, it may easily find application in settling the mining-related legal cases, understanding the longwall mining-related geohazard, and environmental impact assessments.

A geomechanical method for predicting the height of a water-flowing fractured zone in a layered overburden of longwall coal mining

Mine water inrushes result in serious casualties and property losses, and mining-induced water-flowing fractures are important channels for mine water inrushes. The prediction of the height of the water-flowing fractured zone (HWFFZ) is conducive to avoiding such disasters to a certain extent. In this study, the HWFFZ in layered overburden strata was systematically investigated by means of similar simulations, theoretical analyses and engineering verification. The following results were obtained. Fractures occur in overburden strata because the tensile stress exceeds the tensile strength, while fractures penetrate the overburden strata because the compressive stress exceeds the compressive strength. Whether a fracture penetrates the overburden strata can be expressed by the fracture penetration criterion (FPC) value and the compressive-tensile ratio (i.e., the ratio of the compressive stress to the tensile stress). If the FPC value is greater than the compressive-tensile ratio, the fracture penetrates the overburden strata. For both the 31401 mining face of the Bulianta Coal Mine and the 7130 coal mining panel of the Qidong Coal Mine, the FPC values of the water inrush regions are larger than those of the normal regions, and the compressive-tensile ratios of the water inrush regions are smaller than those of the normal regions.

Assessing Gas Leakage Potential into Coal Mines from Shale Gas Well Failures: Inference from Field Determination of Strata Permeability Responses to Longwall-Induced Deformations

This paper summarizes the changes in permeability at three boreholes located above an abutment pillar at a longwall coal mine in southwestern Pennsylvania. The motivation of this study was to better characterize the potential interaction between shale gas wells and the mine environment, through measurement of permeability changes in the coal mine overburden caused by mining-induced deformations. Measuring permeability changes around boreholes affected by longwall mining is an effective method to indicate changes in the fracture network above longwall abutment pillars and estimate the capacity for gas flow from shale gas wells to the mine environment. This study measured permeability through falling-head slug tests at different longwall face positions during the mining of two longwall panels on either side of the test abutment pillar where the test boreholes were located. Three test boreholes were drilled to different depths above the active mining level, and they had screened intervals to evaluate the response of different stratigraphic zones to mining-induced stresses. The results showed that the permeability around the slotted intervals of each borehole increased pre-mining to post-mining, and the permeability increased from mining of the first longwall panel to mining of the second one, adjacent to the pillar.

A New Method for Determining Gob Methane Sources Under Extraction Conditions of Longwall Coal Mines

Underground coal mining operations produce large numbers of gobs that contain methane from mining coal seams and their adjacent coal seams. In longwall mining, gob methane is released to coal faces, presenting safety issues. Therefore, if gob methane sources can by identified accurately under extraction conditions, then efficient methane extraction strategies can be adopted to maximize the capture of methane from gobs and to control effectively methane flow to the coal face, thereby reducing safety risks. In this study, a new method to determine quantitatively gob methane sources under extraction conditions was proposed and tested in China’s Liyazhuang Coal Mine. Firstly, the isotope and gas composition of methane desorbed from coal cores of the mining seams and their adjacent seams, as well as methane collected from methane drainage boreholes at different heights in the gobs, were measured. Then, an isotope source model for calculating gob methane sources was established. Finally, based on isotope and gas compositions, the proportions of sources of gob methane that had been extracted from methane drainage boreholes were determined using the afore-mentioned model, and the extraction range that could maximize efficiently the control of methane emissions from each methane source was obtained.

