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By H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By Yongping Wu

Roadway deformation and stress migration occur with uncertainty and randomness because of geology, tunneling and mining influence. In order to study the surrounding rock deformation and failure mechanisms in three-dimensional stress state, an air return roadway of Yuanyang Lake mine of Shenhua Ningxia Coal Industry Group Co., Ltd. was chosen as a research object. The three-dimensional model was a designed and completed experiment with a dynamic-static coupling load. The model was loaded by hydraulic cylinder, simulating the in-situ rock stress field and the induced stress field of surrounding rock. The deformation monitoring system of the roadway model was constituted by optical total stations, borehole observation instruments, acoustic emission monitoring, and stress sensors. A wealth of data was obtained through an omni observation of surface displacement, characteristics of stress distribution, and deformation and failure of surrounding model material. By acoustic-optic-electric methods, a real-time testing on physical, mechanical, and damage parameters was realized through the experimental process. The results showed that the data of deformation and failure obtained from the deformation monitoring system of the roadway were authentic, which has great significance for warning and forecasting safety hazards of surrounding rock in coal mines.

Jan 1, 2013

By Aarao de Andrade Lima

Mechanical properties typical of coal are used in the comparisons of pillar strengths based on finite elements and the empirical equations of Holland-Gaddy, Salamon-Munro, Obert¬Duvall, and Bieniawski-PSU. A correlation among the strength parameters which are employed in the empirical equations, in Mohr-Coulomb and Hoek-Brown failure criteria is established. Emphasis is given to the important distinction between the mechanical properties of small rock specimens and those related to the rock mass. An elastoplastic version of Hoek-Brown criteria has been developed and implemented in a finite elements code. The strengths calculated using the empirical equations showed a good agreement when the ratio width by height of the pillars varied from I to 8, except for Holland-Gaddy equation which exhibited lower values. Under the hypothesis of associative perfect elastoplasticity used, pillar strength calculated using Mohr¬Coulomb yield criterion became excessively high for all the models having a pillar width by height ratio above 2. The finite elements solution based on Hoek-Brown criterion predicted strengths around fifty percent larger than Bieniawski-PSU equation when the pillars width by height ration varied from 3 to 8, showing good agreement below that range. It has been concluded that, under the hypothesis of percfect plasticity, stress analysis techniques may overestimate the strength of rock structures subjected to high confining stresses.

Jan 1, 1997

By Kot Unrug

Fully grouted tensioned bolts provide a desirable compressive of laminated roof and bondage along the entire bolt length, at the same time the entire surface of the hole is protected against penetration of moisture. This is particularly important in shale strata, where, when left unprotected by a partial grout column, moisture penetrates to the annular space, and weathers rock around the bolt hole, having an easy access to the bedding planes which are strongly hydrophilic. The most desirable are thrust tensioned rebar bolts that can be installed with one procedure step, and can secure a desired pre-stressing. Such bolts have a forged conventional head and offer no hazard to miners in low clearance mines, whose number increase constantly due to the depletion of reserves in thicker seams. The presented field-testing program has been based on instrumented bolts allowing for monitoring of tension at installation and at a chosen time interval after. The tested system is based on rebar bolts installed with two speed Fosroc resin (fast at the top and stow at the bottom), and mechanical compression by the bolting machine at installation. Combination of several different washer -- plate arrangements have been tested. The results are reported with their interpretation, and recommendations directly towards strata control requirements, as well as the operational considerations for efficiency of installation.

Jan 1, 2002

By Abdolreza (Reza) Osouli

Determining the roof stability condition of underground coal mines in the Illinois Basin is sometimes very challenging. The roof stability of Illinois mines are mostly affected by two types of shales: Anna Shale and Energy Shale. These layers are sometimes weak in parallel bedding when subjected to horizontal stresses. The roof rocks in the Illinois Basin are are typically more moisture sensitive than the other coal fields. In this study the geotechnical properties of some of the shale units as well as limestone layers will be presented. The geotechnical properties of roof rocks consist of moisture content, unconfined compressive strength, slake durability, indirect tensile strength, and point load tests (Axial and Diametral). The correlation of indirect tensile strength versus diametral point load strength as well as unconfined compressive strength versus axial point load test are explored and discussed. These correlations will help in identifying the roof mass index and planning appropriate roof control measures. This study also evaluates the application of CMRR rock mass index for an Illinois coal mine.

