Field and numerical modelling on the stability of underground strata in longwall workings

The longwall method is one of the feasible and efficient underground mining methods for greater depth of workings.Recently,Singareni Collieries Company Limited(SCCL),the largest coal-producing government company in the Southern part of India,has deployed a high capacity(2×1152T capacity)power support system in its Adriyala Longwall Project(ALP)at a depth of 375 m.There was a concern about the stability of the longwall workings and the protective pillars.Thus,research work was carried out with the geotechnical instruments and numerical modelling tools to analyze the stability while retreating the longwall panels.The outcomes revealed that the convergence in the gate roads increased with the longwall face advancement and the area of exposure.The pressure of the powered support legs on the dip side was less than the rise side legs,which implies a stable roof condition over the longwall face.An abutment zone was identified ahead of the line of extraction up to 10e25 m and presumed from the Factor of Safety(FoS)criterion that within this zone,the longwall pillar possibly will be of better stability at every stage of extraction.The results obtained from this study would be helpful for the mining engineers to understand the behaviour of underground strata in longwall workings.

Longwall automation:trends,challenges and opportunities

This paper explores the ongoing development and implementation of longwall automation technology to achieve greater levels of underground coal mining performance. The primary driver behind the research and development effort is to increase the safety, productivity and efficiency of longwall mining operations to enhance the underlying mining business. A brief review of major longwall automation challenges is given followed by a review of the insights and benefits associated with the LASC longwall shearer automation solution. Areas of technical challenge in sensing, decision support, autonomy and human interaction are then highlighted, with specific attention given to remote operating centres, proximity detection and systems-level architectures in order to motivate further automation system development.The vision for a fully integrated coal mining ecosystem is discussed with the goal of delivering a highperformance, zero-exposure and environmentally coherent mining operations.

Innovative backfilling longwall panel layout for better subsidence control effect-separating adjacent subcritical panels with pillars

In recent years,field trials of non-pillar longwall mining using complete backfill have been implemented successively in the Chinese coal mining industry.The objective of this paper is to get a scientific understanding of surface subsidence control effect using such techniques.It begins with a brief overview on complete backfill methods primarily used in China,followed by an analysis of collected subsidence factors under mining with complete backfill.It is concluded that non-pillar longwall panel layout cannot protect surface structures against damages at a relatively large mining height,even though complete backfill is conducted.In such cases,separated longwall panel layout should be applied,i.e.,panel width should be subcritical and stable coal pillars should be left between the adjacent panels.The proposed method takes the principles of subcritical extraction and partial extraction;in conjunction with gob backfilling,surface subsidence can be effectively mitigated,thus protecting surface buildings against mining-induced damage.A general design principle and method of separated panel layout have also been proposed.

Systematic principles of surrounding rock control in longwall mining within thick coal seams

Effective surrounding rock control is a prerequisite for realizing safe mining in underground coal mines.In the past three decades, longwall top-coal caving mining(LTCC) and single pass large height longwall mining(SPLL) found expanded usage in extracting thick coal seams in China. The two mining methods lead to large void space left behind the working face, which increases the difficulty in ground control.Longwall face failure is a common problem in both LTCC and SPLL mining. Such failure is conventionally attributed to low strength and high fracture intensity of the coal seam. However, the stiffness of main components included in the surrounding rock system also greatly influences longwall face stability.Correspondingly, surrounding rock system is developed for LTCC and SPLL faces in this paper. The conditions for simultaneous balance of roof structure and longwall face are put forward by taking the stiffness of coal seam, roof strata and hydraulic support into account. The safety factor of the longwall face is defined as the ratio between the ultimate bearing capacity and actual load imposed on the coal wall.The influences provided by coal strength, coal stiffness, roof stiffness, and hydraulic support stiffness,as well as the movement of roof structure are analyzed. Finally, the key elements dominating longwall face stability are identified for improving surrounding rock control effectiveness in LTCC and SPLL faces.