The influence of advance speed on overburden movement characteristics in longwall coal mining: insight from theoretical analysis and physical simulation

Abstract The advance speed of a longwall face is an essential factor affecting the mining pressure and overburden movement, and an effective approach for choosing a reasonable advance speed to realise coal mine safety and efficient production is needed. To clarify the influence of advance speed on the overburden movement law of a fully mechanised longwall face, a time-space subsidence model of overburden movement is established by the continuous medium analysis method. The movement law of overburden in terms of the advance speed is obtained, and mining stress characteristics at different advance speeds are reasonably explained. The theoretical results of this model are further verified by a physical simulation experiment. The results support the following conclusions. (i) With increasing advance speed of the longwall face, the first (periodic) rupture interval of the main roof and the key stratum increase, while the subsidence of the roof, the fracture angle and the rotation angle of the roof decrease. (ii) With increasing advance speed, the roof displacement range decreases gradually, and the influence range of the advance speed on the roof subsidence is 75 m behind the longwall face. (iii) An increase in the advance speed of the longwall face from 4.89 to 15.23 m/d (daily advancing of the longwall face) results in a 3.28% increase in the impact load caused by the sliding instability of the fractured rock of the main roof and a 5.79% decrease in the additional load caused by the rotation of the main roof, ultimately resulting in a 9.63% increase in the average dynamic load coefficient of the support. The roof subsidence model based on advance speed is proposed to provide theoretical support for rational mining design and mining-pressure-control early warning for a fully mechanised longwall face.

Deformation response of roof in solid backfilling coal mining based on viscoelastic properties of waste gangue

Solid backfill mining (SBM) is a form of green mining, the core of which is to control and minimize the deformation and movement of strata above longwall coal mines. Establishing a mechanical model that can reliably describe roof deformation by considering the viscoelastic properties of waste gangue is important as it assists in improving mine designs and reducing the environmental impact on the surface. In this paper, the time-dependent deformation characteristics of gangue under different stress levels were obtained by using lateral confinement compression, that reliably represents the compaction of goaf. The viscoelastic foundation model for gangue mechanical response is different from the traditionally used elastic foundation model, as it considers the time factor and viscoelasticity. A mechanical model using a thin plate on a fractional viscoelastic foundation was established, and the roof deflection, bending moment, time-dependent, viscous and other characteristics of SBM were included and analyzed. Compared with the existing elastic foundation model, the proposed fractional order viscoelastic foundation model has higher accuracy with laboratory data. The plate deflection increases by 50.9% and the bending moment increases by 37.9% after 100 days, which the elastic model would not have been able to predict.

The assessment of the optimal time window for prediction of seismic hazard for longwall coal mining: the case study

The dynamic nature of rock mass damage during mining activity generates seismic events. This article shows, how the time window for the database influences on the actual status of seismic hazard for the longwall mining area in one of Polish coal mines using Gutenberg–Richter law. A time window of 10–90 days was assumed with similar or shorter prediction times forecast on its basis. Additionally, for each seismic database the hazard prediction accuracy was determined. The analysis shows that the 10- and 20-day base periods are too short for prediction purposes. The higher-energy seismic events sometimes do not occur within such a short period of time, preventing regression analysis and parameter b determination. The best time window for the seismic hazard prognosis in given geological and mining conditions seems to be 30–50 days. The shorter periods cause the underestimation of the seismic hazard prognosis. Low range of tremor energies and the relatively low number of seismic events with high energy cause the low probability of prediction of the seismic mining events (10–40%) of the energy of min. 106 J, even for longer day base periods. The accuracy of hazard prediction, obtained from each seismic database period, was determined, using the developed coefficient of hazard autoregression CN. The analysis of the Gutenberg–Richter distribution should serve as complementary tool of seismic hazard prediction only.