Jan 1, 2014

By David P. Conover, Jake Bain, Christopher M. Ross

"Highwall mining of thick (up to 100 ft) steeply dipping (20° or more) coal seams provides many challenges, both geotechnically and operationally because seam dips near or in excess of highwall mining machine capabilities are encountered. Maximizing coal recovery while maintaining highwall stability requires innovative techniques with regard to web and barrier pillar layout, depth of penetration, and choice of mining horizon within the seam. Stability of highwall mining slopes, openings, and pillars are typically analyzed using the ARMPS-HWM program, as well as LAMODEL, UDEC, and Slope-W modeling. Highwall stability can be maintained and highwall mining production optimized by applying design criteria in creative ways, including alternating miner penetration depths and initiating mining of thick seams toward the bottom of the seam. Highwall mining of thick, steeply dipping coal requires careful planning and execution, including close cooperation between geotechnical design engineers, the mining company, and the highwall mining contractor. This paper describes the application of creative design techniques to a specific pit arrangement at the Westmoreland Kemmerer Mine, Kemmerer, Wyoming. Highwall mining was accomplished by UGM ADDCAR Systems, LLC, on a contract basis. INTRODUCTION Highwall mining is a technique for attaining additional coal recovery after the economic strip limit is reached in surface mining. It involves remote deployment of a continuous miner in openings beneath the final highwall, with no personnel entry. Many candidate areas for highwall mining have thick and steeply dipping seams. Mining downdip presents challenges related to the maximum pulling capacity of the machine, traction of the cutting head, and material conveying, all of which limit penetration depth. Maximum penetration is greater for flatter slopes and decreases for slopes nearing the threshold of the maximum machine operating angle. Most highwall mining operations are relatively flat with slight undulations within the seam; therefore, the highwall mining pillar design criteria applies fairly equally to the entire mining area. However, in steeply dipping deposits, design criteria based on higher overburden loads at the far end of the penetration are excessively conservative for the shallower portions of the openings near the highwall."

Jan 1, 2017

By Hongwei Zhang, Sen Yang, Shimin Liu

"This laboratory study investigates the damage evolution of sandstone specimens under two types of cyclic loading conditions through monitoring and analyzing the variations of elastic moduli and ultrasonic velocities. We adopted the rigidity degradation method for evaluating the damage accumulation through the rock dynamic ultrasonic velocity measurements. Compared to the ultrasonic test, the method of rigidity degradation shows deficiencies in the detection of damage development. The cyclic loading within the elastic limit led to a stiffening trend for the loading process but a softening trend for unloading process. The method of rigidity degradation was unable to describe the damage accumulation in this situation, while P-wave velocity clearly reflected the damage development. Moreover, a simplified model is established for interpreting the behaviors of the modulus variations and ultrasonic velocity. The cyclic loading within elastic limit enhances the anisotropy of rock, which causes a denser packing pattern in the direction of axial stress. The persistent damage accumulation under the cycling with an increasing upper limit is the consequence of the tensile failure in the microstructure of sandstone. INTRODUCTION The damage accumulation due to dynamic (cyclic) loading is highly detrimental in many engineering structures, such as mine openings, petroleum and natural gas boreholes, tunnels, rock foundations, and underground chambers. The sources of cyclic or repetitive loadings can be roughly divided into two types: (1) periodic operation, including drilling, blasting, and mining; (2) random vibration, including earthquakes and traffic loads. The repetitive loading-unloading process opens and closes the micro-pores and micro-cracks within the rock, further inducing the growth of cracks. As expected, the accumulating damage becomes a potential factor to trigger the rapid and violent failures of large-scale engineering rock structure."