Heavy rockbursts due to longwall mining near protective pillars:A case study

Rockburst represents a very dangerous phenomenon in deep underground mining in unfavourable conditions such as great depth, high horizontal stress, proximity of important tectonic structures, and unmined pillars. The case study describes a recorded heavy rockburst in the Czech part of the Upper Silesian Coal Basin, which occurred during longwall mining near the protective pillar. The artificial dividing of geological blocks and creation of mining protective pillars(shaft pillars, crosscut pillars etc.) is a dangerous task in light of rockbursts occurring mainly due to overstressing of remaining pillars. A simple model of this situation is presented. Natural and mining conditions are analysed and presented in detail as well as registered seismicity during longwall mining in the area. Recorded rockbursts in the area of interest are described and their causes discussed. Many rockbursts near protective pillars were recorded in this mining region. Methodical instructions for rockburst prevention in proximity of protective pillars as well as for gates driving were devised based on the evaluation of rockburst causes. The paper presents these principles for prevention.

Fracturing,caving propagation and influence of mining on groundwater above longwall panels–a review of predictive models

Historically there have been a number of different hypotheses and empirical models developed in an attempt to describe the nature of fracturing above longwall panels in underground coal mining.The motivation for suchmodels varies,ranging fromunderstanding the impact ofmining on surface subsidence,to back-analysis of caving behaviour in the immediate roof behind the longwall face.One of the most critical motivating factors that is taking on increased importance in many coalfields,is the need for better understanding,and hence prediction of the impact of mining on overlying strata,particularly strata units acting as aquifers for different groundwater horizons.This paper reviews some of the major prediction models in the context of observed behaviour of strata displacement and fracturing above longwall panels in the southern coalfields of New South Wales,south of Sydney.The paper discusses the parameter often referred to as"height of fracturing"in terms of the critical parameters that influence it,and the relevance and appropriateness of this terminology in the context of overlying sub-surface subsidence and groundwater impact.The paper proposes an alternative terminology for this parameter that better reflects what it is and how it is used.The paper also addresses the potential role of major bedding shear planes mobilised by mining and their potential influence on overlying subsidence and groundwater interference.

Developing selective mining capability for longwall shearers using thermal infrared-based seam tracking

Longwall mining continues to remain the most efficient method for underground coal recovery. A key aspect in achieving safe and productive longwall mining is to ensure that the shearer is always correctly positioned within the coal seam. At present, this machine positioning task is the role of longwall personnel who must simultaneously monitor the longwall coal face and the shearer's cutting drum position to infer the geological trends of the coal seam. This is a labour intensive task which has negative impacts on the consistency and quality of coal production. As a solution to this problem, this paper presents a sensing method to automatically track geological coal seam features on the longwall face, known as marker bands, using thermal infrared imaging. These non-visible marker bands are geological features that link strongly to the horizontal trends present in layered coal seams. Tracking these line-like features allows the generation of a vertical datum that can be used to maintain the shearer in a position for optimal coal extraction. Details on the theory of thermal infrared imaging are given, as well as practical aspects associated with machine-based implementation underground. The feature detection and tracking tasks are given with real measurements to demonstrate the efficacy of the approach. The outcome is important as it represents a new selective mining capability to help address a long-standing limitation in longwall mining operations.

Automation in U.S. longwall coal mining: A state-of-the-art review

This paper reviews the development of U.S. longwall mining from an unknown to became the world standard in the past five decades with emphasis on automation. Large scale longwall face equipment were imported from Germany and United Kingdom to increase production in the 1970 s and great effort was made to improve them to suit U.S. conditions, rather than domestic market. Automation began with the development of electrohydraulic shields in 1984 and continue to present. Introduction of first generation semi-automated longwall system occurred in 1995 and step-to-step improvement continues to present following the development of sensor technology and internet of things(IOT). Since then, emphasis on new development has been concentrated on the improvement of equipment reliability, miner's health and safety as well as production, including dust control techniques, proximity sensor, anti-collision and remote control. Automation is classified into two categories: automation of individual face equipment and automation of longwall system. The automation development of longwall system is divided into three stages: shearer-initiated-shield-advance(SISA), semi-automated longwall system, and remote control shearer.

