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By Xicai Gao

With the intensification and expansion of deep coal mining, the surrounding rock stress environment becomes further deteriorated because of the complicated geological environment and powerful mining effect, so the major deformation of roadway appeared frequently. The proportions of floor heave are increasing and , the most serious problem is the floor heave of preparatory workings. The coal seam of Binchang coal area in Shaanxi province is at a depth of 621.6 ~ 817.0 m, the average thickness of the coal seam is 25 m, and the overburden and coal measure strata are the main regional aquifers and are highly permeable. The properties of water-weakening are obvious in the floor strata. The main preparatory workings or chambers were driven in the coal seam. The severe floor heave of the water chamber appeared during early mining and the amount was 180 ~ 910mm, and seriously affected the normal installation and use of the water chamber. Theoretical study and numerical simulation as well as the in-situ measurements were carried out to investigate the deformation and failure characteristic of the surrounding rock with non-support in the floor and the main reasons which caused the floor heave were determined. A new combined support technology was proposed which consists of overcutting, grouting, backfilling and invertedarch and bolt-grouting. The field monitoring showed that the cumulative convergence of the water chamber was 20mm, the maximum convergence speed is 1.2 ~ 2mm/d, the accumulative convergence of two ribs was 36mm and the maximum convergence speed is 2mm/d. The combined support technology will enhance the support intensity between the roof and floor, the two sides of the entry and improve the loading capacity of the floor effectively and the floor heave can be controlled effectively.

Jan 1, 2013

By James D. Pile

Following a methane ignition in the active panel of a longwall coal mine, the affected panel was successfully isolated from the rest of the mine. This allowed for the remainder of the mine to return to regular operation, and the conventional practice of having to seal the portals for a defined period of time was avoided. After the affected panel had remained isolated for a regulatorily mandated period of time, it was reopened in preparation for longwall face recovery. As it was not possible to move the shields from their existing positions to facilitate the installation of recovery mesh prior to shield recovery, alternative methods had to be considered to stabilise the gob and minimise flushing, thereby improving operator safety. The objective of the paper is to show how stabilisation of the gob was achieved during shield recovery, thereby minimising flushing, and improving operator safety. Phenolic foam was chosen and injected from between the shields into the gob behind them. This is a technique that has been used to great effect in Germany, to a lesser extent in Australia, but not knowingly to date in the United States. To ensure nothing was left to chance and thereby guarantee the project?s success, the procedure used in Germany was employed, down to the application of equivalent hardware, which was sourced through the North American supplier.

Jan 1, 2013

By J. L. Condon

The Bureau of Mines, through in-house and contract research, monitored mountain bump-prone areas of the Olga #2 Mine, near Welch, WV, using microseismic techniques for 15 months during 1985 and 1986. During this period, two ground failures occurred that were considered "mountain bumps" owing to excessive stress buildups. One failure damaged the roof and 4 pillars around an intersection; the second failure, which released much greater energy, resulted in significant damage to over 100 coal pillars. Analysis of the microseismic data collected provided significant insights into the magnitude and location of each failure. The actual failure zone was clearly delineated prior to the mountain bump occurrences through analysis of source locations of each microseismicevent. Trends in cumulative rate plots indicated the onset of structural instability. Although the data could not be analyzed automatically, the significance of the data interpretation cannot be overemphasized. Microseismic monitoring of bump-prone areas can be successful in providing a means to actually observe the structural stability occurring in underground coal mines. This paper discusses data collection techniques, analysis methods, and implications of the monitoring technique, with special emphasis on actual data from two large mountain bumps in West Virginia.

Jan 1, 1987

By Anton Sroka, Antoni Tajdus, Axel Preusse, Krzysztof Tajdus

"In Europe, the magnitude and range of mining deformation predicted on the surface have been, in most cases, determined using methods based on Gauss' influence function (e.g., Knothe's theory, Ruhrkohle method, Stochastic method). The increase in the application of numerical methods (finite element method FEM, finite difference method FDM, etc.) provides an opportunity to offer a different approach in describing mining deformations. However, this phenomenon is very complex, and due to numerical limitations, a two dimensional analysis was made (in plane strain) in most cases. Such 2D models could not accurately describe the mining geometry and geological conditions. The authors of this paper carried out the calculations in 3D models which describe a mining situation and the character of the strata in one Upper Silesian Polish mine. Furthermore, the authors include some tips for three-dimensional modeling.INTRODUCTIONThrough the years, many numerical models have been used to describe a phenomenon of underground mining exploitation influence on the ground surface. However, such rock masses are highly disturbed (cracks, fractures, changes in water conditions, etc.), and it is very difficult to suitably describe, in a mathematical manner, such materials. Therefore, many of the numerical models created have been corrected in various degrees.At first, models widely used to describe the behavior of strata in the region of exploitation appeared as elastic models, where scientests assumed that a whole strata behaved in a linearly elastic manner, according to Hook's law. Those models have been modifed to received suitable VALUES (of settlement and range of subsidence troughs. Scientests created this amongst others: elastic models with contact between the rocks layers (Shippam, 1975; Salamon, Chugh and Yang, 1993), elastic models with no tension (Chrzanowska-Szostak, 1988), and elastic models with a contact on the subsidence limit plane (Tajdus K., Tajdus A., 2005). However, in most cases, those models did not bring the expected results and the differences between shapes of measured and the calculated subsidence trough were substantial."

