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.     

Sensing for advancing mining automation capability: A review of underground automation technology development

This paper highlights the role of automation technologies for improving the safety, productivity, and environmental sustainability of underground coal mining processes. This is accomplished by reviewing the impact that the introduction of automation technology has made through the longwall shearer automation research program of Longwall Automation Steering Committee (LASC). This result has been achieved through close integration of sensing, processing, and control technologies into the longwall mining process. Key to the success of the automation solution has been the development of new sensing methods to accurately measure the location of longwall equipment and the spatial configuration of coal seam geology. The relevance of system interoperability and open communications standards for facilitating effective automation is also discussed. Importantly, the insights gained through the longwall automation development process are now leading to new technology transfer activity to benefit other underground mining processes.     

Modern experience of low-coal seams underground mining in Ukraine

Coal is the main energy resource in Ukraine. However geotechnological aspects of coal seams development and Ukrainian crisis have a negative influence on the mining industry. This article analyzes the experience in the development of very low and low-coal seams with 0.7–1.0 m thickness, as well as advanced technological solutions that allowed private coal enterprises, despite the difficult situation in the country, to maintain sufficient(more than 75% of all production) level of steam coal extraction for Ukrainian society. Given that Ukrainian's mining sector development is a huge task, we hope this review will add some discussions into the ongoing conversation.     

Assessing risks from mining-induced ground movements near gas wells

The proliferation of unconventional gas well development in the Northern Appalachian coalfields has raised a number of mine safety concerns. Unconventional wells, which extract gas from deep shale formations, are characterized by gas volumes and pressures that are significantly higher than those observed at many conventional wells. The gas is composed largely of methane as well as other hydrocarbons. Hundreds of planned and actively producing wells penetrate protective coal pillars or barriers within active mine boundaries, including chain pillars located between longwall panels. Gas released from a well damaged by mining-induced ground movements could pose a risk to miners by flowing into the mine atmosphere. The mining-induced ground movements that may cause well damage include conventional subsidence, non-conventional subsidence(e.g. bedding plane slip), pillar failure, and floor instability. This paper describes the known risk factors for each of the four failure mechanisms. It includes a framework that can guide the risk assessment process when mining takes place near gas or oil wells.     

Influence of longwall mining on the stability of gas wells in chain pillars

Longwall mining has a significant influence on gas wells located within longwall chain pillars. Subsurface subsidence and abutment pressure induced by longwall mining can cause excessive stresses and deformations in gas well casings. If the gas well casings are compromised or ruptured, natural gas could migrate into the mine workings, potentially causing a fire or explosion. By the current safety regulations,the gas wells in the chain pillars have to be either plugged or protected by adequate coal pillars. The current regulations for gas well pillar design are based on the 1957 Pennsylvania gas well pillar study. The study provided guidelines for gas well pillars by considering their support area and overburden depth as well as the location of the gas wells within the pillars. As the guidelines were developed for room-andpillar mining under shallow cover, they are no longer applicable to modern longwall coal mining, particularly, under deep cover. Gas well casing of failures have occurred even though the chain pillars for the gas wells met the requirements by the 1957 study. This study, conducted by the National Institute for Occupational Safety and Health(NIOSH), presents seven cases of conventional gas wells penetrating through longwall chain pillars in the Pittsburgh Coal Seam. The study results indicate that overburden depth and pillar size are not the only determining factors for gas well stability. The other important factors include subsurface ground movement, overburden geology, weak floor, as well as the type of the construction of gas wells. Numerical modeling was used to model abutment pressure, subsurface deformations, and the response of gas well casings. The study demonstrated that numerical models are able to predict with reasonable accuracy the subsurface deformations in the overburden above,within, and below the chain pillars, and the potential location and modes of gas well failures, thereby providing a more quantifiable approach to assess the stability of the gas wells in longwall chain pillars.     

Effect of longwall-induced subsurface deformations on shale gas well casing stability under deep covers

This paper presents the results of a 2017 study conducted by the National Institute for Occupational Safety and Health(NIOSH), Pittsburgh Mining Research Division(PMRD), to evaluate the effects of longwall-induced subsurface deformations within a longwall abutment pillar under deep cover. The 2017 study was conducted in a southwestern Pennsylvania coal mine, which extracts 457 m-wide longwall panels under 361 m of cover. One 198 m-deep, in-place inclinometer monitoring well was drilled and installed over a 45 m by 84 m center abutment pillar. In addition to the monitoring well, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 457 m-wide longwall panel on the south side of the abutment pillar was being mined. Prior to the first longwall excavation, a number of simulations using FLAC3D~(TM) were conducted to estimate surface subsidence, increases in underground coal pillar pressure, and subsurface horizontal displacements in the monitoring well. Comparisons of the pre-mining FLAC3D simulation results and the surface, subsurface,and underground instrumentation results show that the measured in-place inclinometer casing deformations are in reasonable agreement with those predicted by the 3D finite difference models. The measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models.Results from this 2017 research clearly indicate that, under deep cover, the measured horizontal displacements within the abutment pillar are approximately one order of magnitude smaller than those measured in a 2014 study under medium cover.     