Longwall surface subsidence control by technology of isolated overburden grout injection

Surface subsidence is a typical ground movement due to longwall mining, which causes a series of environmental problems and hazards. In China, intensive coal extractions are commonly operated under dense-populated coalfields, which exacerbates the negative subsequences resulted from surface settlement. Therefore, effective approaches to control the ground subsidence are in urgent need for the Chinese coal mining industry. This paper presents a newly developed subsidence control technology: isolated overburden grout injection, including the theory, technique and applications. Relevant procedures such as injection system design, grouting material selection, borehole layout, grout take estimation and injection process design are proposed. The applicability of this technology has been demonstrated through physical modelling, field measurements, and case studies. Since 2009, the technology has been successfully applied to 14 longwall areas in 9 Chinese coal mines. The ultimate surface subsidence factors vary from 0.10 to 0.15. This method has a great potential to be popularized and performed where longwall mining are implemented under villages and ground infrastructures.

Simulation of recovery of upper remnant coal pillar while mining the ultra-close lower panel using longwall top coal caving

With the depletion of easily minable coal seams,less favorable reserves under adverse conditions have to be mined out to meet the market demand.Due to some historical reasons,large amount of remnant coal was left unrecovered.One such case history occurred with the remnant rectangular stripe coal pillars using partial extraction method at Guandi Mine,Shanxi Province,China.The challenge that the coal mine was facing was that there is an ultra-close coal seam right under it with an only 0.8–1.5 m sandstone dirt band in between.The simulation study was carried out to investigate the simultaneous recovery of upper remnant coal pillars while mining the ultra-close lower panel using longwall top coal caving(LTCC).The remnant coal pillar was induced to cave in as top coal in LTCC system.Physical modelling shows that the coal pillars are the abutments of the stress arch structure formed within the overburden strata.The stability of overhanging roof strata highly depends on the stability of the remnant coal pillars.And the gob development(roof strata cave-in)is intermittent with the cave-in of these coal pillars and the sandstone dirt band.FLAC3D numerical modelling shows that the multi-seam interaction has a significant influence on mining-induced stress environment for mining of lower panels.The pattern of the stress evolution on the coal pillars with the advance of the lower working face was found.It is demonstrated that the stress relief of a remnant coal pillar enhances the caveability of the pillars and sandstone dirt band below.

Numerical simulation to determine the gas explosion risk in longwall goaf areas:A case study of Xutuan Colliery

Underground gassy longwall mining goafs may suffer potential gas explosions during the mining process because of the irregularity of gas emissions in the goaf and poor ventilation of the working face,which are risks difficult to control.In this work,the 3235 working face of the Xutuan Colliery in Suzhou City,China,was researched as a case study.The effects of air quantity and gas emission on the three-dimensional distribution of oxygen and methane concentration in the longwall goaf were studied.Based on the revised Coward’s triangle and linear coupling region formula,the coupled methane-oxygen explosive hazard zones(CEHZs)were drawn.Furthermore,a simple practical index was proposed to quantitatively determine the gas explosion risk in the longwall goaf.The results showed that the CEHZs mainly focus on the intake side where the risk of gas explosion is greatest.The CEHZ is reduced with increasing air quantity.Moreover,the higher the gas emission,the larger the CEHZ,which moves towards the intake side at low goaf heights and shifts to the deeper parts of the goaf at high heights.In addition,the risk of gas explosion is reduced as air quantities increase,but when gas emissions increase to a higher level(greater than 50 m3/min),the volume of the CEHZ does not decrease with the increase of air quantity,and the risk of gas explosion no longer shows a linear downward trend.This study is of significance as it seeks to reduce gas explosion accidents and improve mine production safety.

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.

Ground response to high horizontal stresses during longwall retreat and its implications for longwall headgate support

Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site.The roof behavior in the headgate near the face was carefully observed during longwall retreat.Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.