Jan 1, 2017

By Zbigniew Lubosik, Andrzej Walentek, Stanislaw Prusek, Aleksander Wrana

"The Carboniferous coal measures, mined across Europe including the major European underground coal mining countries of Poland, UK, Germany, and Czech Republic, suffer from increasing extraction depth. This results in major geomechanics problems associated with ground control and support design.With production being concentrated into ever fewer high performance longwalls, located at greater and greater depth, pressure to ensure the roadways’ functionality (required dimensions) continues to increase. Mining in deeper and more highly stressed mining environments creates several problems. The high stress and deterioration in geological and mining conditions in many cases causes problems with gateroads maintenance (due to excess convergence). That leads to costly production gaps, longwall slowing, transport problems, an increase in the methane hazard and in the general health and safety of the workforce, and deterioration in climatic conditions with major economic risk associated with interruption of the revenue stream. Time—and thus production—delays due to maintenance often result from insufficient geological and geotechnical data acquisition and misinterpretation combined with inadequate support for the type of rock mass.longwall face on the gateroad stability (described by the gateroad convergence, support load, and shape and height of the fractured zone around the longwall gateroad) are presented. The investigations were carried out in the longwall panel with roof caving located at the great depth, in conditions of the Upper Silesian Coal Basin in Poland.The measurement results showed that the gateroad fractures in roof strata began to occur at the distance of about 1000 m ahead of the face. Additionally, the load on support and gateroad convergence increased as the longwall face got closer.It is very important for coal mining research institutes and mine operators to investigate and develop practical solutions and apply innovative support technologies at a great depth (particularly for reinforcement and stabilization of roadways and other openings).Underground measurements of stability conditions around the gateroad is of great importance to mining practice, especially in planning and designing supports for longwall gateroads.The influence of the front abutment pressure results in an increase of support load, gateroad convergence, and shape of fractured rock zone."

Jan 1, 2015

By WenFeng Li

A 3D roof bolt model with the consideration of rebar, resin, bearing plate, and resin/rock interface was developed for studying the bolt/rock interactions of the tensioned and fully-grouted resin bolts. For the tensioned bolts, 2 ft compressive zone above the rooflineis generated by the 10 tons pretension. Since the free portion of the tensioned bolts displays a higher vertical stress, the increase of the bearing capacity of the rocks in such range is much larger. For the fully grouted resin bolts, despite no pretension is generated during installation, the roof sag can indirectly generate a compressive zone near the roof line. In addition, the analysis on the differences in bolt/rock interactions when employing the tensioned or fully-grouted resin bolts indicates that the fully-grouted resin bolts are preferred as the primary support when weak and thinly-bedded immediate-roof is present. Meanwhile, it is found that both the tensioned and fully-grouted resin bolts may have very limited effect in preventing and/or control the cutter roof.

Jan 1, 2014

By John C. Stankus

ldentifcation of potential roof problems in coal mines has long been a complex issue due to the wide variety of mining and geological conditions. It is well known that mine roof falls are related to many different factors such as roof strata properties, sandstone channels, regional and localized horizontal stress, vertical stress, and tectonic stress. Many times, a combination of these factors causes roof problems. Through years of ground control experiences, Keystone Mining Services LLC, (KMS) has developed a Roof Instability Rating (TUR) system to predict potential roof problems. The first RIR analysis was presented at the 19th Ground Control Conference (Watts, et al., 2000). The RIR system includes: quantifying each individual factor, weighting each factor, combining individual factors, finalizing the RIR, and setting the criteria for the areas where special support is needed. In this paper, the application of the Roof Instability Rating (RIR) system at Enlow Fork mine will be presented.

Jan 1, 2001

By Ryan Garvey

A numerical investigation is made into unstable failures of coal pillars (i.e. coal bumps) using the finite difference software FLAC3D. A static energy balance is first derived to calculate the excess energy released as a consequence of unstable failure in rock. Two- and three-dimensional mining layouts are then assessed on their bump-potential based on the magnitudes of excess energy released during simulated pillar failures. A three dimensional pillar model is developed to represent tributary area loading of an infinite room-and-pillar layout. Pillar strength and brittleness characteristics are calibrated using a Mohr-Coulomb strain-softening constitutive law to simulate width-to-height ratio coal pillars ranging from 1:1 to 5:1. Width-to-height ratio pillars 1:1 to 3:1 release large magnitudes of excess energy as the pillars fail, representing massive collapse of room-and-pillar layouts. The larger pillars maintain their residual strength during failure; however significant excess energy is still released during temporary reductions in pillar strength. A two-dimensional model is then constructed of a gate road pillar failing due to elastic rebound of the surrounding rock mass. Soft loading conditions initiate unstable failure for all pillar widths, while the total magnitudes of excess energy increase with the size of pillar being modeled. The results from these tests indicate that excess energy may be used as a direct assessment of the bump potential of simulated mine layouts.