Strata movement and stress evolution when mining two overlapping panels affected by hard stratum

Mining causes stress redistribution and stratum movement. In this paper, a numerical model was built according to the geological conditions in the 12 th coal mine in Pingdingshan city to study the strata movement and the evolution of stress when mining two overlapping longwall panels, named panels#14 and #15. The strata close to the mined panel move directly towards the gob, while the strata that are farther away swing back and forth during the mining process. The directed movement and swinging can break the transverse boreholes for gas extraction; a surface borehole should not be within the range of directional movement. The stress evolution suggested that the mining of the lower panel #15 after the upper panel #14 would further increase the de-stressed range, while the stress concentration around the mined panel would be increased. Hard strata usually carry a greater stress than adjacent rocks and soft coal seams. The stress in a hard stratum increases greatly, and the stress decreases greatly in the coal seams below the hard stratum. This study supplies a reference for similar coal mines and is useful for determining the de-stressed range and transverse borehole arrangement for gas extraction.     

Strategic mining options optimization:Open pit mining,underground mining or both

Near-surface deposits that extend to considerable depths are often amenable to both open pit mining and/or underground mining. This paper investigates the strategy of mining options for an orebody using a Mixed Integer Linear Programming(MILP) optimization framework. The MILP formulation maximizes the Net Present Value(NPV) of the reserve when extracted with(i) open pit mining,(ii) underground mining, and(iii) concurrent open pit and underground mining. Comparatively, implementing open pit mining generates a higher NPV than underground mining. However considering the investment required for these mining options, underground mining generates a better return on investment than open pit mining. Also, in the concurrent open pit and underground mining scenario, the optimizer prefers extracting blocks using open pit mining. Although the underground mine could access ore sooner, the mining cost differential for open pit mining is more than compensated for by the discounting benefits associated with earlier underground mining.     

Prevention of gob ignitions and explosions in longwall mining using dynamic seals

Most, if not all longwall gob areas accumulate explosive methane-air mixtures that pose a deadly hazard to miners. Numerous mine explosions have originated from explosive gas zones(EGZs) in the longwall gob. Since 2010, researchers at the Colorado School of Mines(CSM) have studied EGZ formation in longwall gobs under two long-term research projects funded by the National Institute for Occupational Safety and Health. Researchers used computational fluid dynamics along with in-mine measurements. For the first time, they demonstrated that EGZs form along the fringe areas between the methane-rich atmospheres and the fresh air ventilated areas along the working face and present an explosion and fire hazard to mine workers. In this study, researchers found that, for progressively sealed gobs, a targeted injection of nitrogen from the headgate and tailgate, along with a back return ventilation arrangement, will create a dynamic seal of nitrogen that effectively separates the methane zone from the face air and eliminates the EGZs to prevent explosions. Using this form of nitrogen injection to create dynamic seals should be a consideration for all longwall operators.     

Partitioning characteristics of gas channel of coal-rock mass in mining space and gas orientation method

In order to research the influence of coal-rock mass morphology of mining space on the flow law of gas, the laboratory physical model and numerical computation methods were adopted to simulate coal mining activities. The simulation results indicate that, after coal seam mining, the loose rock accumulation body of free caving, ordered rock arrangement body of plate damage rich in longitudinal and transverse fractures and horizontal fissure body formed by rock mass deformation imbalance are formed from bottom to top in the mining space. For these three types of accumulation bodies, there are essential differences in the accumulation state, rock size and gas breakover characteristics. According to this, the coal-rock mass in the mining space is classified into gas turbulence channel area, gas transitional flow channel area and gas seepage channel area. In the turbulence channel area, the gas is distributed transversely and longitudinally and gas diffuses in the form of convection with Reynolds number R_e more than 100; in the transitional flow channel area, one-way or two-way gas channels are crisscross and gas is of transitional flow regime with R_e between 10 and 100. In the seepage channel area, there are a few vertical gas channels with R_e less than 10. In this paper, the researches on the gas orientation method in different partitions were further carried out, gas orientation methods of low-level pipe burying, middle-level interception and high-level extraction were determined and an on-site industrial test was conducted, achieving the effective diversion of gas and verifying the reasonableness of gas channel partition.     