Physical and numerical investigations of bedding adhesion strength on stratified rock roof fracture with longwall coal mining

Physical and numerical investigations of bedding adhesion strength on stratified rock roof fracture with longwall coal mining

Finite-difference based response surface methodology to optimize tailgate support systems in longwall coal mining

Designing a suitable support system is of great importance in longwall mining to ensure the safe and stable working conditions over the entire life of the mine. In high-speed mechanized longwall mining, the most vulnerable zones to failure are roof strata in the vicinity of the tailgate roadway and T-junctions. Severe roof displacements are occurred in the tailgate roadway due to the high-stress concentrations around the exposed roof span. In this respect, Response Surface Methodology (RSM) was utilized to optimize tailgate support systems in the Tabas longwall coal mine, northeast of Iran. The nine geomechanical parameters were obtained through the field and laboratory studies including density, uniaxial compressive strength, angle of internal friction, cohesion, shear strength, tensile strength, Young’s modulus, slake durability index, and rock mass rating. A design of experiment was developed through considering a Central Composite Design (CCD) on the independent variables. The 149 experiments are resulted based on the output of CCD, and were introduced to a software package of finite difference numerical method to calculate the maximum roof displacements (dmax) in each experiment as the response of design. Therefore, the geomechanical variables are merged and consolidated into a modified quadratic equation for prediction of the dmax. The proposed model was executed in four approaches of linear, two-factor interaction, quadratic, and cubic. The best squared correlation coefficient was obtained as 0.96. The prediction capability of the model was examined by testing on some unseen real data that were monitored at the mine. The proposed model appears to give a high goodness of fit with the accuracy of 0.90. These results indicate the accuracy and reliability of the developed model, which may be considered as a reliable tool for optimizing or redesigning the support systems in longwall tailgates. Analysis of variance (ANOVA) was performed to identify the key variables affecting the dmax, and to recognize their pairwise interaction effects. The key parameters influencing the dmax are respectively found to be slake durability index, Young’s modulus, uniaxial compressive strength, and rock mass rating.

Persistent effects of underground longwall coal mining on freshwater wetland hydrology

More than half of global wetlands have been lost because of anthropogenic disturbance, with the trend of decline continuing in the 21st century. While much of this loss relates to changes in surface flows, groundwater is also critical to sustaining wetland hydrology. Underground longwall mines extract coal seams, in turn fracturing the overlying stratigraphy, influencing aquifer connectivity and affecting surface flows via subsidence disturbance. Crucially, this subterranean disturbance may disrupt the hydrological processes that sustain freshwater wetlands at the surface. Here we present a new designed empirical study that compares the persistence of soil moisture after a rainfall event in wetlands subject to underground longwall coal mining to that in unmined reference wetlands. Accelerated Failure Time models showed that mined wetlands were persistently drier, retained water for shorter durations and exhibited less spatial differentiation than unmined wetlands. This quantitative evidence of severe, persistent hydrological change following resource extraction reinforces earlier observations and has important implications for biodiversity and provision of ecosystem services to a large urban population. If Ecologically Sustainable Development (ESD) outcomes and effective deployment of the mitigation hierarchy are to be achieved in line with current legislative and policy paradigms, our results highlight the need for more emphasis on impact avoidance and minimisation than restoration or offsetting to protect water and biodiversity values. Given severe constraints on restoration success, greater emphasis on avoidance in mine design and approval processes offers realistic opportunities for an improved balance between sustaining irreplaceable public assets and short-term benefits from non-renewable resource extraction.

Re-analysis of Abutment Angle Method for Moderate and Deep Cover Retreat Room and Pillar Mines and Investigation of Loading Mechanics Using Finite Volume Modeling