Characteristics of evolution of mining-induced stress field in the longwall panel:insights from physical modeling

The evolution of mining-induced stress field in longwall panel is closely related to the fracture field and the breaking characteristics of strata.Few laboratory experiments have been conducted to investigate the stress field.This study investigated its evolution by constructing a large-scale physical model according to the in situ conditions of the longwall panel.Theoretical analysis was used to reveal the mechanism of stress distribution in the overburden.The modelling results showed that:(1)The major principal stress field is arch-shaped,and the strata overlying both the solid zones and gob constitute a series of coordinated load-bearing structures.The stress increasing zone is like a macro stress arch.High stress is especially concentrated on both shoulders of the arch-shaped structure.The stress concentration of the solid zone in front of the gob is higher than the rear solid zone.(2)The characteristics of the vertical stress field in different regions are significantly different.Stress decreases in the zone above the gob and increases in solid zones on both sides of it.The mechanical analysis show that for a given stratum,the trajectories of principal stress are arch-shaped or inverselyarched,referred to as the‘‘principal stress arch’’,irrespective of its initial breaking or periodic breaking,and determines the fracture morphology.That is,the trajectories of tensile principal stress are inversely arched before the first breaking of the strata,and cause the breaking lines to resemble an inverted funnel.In case of periodic breaking,the breaking line forms an obtuse angle with the advancing direction of the panel.Good agreement was obtained between the results of physical modeling and the theoretical analysis.

Design of a water curtain to reduce accumulations of float coal dust in longwall returns

Accumulation of float coal dust(FCD)in underground mines is an explosion hazard that affects all underground coal mine workers.While this hazard is addressed by the application of rock dust,inadequate rock dusting practices can leave miners exposed to an explosion risk.Researchers at the National Institute for Occupational Safety and Health(NIOSH)have focused on developing a water curtain that removes FCD from the airstream,thereby reducing the buildup of FCD in mine airways.In this study,the number and spacing of the active sprays in the water curtain were varied to determine the optimal configuration to obtain peak knockdown efficiency(KE)while minimizing water consumption.

Monitoring strata behavior due to multi-slicing top coal caving longwall mining in steeply dipping extra thick coal seam

For studying the strata behavior due to multi-slicing top coal caving longwall mining along-the-strike direction in steeply dipping extra thick coal seams,the shield support pressures of the upper and lower slices of panel 37220 in Dongxia Coal Mine were monitored using the KJ513 dynamic monitoring system.The set up rooms adopted the "horizontal line-arc segment-inclined line" form and used different types of shield supports.The results show that the strata pressure of upper slice panel 37220-1 changed slightly along the strike direction,while along the dip direction it exhibited strong to weak pressure from bottom to top.The first weighting interval of lower slice panel 37220-2 was about 60.8 m,and the average periodic weighting interval were about 22.6 m.The strata behavior of panel 37220-2 exhibited a spatiotemporal characteristic in that periodic weighting occurred first in the middle-upper part,followed by the middle and upper parts,arc segment,and finally the lower part.During the periodic weighting,the weighting interval and intensity also exhibited strong space characteristics.The average dynamic load coefficient was 1.48 and the maximum lateral load of the side shield was 20-25 MPa.

Theoretical description of drawing body shape in an inclined seam with longwall top coal caving mining