The current perspective of the PA 1957 gas well pillar study and its implications for longwall gas well pillars

Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells. The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span. At the time, coal was mined using the room-and-pillar mining method with full or partial pillar recovery, and square or rectangle pillars surrounding the gas wells were left to protect the wells. The study provided guidelines for pillar sizes under different overburden depths up to213 m(700 ft). The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s. The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover. However, under deep cover, severe deformations in gas wells have occurred in longwall chain pillars. Presently, with a better understanding of coal pillar mechanics, new insight into subsidence movements induced by retreat mining, and advances in numerical modeling, it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars. In this paper, the data from the 1957 study is analyzed from a new perspective by considering various factors, including overburden depth, failure location, failure time, pillar safety factor(SF), and floor pressure. The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery. A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining. Through analyzing the data from the 1957 study, the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining. Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings. Results of this study indicate that the guidelines in the 1957 study may be appropriate for pillars protecting conventional gas wells in both room-and-pillar mining and longwall mining under overburden depths up to 213 m(700 ft),but may not be sufficient for protective pillars under deep cover. The current evaluation of the 1957 study provides not only insights about potential gas well failures caused by retreat mining but also implications for what critical considerations should be taken into account to protect gas wells in longwall mining.     

Creep behaviour and constitutive model of coal filled with gas

Coal exhibits different creep behaviours when filled with different amounts of gas. Creep tests of coal filled with 0 and 0.5 MPa gas were performed, and strain under different axial stress was compared.The three creep constitutive models which were analysed using the method fitting experimental data for determining which creep model can reflect the creep process of the test best. The results show that the deformation of coal filled with 0.5 MPa gas is more higher than that of coal filled with 0 MPa gas under the same axial stress. Gas plays a positive effect on the deformation of coal process and will accelerate creep process. And gas will reduce coal intensity and change coal creep properties.Compared with Nishihara Model and Extensional Nishihara Model, Burgers Model can reflect the three stages of creep process of coal filled with gas better. The research results can contribute to reveal coal and gas outburst mechanism.     

Highlights from the literature on risk assessment techniques adopted in the mining industry:A review of past contributions,recent developments and future scope

In this paper a comprehensive review is presented of risk assessment techniques adopted in the mining industry worldwide;those techniques applied in other hazardous industries and potential techniques which are robust,mature and holistic and can be implemented for the Indian mining industry in future to enhance workplace safety are also presented.Findings from the review are indicative of the fact that socio-technical complexity of industrial systems has increased.Recent developments in the area of risk management highlight the need for implementation of the latest robust techniques of risk assessment in the mining sector.In consideration of the present scenario,the development of a model for risk analysis having an interface between hazard identification and risk assessment,along with an interface between risk assessment and accident causation to predict if an accident will occur under given conditions,has become dire necessity.This will increase hazard awareness and enable mine management to select and prioritize problem areas and identify safety system weaknesses in both underground and surface mining.This will ultimately help decision makers,risk analysts and safety managers make a major contribution in the development of workplace safety with a near-to-zero accident rate.     

The future of underground spatial planning and the resulting potential risks from the point of view of mining subsidence engineering

The topic of ground movements in Germany has been studied extensively in the past, especially in the field of active mines. The active hard coal mines in Germany were finally shut down in 2018 and lignite mining is expected to take place only until 2038. The so-called long-term liabilities of the mine operators in Germany include, among other things, the long-term guarantee of stability and thus the monitoring of ground motion. So far, the economic use of underground mining in Germany was mainly the supply of raw materials. In the future, the underground storage of compressed air, methane or hydrogen will play an important role in renewable energy supply and climate change. Therefore, the underground storage space will become more important and the spatial planning is essential to ensure availability of safe underground openings for the various options of environmentally friendly energy storage. However, this renewed usage of underground openings may also bring new and sometimes unknown challenges of geomechanical influence. The aftermath of hard coal and lignite mining will be an increasing challenge in mining subsidence engineering. On the other hand, new possibilities due to underground spatial planning may lead to subsidence and/or heaving of the upper surface.     