Mining-induced stresses in underground coal mines play a significant role in pillar and ground support design, hence in the safety of mining operations. In the US, Analysis of Longwall Pillar Stability (ALPS) and in Australia, Analysis of Longwall Tailgate Serviceability (ALTS) software are used for designing Longwall coal mine layouts; and in the US, Analysis of Retreat Mining Pillar Stability (ARMPS) software is used to design retreat room-and-pillar mine layouts. All these software determine the adequacy of the design by comparing the estimated loads to the load-bearing capacity of the pillars and they use the “abutment angle” concept and a square decay stress distribution function to calculate the magnitude and distribution of the mining-induced loads. The abutment angle concept has been successfully applied to US longwall coal mines with the use of ALPS and ALTS in Australia. ARMPS uses the same concept for retreat room and pillar coal mine design in the US. The suggested abutment angle for coal mines in the US was derived as 21° by the back analysis of underground stress measurements from the 1990s and implemented in ALPS and ARMPS. The ALPS methodology was re-examined and calibrated for Australian conditions with additional Australian stress measurements and resulted in the original ALTS methodology which has been continually improved and expanded with additional cases. In this paper, some recent stress measurements are back-analyzed, and the abutment angles are investigated to verify the applicability of using 21° in retreat room and pillar mines with different depths and mining dimensions. For shallow mines, the derivation of the 21° abutment angle is supported by the new case histories. However, at depths greater than 200 m, the abutment angle was found to be decreasing with increasing depth. In this study, a new equation for the calculation of abutment angle for moderate and deep cover cases was constructed and tested for its applicability in retreat room and pillar mines. The differences in the mechanism of complete side abutment loads in shallow and deep cover mines are further analyzed by applying the finite volume modeling (FVM) approach to two case study mines, one shallow, and one deep cover. A 2D model of each mine is created and one-side and two-side abutment loads of consecutive panels are analyzed. Analysis of the deep cover mine indicated that the prior panel gobs provide a considerable amount of support to the overburden strata. These higher gob loads prevent a higher percentage of overburden loads from being transferred to the active panel workings, and this is in agreement with the lower abutment angles observed for deep cover mines. The findings of this study should only be used for retreat room-and-pillar mines’ production pillar loads since these are calculated geometrically using the abutment angle concept.

Mine subsidence prediction using gene expression programming based on multivariable symbolic regression

Accurate prediction of surface subsidence becomes a significant challenge for active industrial companies in coal mining fields due to the importance of the economic impacts of longwall mining-induced subsidence. This article explores a new variant of genetic programming, namely gene expression programming (GEP). The GEP-based method is utilized to present a new mathematical formula for subsidence prediction in longwall coal mining. The derived model includes both geometrical and geological variables. The data set consists of field measurements obtained through 37 longwall panels of Ulan coal mine, NSW, Australia. The GEP-based model concluded satisfactory subsidence prediction outcomes compared to other empirical methods such as NCB, DMR, ACARP, and IPM. The predictive ability of the GEP-based models, which captures the complex nonlinear effects of the critical factors on the magnitude of subsidence, resulted in a statistically significant improvement in predictive capacity compared to the aforementioned empirical methods. The sensitivity analysis results indicated that Panel width and cover thickness with 31% and 23% were the most influential parameters in the proposed model. Also, the extracted seam thickness, thick layer location, and thick layer thickness had 19%, 16%, and 11% impact on the GEP proposed model, respectively.

Assessment of Relationship of Some Causal Factors Associated with Productivity of Longwall Mining Using Structured Equation Modeling

Enhancement of productivity optimization is steadily gaining the priority in mining companies especially in the underground coal mining industry which faces a daunting task to balance marginal profit generation with a comparatively high cost of production, volatile market price and rapid grade variation. This paper is aimed to analyze some of the causal factors both technical and site specific which are directly or indirectly impacting the productivity of the longwall coal mining system such as downtime of equipment system in longwall panel, overloading of conveyors and bin, preventive maintenance, gas management practices and injury severity rate. Structured Equation Modelling (SEM) was used to study the causal relationships between the above-mentioned factors and mine productivity. The equipment considered for analysis included shearer, armored face conveyor, crusher, bridge stage loader, chock supports, main gate drive, gate conveyor, hydraulic pumps and crusher. SEM was applied to relate the correlations existing among these causal variables in order to assess their direct or indirect impact on mining productivity. Based on the data extracted over a period of 10 months which included the extraction of 2 longwall panels, the study revealed a significant negative causal relationship between injury severity rate (p < 0.01) and equipment downtime (p = 0.001), with mine productivity. However, preventive maintenance delay time, gas management delay time and conveyor overloading delay time were found to have an insignificant direct influence on mine productivity but indirectly modify it through a significant mediator relationship with equipment downtime. This information would assist mine management to take proper preventive measures.