Jan 1, 2013

By Shuren Wang, Wenbing Guo, Paul C. Hagan, Yanhai Zhao

"The overlying strata is often destroyed in large scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. The typical shallow coal mining in Shendong Mining Area in China was used as engineering background; the load distribution characteristics of the overlying strata were analyzed based on the monitoring data of the support resistance. Through theoretical analysis and numerical simulation, the formation and changing process of the pressure-arch in the overlying strata were studied, and the instability mechanism of the overlying strata was revealed by focusing on the accumulation and release law of the elastic energy in the pressure-arch. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The elastic energy was accumulated in the pressure-arch, and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining, and the shear zone could be formed. Therefore, the sliding of the caved zone along the shear zone induced strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct similar engineering practice.INTRODUCTIONThe instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju and Xu, 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al., 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mine and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch."

Jan 1, 2018

By Zbigniew Lubosik, Andrzej Walentek, Stanislaw Prusek, Sylwester Rajwa, Aleksander Wrana

"Underground coal gasification (UCG) provides a solution to supply electricity from high ash coal or deep deposited seams. The coal is consumed in situ, and, accordingly, the ash is left underground. UCG could play a major role in the future of coal, which is expected to fuel 44% of the world’s energy in 2030, in a more environmentally friendly and commercially viable manner. Nevertheless, it is known that the UCG can impact the environment. One of the most important factors in safe UCG process should be the evaluation of rock mass quality and the monitoring of its behaviour. The changes of rock mass parameters and quality have an influence on numerous aspects, such as subsidence and the stability of underground workings in georeactor vicinity, however, they also increase the possibility of pollutant migration through fractures and pores. These issues are especially important in cases of gasification conducted in an operating mine or on areas of high ecological vulnerability. This paper details observations made in two underground trials of UCG performed in Poland recently. The first of which is the Experimental Mine Barbara (under the project entitled HUGE-2 Hydrogen oriented underground coal gasification for Europe – Environmental and Safety aspects, (Wiatowski et al., 2015). The other trial has been conducted at an operating mine, Wieczorek, which is in the framework of a research project on the development of coal gasification technology for highly efficient fuel and electricity production (Czaplicka-Kolarz et al., 2013). During these trials, investigations were carried out aimed at assessing geomechanical parameters of rock mass and support behaviour in surroundings of UCG geaoreactors. Investigation comprises assessment of geomechanical parameters of coal, floor and roof strata, roof displacements, and concrete support strength parameters variation, as well as standing steel support load. Hydraulic penetrometer, endoscopic camera, Schmidt hammer, pull-out method, telltale, and ground penetration radar (GPR) were used in the investigation. Research conducted at both EM Barbara and Wieczorek Collieries revealed that the UCG process did not adversely affected the competency of rock mass (expressed by number of fissures and strength parameters of rocks), the stability of roadways and the support performance in the areas located in the vicinity of the georeactors."