Effects of longwall-induced stress and deformation on the stability and mechanical integrity of shale gas wells drilled through a longwall abutment pillar

This paper presents the results of a comprehensive study conducted by CONSOL Energy, Marcellus Shale Coalition, and Pennsylvania Coal Association to evaluate the effects of longwall-induced subsurface deformations on the mechanical integrity of shale gas wells drilled over a longwall abutment pillar.The primary objective is to demonstrate that a properly constructed gas well in a standard longwall abutment pillar can maintain mechanical integrity during and after mining operations. A study site was selected over a southwestern Pennsylvania coal mine, which extracts 457-m-wide longwall faces under about 183 m of cover. Four test wells and four monitoring wells were drilled and installed over a 38-m by84-m centers abutment pillar. In addition to the test wells and monitoring wells, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 457-m-wide longwall panels on the south and north sides of the abutment pillar were mined by. To evaluate the resulting coal protection casing profile and lateral displacement, three separate 60-arm caliper surveys were conducted. This research represents a very important step and initiative to utilize the knowledge and science obtained from mining research to improve miner and public safety as well as the safety and health of the oil and gas industries.     

Effects of coal damage on permeability and gas drainage performance

Coal permeability is a measure of the ability for fluids to flow through coal structures. It is one of the most important parameters affecting the gas drainage performance in underground coal mines. Despite the extensive research conducted on coal permeability, few studies have considered the effect of coal damage on permeability. This has resulted in unreliable permeability evaluation and prediction. The aim of this study is to investigate the effect of coal damage on permeability and gas drainage performance. The Cui-Bustin permeability model was improved by taking into account the impact of coal damage on permeability. The key damage coefficient of the improved permeability model is determined based on the published permeability data. A finite-element numerical simulation was then developed based on the improved permeability model to investigate the damage areas and the permeability distribution around roadway. Results showed that the tensile failure occurs mainly on the upper and lower sides of the roadway while the shear failure symmetrically occurs on the left and right sides. With the increase in the friction angle value, the damage area becomes small. A good agreement was obtained between the results of the improved permeability model(c = 3) and the published permeability data. This indicated a more accurate permeability prediction by the improved permeability model. It is expected that the findings of this study could provide guidance for in-seam gas drainage borehole design and sealing, in order to enhance the gas drainage performance and reduce gas emissions into underground roadways.     

Characteristics of mining gas channel expansion in the remote overlying strata and its control of gas flow

The technology of pressure relief gas drainage is one of the most effective and economic for preventing gas emissions in underground mines. Based on current understanding of strata breakage and fracture development in overlying strata, the current study divides the overlying strata into the following three longitudinal zones in terms of the state of gas flow: a turbulent channel zone, a transitional circulation channel zone and a seepage channel zone. According to the key strata discrimination theory of controlling the overlying strata, the calculation method establishes that the step-type expansion of the mining gas channel corresponds to the advancing distance of working face, and this research also confirms the expanding rule that the mining gas channel in overlying strata follows the advancing distance of mining working face. Based on the geological conditions of Xinjing Coal Mine of Yangquan, this paper researches the expanding rule of mining gas channel as well as the control action of the channel acting on the pressure relief flow under the condition of the remote protective layer, and got the distance using inversion that the step-type expanding of mining gas channel is corresponding to the advancing distance of working face, which verifies the accuracy and feasibility of theoretical calculation method proposed in this study. The research provides the theoretical basis for choosing the technology of pressure relief gas drainage and designing the parameters of construction.     

Advances in gas content based on outburst control technology in Huainan,China

The sudden and violent nature of coal and gas outbursts continues to pose a serious threat to coal mine safety in China. One of the key issues is to predict the occurrence of outbursts. Current methods that are used for predicting the outbursts in China are considered to be inadequate, inappropriate or impractical in some seam conditions. In recent years, Huainan Mining Industry Group (Huainan) in China and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia have been jointly developing technology based on gas content in coal seams to predict the occurrence of outbursts in Huainan. Significant progresses in the technology development have been made, including the development of a more rapid and accurate system in determining gas content in coal seams, the invention of a sampling-while-drilling unit for fast and pointed coal sampling, and the coupling of DEM and LBM codes for advanced numerical simulation of outburst initiation and propagation. These advances are described in this paper.     

Response characteristics of gas pressure under simultaneous static and dynamic load: Implication for coal and gas outburst mechanism

Coal and gas outbursts are dynamic disasters in which a large mass of gas and coal suddenly emerges in a mining space within a split second. The interaction between the gas pressure and stress environment is one of the key factors that induce coal and gas outbursts. In this study, first, the coupling relationship between the gas pressure in the coal body ahead of the working face and the dynamic load was investigated using experimental observations, numerical simulations, and mine-site investiga...     

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation,gas diffusion and seepage,gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining.The research results indicate:(1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase,thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient,which will cause a decrease in gas pressure.(2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam,which can effectively reduce the coal and gas outburst risk.The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.