Understanding the characteristics of drawing body shape is essential for optimization of drawing parameters in longwall top coal caving mining.In this study,both physical experiments and theoretical analysis are employed to investigate these characteristics and derive a theoretical equation for the drawing body shape along the working face in an inclined seam.By analyzing the initial positions of drawn marked particles,the characteristics of the drawing body shape for different seam dip angles are obtained.It is shown that the drawing body of the top coal exhibits a shape-difference and volume-symmetry characteristic,on taking a vertical line through the center of support opening as the axis of symmetry,the shapes of the drawing body on the two sides of this axis are clearly different,but their volumes are equal.By establishing theoretical models of the drawing body in the initial drawing stage and the normal drawing stage,a theoretical equation for the drawing body in an inclined seam is proposed,which can accurately describe the characteristics of the drawing body shape.The shape characteristics and volume symmetry of the drawing body are further analyzed by comparing the results of theoretical calculations and numerical simulations.It is shown that one side of the drawing body is divided into two parts by an inflection point,with the lower part being a variation development area.This variation development area increases gradually with increasing seam dip angle,resulting in an asymmetry of the drawing body shape.However,the volume symmetry coefficient fluctuates around 1 for all values of the seam dip angle variation,and the volumes of the drawing body on the two sides are more or less equal as the variation development volume is more or less equal to the cut volume.Both theoretical calculations and numerical simulations confirm that the drawing body of the top coal exhibits the shape-difference and volume-symmetry characteristic.

Numerical investigation on the assessment and mitigation of coal bump in an island longwall panel

This study presents a numerical investigation to assess the risk of coal bumps and produces a stress–relief technology using boreholes to mitigate risk during the extraction of an island longwall panel.Based on the geological condition in an island longwall panel in the Tangshan Coal Mine,Tangshan,China,a numerical FLAC3D(Fast Lagrangian Analysis of Continua in 3 Dimensions) model was established to determine and to map the zones in the panel with a high risk for coal bumps.The results of the numerical modeling show that the roof deformation starts to occur at more than 30 m ahead of the longwall face and the deformation starts to accelerate after a distance of 10 m in front of the longwall face.Large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel.Based on the numerical results,a stress–relief technology using boreholes,which was employed to release abutment pressure,was investigated through numerical methods.The modeled results suggest that the peak stress concentration could be released by drilling boreholes in the zones prone to coal bumps.The effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing.

Quantifying relationships between subsidence and longwall face advance using DInSAR

Surface subsidence that results from longwall mining can be large magnitude and can affect significant areas.Conventional methods for subsidence monitoring include leveling,global positioning system(GPS),and photogrammetric surveys.Remote sensing techniques including,aerial LiDAR,terrestrial laser scanning,and satellite-based Differential Interferometric Synthetic Aperture Radar(DInSAR),are also used to measure deformation associated with subsidence.DInSAR data are different than data from conventional subsidence surveys.Images capture data over large areas(hundreds of kilometers),and each pixel(data point)in an image quantifies the average displacement over an area of square meters.DInSAR data can have fairly high time resolution;imaging periods typically range from weeks to months.DInSAR data can be useful to monitor subsidence sequentially over short periods.Regularly monitoring subsidence may help define if caving is progressing normally and can establish relationships between surface deformation and longwall face advance,which has potential to help quantify possible risks to mine stability.In this study,subsidence at a longwall trona mine is monitored over short periods,typically 12 days,as the longwall face is advanced through a panel.C-band interferometric wide swath synthetic aperture radar(SAR)images from the sentinel satellites are used to quantify the subsidence.The onset of subsidence occurs close in time to the beginning of the longwall face advance,and overall,the development of subsidence closely follows the longwall face advance.

Analysis of gateroad stability at two longwall mines based on field monitoring results and numerical model analysis

Coal mine longwall gateroads are subject to changing loading conditions induced by the advancing longwall face. The ground response and support requirements are closely related to the magnitude and orientation of the stress changes, as well as the local geology. This paper presents the monitoring results of gateroad response and support performance at two longwall mines at a 180-m and 600-m depth of cover.At the first mine, a three-entry gateroad layout was used. The second mine used a four-entry, yieldabutment-yield gateroad pillar system. Local ground deformation and support response were monitored at both sites. The monitoring period started during the development stage and continued during first panel retreat and up to second panel retreat. The two data sets were used to compare the response of the entries in two very different geotechnical settings and different gateroad layouts. The monitoring results were used to validate numerical models that simulate the loading conditions and entry response for these widely differing conditions. The validated models were used to compare the load path and ground response at the two mines. This paper demonstrates the potential for numerical models to assist mine engineers in optimizing longwall layouts and gateroad support systems.