Jan 1, 2017

By Joe E. Bryan, John P. McDonnell

"Underground mine rib stability is an ongoing safety issue in all mining applications. Rib-related accidents present at least as many problems as roof-related ground control. Underground coal mine rib safety is continually emphasized as a primary safety concern at every mining operation. The application of rib support materials is increasing as more difficult mining conditions are experienced due to deeper cover, more variable depositional environments, thicker seam heights, and multi-seam mining effects. As the coal seams become thicker, the need for rib control increases. A variety of rib control methodologies has been documented and research continues on mine design methods and support techniques to reduce or eliminate rib-related accidents.This paper presents an alternative rib bolting technology using proven conventional rib support systems that allows mine personnel to more efficiently install rib support during development operations. This alternative rib bolting method allows for additional rib support with a single support installation.INTRODUCTIONRib control continues to garner the watchful eye of both concerned operators and government officials due to many factors: inherent safety within entries and crosscuts, actual rib stability history, safety improvement, cost control, improved mining methods, and maintaining production goals. As demonstrated through many years of actual practice, desired outcomes can be achieved by scrutiny, innovation, and combining improved practice methodologies.Rib surface control continues to be a necessary part of the production process. However, production and safety issues persist at the point of installation using current control apparatuses and methods to install rib controls. Among others, current issues of rib control are quantity of bolts required, overall time required for installation, ease of apparatus installation, flexibility of the system for appendage installation and operator exposure. A need exists to decrease the time miners spend installing rib control apparatuses, while increasing the safety of such installation processes. At present, installing rib control devices (primary rib bolts, plates, mesh, and, in most cases, secondary rib bolts and plates) is performed in a very congested area and is time-consuming process at the primary coal-winning area: the face. The MatLok system addresses these problems."

Jan 1, 2015

By Anthony T. Iannacchione

This study, funded by the Appalachian Research Initiative for Environmental Science (ARIES), examines how mine layout, mining method, and geology interacted to allow water from the Grove No.1 mine pool to discharge into local surface waters. Federal and State regulations prohibit the discharge of acid or iron-producing waters from underground coal mines. Mining companies design barriers around their operations to prevent mine waters from discharging to the surface. The Grove No.1 represents one mine where these designs proved inadequate. Unique geologic conditions at the Grove No.1 mine pool, combined with retreat room-and-pillar mining, contributed to a discharge that required passive water treatment.

Jan 1, 2013

By Gang Ren, Behrooz Ghabraie

"Observational data suggest that the ground surface subsidence profile from multiple-seam longwall mining is rather different from that of single seam. Despite the increase of coal production from multiple-seam mining, available predictive methods are mostly unable to predict a reliable subsidence profile for multiple-seam extractions. In this paper, a case study of multiple-seam subsidence is modelled using physical modelling techniques. The subsidence mechanism as revealed in the physical model is explained and the differences between multiple and single-seam subsidence are highlighted. On the same case study, a commonly used influence function method (IFM) and Generalised Influence Function Method (GIFM) are utilised to predict the subsidence profile. The results are compared with predictions achieved by a newly developed predictive method based on the understanding of mechanism of the multiple-seam subsidence. Results show a significantly improved correlation between the newly developed method in comparison with the conventional IFM and GIFM. INTRODUCTIONField observations and subsidence monitoring data from multiple-seam coal mining cases show a different subsidence profile as compared with single-seam ones (Sheorey et al., 2000; Li, et al, 2007; Luo and Qiu, 2012; Unlu et al, 2013). The multiple-seam subsidence curve is generally deeper and steeper than of single-seam cases, and varies from one mining configuration to another. Various researchers had made efforts to understand the mechanism behind this phenomenon and the interaction between the two mining horizons (Li et al., 2010, Sui et al., 2014; Zhang et al., 2014). However, the mechanism of the multiple-seam subsidence is yet to be comprehensively understood (Suchowerska, 2014; Ghabraie et al., 2015b).In addition, differences between the single and multiple-seam subsidence profiles make it difficult to adapt single-seam subsidence prediction tools for predicting the subsidence over multiple-seam cases, unless significant modifications are applied This requires understanding the mechanism of the multiple-seam subsidence as the first step to achieve reliable predictive results.The mechanism of the subsidence due to a multiple-seam case study can be investigated via physical modelling. The physical model allows observing the substrata movement as well as the interaction of the two mining activities. An understanding of the mechanism of the multiple-seam subsidence can be developed based on these observations, which would provide basis for developing a subsidence prediction tool."