Impacts of longwall mining speeds on permeability of weakly cemented strata and subsurface watertable: a case study

The negative impacts of underground longwall coal mining on water resources and ecological environment are associated with changing overburden permeability and subsurface watertable. For weakly cemented strata characterized by short diagenesis, low strength, easy swelling and mudding with water, overburden permeability and subsurface watertable are highly sensitive to the advancing speed of the longwall. In this paper, the longwall panel 21103 in Yili No.4 Coal Mine, Xinjiang, China, where weakly cemented strata exist in Jurassic and Palaeogene systems, was taken as the case to study the response of overburden permeability and subsurface watertable to different advancing speeds. Results show that there is a butterfly-shaped permeability-increasing area comprised of a trapezoid and double V-shaped subareas in overburden. Generally, the permeability curves of aquiclude are of double peaks. In shallow aquifer, there is an exponential decline in the maximum drawdown of watertable, while a linear decline in lateral influence range as the speed goes up. If constrained by the ultimate watertable drawdown still being able to sustain the regular growth of vegetative covers, the optimal minimum advancing speed of panel 21103 is determined as 8 m/d, which is demonstrated by field verification.

Collision Detection of Virtual Powered Support Groups Under Complex Floors

Abnormal relative position and attitude offset may cause powered supports to collide during movement, affecting the stability of a longwall operation. This study proposed a collision detection method for virtual powered support groups in the undulating strata condition. First, a support workspace was developed by defining key points, and the Denavit-Hartenberg (DH) matrix coordinate system of powered supports was established to solve the key points. Then, based on the intersection rule of plane line segments, collision detection between powered supports was realized by calculating the coordinates of the key points. According to actual coal face data, a virtual simulation platform is established, and the sequential movement of support groups on the virtual floor was realized. Finally, contrast experiments of different virtual floor models were conducted, and the Mesh floor, which is closer to the actual coal seam, was selected for subsequent virtual simulation. The collision detection experiments of powered supports were conducted in both virtual and physical environments, and the results validated the feasibility of the collision detection method. This method improves the virtual simulation degree of longwall coal mining, and provides simulation reference for the attitude adjustment of actual powered support groups before moving.

Investigating different methods used for approximating pillar loads in longwall coal mines

Accurately estimating load distributions and ground responses around underground openings play a significant role in the safety of the operations in underground mines. Adequately designing pillars and other support measures relies highly on the accurate assessment of the loads that will be carried by them, as well as the load-bearing capacities of the supports. There are various methods that can be used to approximate mining-induced loads in stratified rock masses to be used in pillar design. The empirical methods are based on equations derived from large databases of various case studies. They are implemented in government approved design tools and are widely used. There are also analytical and numerical techniques used for more detailed analysis of the induced loads. In this study, two different longwall mines with different panel width-to-depth ratios are analyzed using different methods. The empirical method used in the analysis is the square-decay stress function that uses the abutment angle concept, implemented in pillar design software developed by the National Institute for Occupational Safety and Health (NIOSH). The first numerical method used in the analysis is a displacement-discontinuity (DD) variation of the boundary element method, LaModel, which utilizes the laminated overburden model. The second numerical method used in the analysis is Fast Lagrangian Analysis of Continua (FLAC) with the numerical modeling approach recently developed at West Virginia University which is based on the approach developed by NIOSH. The model includes the 2D slice of a cross-section along the width of the panel with the chain pillar system that also includes the different stratigraphic layers of the overburden. All three methods gave similar results for the shallow mine, both in terms of load percentages and distribution where the variation was more obvious for the deep cover mine. The FLAC3D model was observed to better capture the stress changes observed during the field measurements for both the shallow and deep cover cases. This study allowed us to see the shortcomings of each of these different methods. It was concluded that a numerical model which incorporates the site-specific geology would provide the most precise estimate for complex loading conditions.