Jan 1, 2016

By Chris Kelly

The Monroe County Quarry is an open pit granite quarry located near the town of Bolingbroke. Monroe County. Georgia. The quarry has been in continuous operation for approximately 10 years. The quarry covers approximately 27 acres and has an annual production rate of 1.0 million tons. On the morning of April 26. 2000 a sudden rapid highwall failure occurred at the west end of the north wall of the quarry pit. The failure occurred approximately one hour after a three inch-wide crack was observed in an access road located at the crest of the highwall. No indications of failure had been observed prior to the development of the crack. The active mining area had been at the base of the failed area. The pit was successfully evacuated prior to the failure and there were no losses of either personnel or equipment. The amount of failed material was estimated to be in excess of two million tons. In order to ensure a safe working environment at the quarry a slope monitoring program was established in the area of the failure. The program consisted of using an EDM survey instrument to periodically measure the location of survey prisms installed at carious points on the north wall. The locations of the prisms were recorded over several months and changes in location were analyzed in order to identify and characterize the movement of the failed mass. The results of the analysis provided a basis for recommendations regarding access to various areas of the quarry. This paper discusses the factors contributing to the failure, the method of post-failure slope monitoring, anti the interpretation of the monitoring data.

Jan 1, 2002

By Chengguo Zhang, Bruce Hebblewhite, Faham Tahmasebinia, lsmet Canbulat

"Coal burst is a dynamic release of energy within the rock (or coal) mass leading to high velocity expulsion of the broken/ failed material into mine openings. This phenomenon has been recognised as one of the most catastrophic failures associated with the coal mining industry, which can often lead to injuries and fatalities of miners as well as significant production losses. This paper aims to examine the mechanisms contributing to coal burst occurrence, with an emphasis on the energy release concept. In this study, a numerical modelling study has been conducted to evaluate the roles and contributions of difference energy components. The energy analysis presented in this paper can help to improve the understanding of energy release mechanisms especially under Australian conditions.INTRODUCTIONCoal burst is one of the most hazardous problems encountered in underground coal mines, which occurs at different locations in a variety of mining systems and operations. This phenomenon always involves a violent and dynamic energy release with large rock mass/coal deformation and ejection that can cause severe damage to openings, equipments and may result in fatalities and injuries.The first coal burst occurrence was reported in Britain in 1738 (Dou 2001). Since then, international experiences are available in Canada, South Africa, USA, former Soviet Union, China, India, France, Germany, Poland and Czechoslovakia. Rice (1935) classified coal bursts as two general types, namely excessive pressure bumps and shock bumps and he further mentioned that pressure bumps are caused when pillar stress exceeds bearing strength. Shock bumps are induced by the breaking of thick, massive strata at a considerable distance above the coal-bed, causing the immediate roof to transmit a shock wave to the coal."

Jan 1, 2016

By Peter Zhang

"Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground-fall fatalities in the underground coal mines of the United States. The risk of ground falls during pillar recovery increases in multiple-seam mining conditions. The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave. When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction. This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 70 ft of interburden under about 1,000 ft of depth of cover.The study finds that, for interburden thickness of 70 ft, the multiple seam mining influence zone in the lower seam is directly under the barrier pillar within about 100 ft from the gob edge of the upper seam. The peak stress in the interburden transfers down at an angle of approximately 20° away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat. The study also suggests that, for full pillar recovery in close distance multiple seam scenarios, it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars. If the entries and / or crosscuts in the lower seam are developed outside the gob line of the upper seam, additional roof and rib support needs to be considered to account for the elevated stress in the multiple seam influence zone. INTRODUCTION Room-and-pillar mining accounted for about 40% of underground coal production in the United States in 2016. Pillar recovery, practiced in about one-third of the room-and-pillar mines, represents about 10% of the coal mined underground, yet it has historically been associated with more than 25% of all ground fall fatalities (Mark, Chase, and Pappas, 2003). In some U.S. coal fields, particularly central Appalachia, many coal mines are operating under geological conditions with multiple coal seams. The risk of ground falls during pillar recovery increases under multiple-seam mining conditions. The hazards of pillar recovery associated with multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave (Mark and Tuchman, 2007; NIOSH, 2010b). Pillar retreating creates abutment pressure, not only in the currently mined seam, but also in the overlying or underlying seams. Multiple-seam interactions become more pronounced as overburden depth increases and interburden thickness decreases. To safely recover the pillars in multiple seams, it is critical to properly plan the mining sequence and panel layout to minimize potential multiple-seam interaction."

Jan 1, 2017