Evolution Mechanism of Interconnected Vertical Fractures in the Overburden of Longwall Coal Mining

Whether a tensile failure fracture will penetrate a stratum is difficult to ascertain at present. In view of this, the method of similar simulation and field verification are used to carry out a systematic study. Similar simulations show that tensile failure fractures will penetrate the layered strata if the compressive stress is greater than the compressive strength. Theoretical analysis points out that whether the tensile failure fractures will penetrate the layered strata can be expressed by the value of criterion of interconnected vertical fractures and the compression-tension ratio. When the value of criterion of interconnected vertical fractures is greater than the compression-tension ratio, the layered strata will break. This criterion was qualitatively verified with a field test. The results of this paper are of great significance for the prevention of water inrush in coal mines, and it can also promote the understanding of the law of strata movement.

A Probabilistic Model to Determine Main Caving Span by Evaluating Cavability of Immediate Roof Strata in Longwall Mining

Caving process is a complex dynamic phenomenon influences safety and productivity of coal longwall mining. It improves safety due to reduction of load on support, face convergence and abutment stresses. Proper caving with respect to the quality and time of occurrence ensures continuity of operation and subsequently, the productivity of coal extraction. Therefore, a reliable prediction of strata behaviour and its caving potential are imperative in design of longwall projects. This paper presents a hybrid probabilistically qualitative–quantitative model to evaluate cavability of immediate roof and to estimate main caving span in longwall mining by combining empirical model and numerical solution. For this purpose, numerical simulation was incorporated to Roof Strata Cavability index (RSCi) as summation of ratings for nine significant parameters. Distinct element code was used to simulate numerically main caving span corresponding to various RSCi classes probabilistically. The newly proposed model was verified against actual field data collected from different longwall panels around the world. The results of proposed model agreed well with those of collected data.

A physics-based probabilistic forecasting methodology for hazardous microseismicity associated with longwall coal mining

Mining-induced microseismicity is widely considered as a result of slippage of pre-existing critically stressed fractures caused by stress perturbations around an advancing face. An in-depth analysis of the recorded microseismicity associated with longwall top coal caving mining at Coal Mine Velenje in Slovenia has been previously carried out and reported by the authors. It has been concluded that while microseismic event rate is affected by mining intensity (longwall face daily advance rate) as well as local abundance of pre-existing fractures, spatial and magnitude characteristics of microseismicity are predominantly influenced by the latter. Based upon this improved understanding of fracture-slip seismic-generation mechanism, the current work aimed at establishing a data-driven yet physics-based probabilistic forecasting methodology for hazardous microseismicity using microseismic monitoring data with concurrent face advance records. Through performing statistical analyses and probability distribution fitting for temporal, magnitude and spatial characteristics of microseismicity within a time window, a short-term forecasting model is developed to estimate the probability of potentially hazardous microseismicity over the next time interval in the form of a joint probability. The real time forecasting of hazardous microseismicity during longwall coal mining is realised through regularly updating the statistical model using the most recent microseismic sequence datasets and face advance records. This forecasting methodology is featured by the physical basis which provides a good explicability of forecasting results, and the probabilistic perspective which accounts for the stochastic nature of mining-induced microseismicity. This model has been employed to make time-varying forecasts of hazardous microseismicity around two longwall panels over a one-year coal production period at Coal Mine Velenje, and satisfactory results at both panels were achieved. In addition, the analysis suggested that the energy magnitude distribution of microseismicity is a dominant factor in contributing to the potential of hazardous microseismicity. This statistical model using microseismic monitoring data has important implications in the evaluation of mining-induced hazards and optimal control of longwall face advance in burst-prone deep-level mining sites.