Jan 1, 2010

By Thomas M. Barczak

A survey of the longwall industry was conducted to examine the performance of modern shield technology. The results of this survey indicate that state-of-the-art shields perform better and last longer than ever before, but premature failures do still occur. In addition, longwalI operators are now using shields longer than ever before and a greater number of fatigue related failures on aging shields are now occurring. A generic assessment of these shield failures is provided, describing the nature of the failures and how widespread they are throughout the industry. An engineering assessment of shield design and factors that cause shield failures is made using both strength of materials and fracture mechanics principles. From this analysis, design practices to improve structural margins of safety and extend the life of the shield are proposed. The existence of unexpected shield failures indicate that current shield performance testing is at times inadequate. Recommendations are made relative to improved shield performance testing protocols including the benefits of testing in an active load frame such as the National Institute for Occupational Safety and Health's (NIOSH) Mine Roof Simulator. Even the most rigorous shield testing procedures, which strive to simulate in-service loading conditions, fail to consider key environmental issues, such as corrosion which is often the cause of structural fatigue failures in modern shields. The survey also indicated that hydraulic failures occur much sooner in the life cycle of a shield than structural failures, and although they can significantly degrade support capability, often go undetected for long periods of time. Methods to detect the hydraulic failures are also provided. The paper concludes with key points to maximizing shield design and life expectancy and ideas for future generation shield designs.

Jan 1, 1999

By Zach G. Agioutantis

The Illinois Basin has an extensive history with the use of mechanized longwall mining dating back to the early 1970?s. In recent times, technological advances have resulted in increased panel widths from 800 feet to over 1400 feet and daily panel advance rates have also increased significant l. The prediction and control of subsidence and surface deformation due to underground mining are important considerations in the permitting, planning, and monitoring of these coal mining operations. The prediction of mining-induced ground movements using the influence function method is a mature technology, well utilized in the Illinois basin, and widely used by researchers and planning engineers around the world. This paper utilizes subsidence data, recently measured in longwall operations in Illinois, to develop an updated ground movement prediction capability implemented by the Surface Deformation Prediction System (SDPS) software package.

Jan 1, 2014

By P. A. Gray

Artificial ground support has developed into a relined science over the past 20 years. From reactive support systems such as timber props and steel supports, to active systems such as roof bolts and cable anchors, ground support technology has now developed efficient systems that work well in many instances. However this paper demonstrates that there is still a long way to go to develop optimum ground support systems. Some conventional rock bolts and cable anchors emphasise their tensile capacity only, without considering other factors such as their shear strength or shear modulus, or if the support member can transfer all of its support capacity to the surrounding rock. Examples are given of the use of ground anchors with high tensile and shear capacity in underground coal mining as well as examples of slope failures in open pits where the cable bolts have remained intact because they have been unable to transfer their full capacity to the surrounding weak rock mass. For underground coal mines, speed of installation is also of primary concern, but it must also have high tensile and shear capacity both in terms of strength and stiffness. Ground support systems of the future must therefore address these problems. An innovative new rock bait is discussed which goes some way towards solving these problems. This new rock bolt is screwed into the rock and thus has a direct mechanical interlock with the rock. Initial field results show that it has an extremely high anchor capacity and can be fully installed in approximately 30 seconds. It has considerable capacity to improve the productivity of the coal mining industry.

Jan 1, 1992

By Pierre-Yves Declercq, Andre Vervoort

"After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement. Although most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated. After the closure and flooding of underground excavations and surrounding rock, this movement is reversed. This paper focuses on quantifying the upward movement in two neighboring coal mines (Winterslag and Zwartberg, Belgium). The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar (INSAR). The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average. The upward movements are not linked directly to the past exploitation directly underneath a location. The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings. Local features, such as geological faults, can have a secondary effect on the local variation of the uplift. The processes of subsidence and uplift are based on completely different mechanisms. Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding. INTRODUCTION After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—upward movement in the area above the past exploitation and in the immediate surroundings. Of course, most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method (Peng, 1986). This downward movement continues to occur at a smaller rate many decades after mining has been terminated when the water in the underground excavations is pumped to the surface. This is the case as long as a mine is in operation. However, after the closure and flooding of the underground excavations and the surrounding rock, this movement is reversed, and the surface is uplifted. A total uplift of about half a meter has been recorded to date. This phenomenon has been described in several different coal basins in Europe, for example, in Belgium (Devleeschouwer et al., 2008), France (Samsonov, d’Oreye, and Smets, 2013), Germany (Baglikow, 2011), the Netherlands (Caro Cuenca, Hooper, and Hanssen, 2013), and Poland (Preusse, Kateloe, and Sroka, 2013). To date, research has focused mainly on understanding the phenomenon (e.g., Herrero et al., 2012) and identifying general trends, whereby the link with the rise in water levels is an important issue (Caro Cuenca, Hooper, and Hanssen, 2013). The most accepted explanation is linked to the swelling of clay minerals in the argillaceous rocks in the coal strata (Herrero et al., 2012). Without doubt, the full explanation is complex, and swelling is probably not the only cause of the residual movement. Most coal strata rock is weakened upon contact with water. The increase in water pressure also results in a decrease in the effective stress. Both aspects facilitate the fracturing of rock, which normally would result in further subsidence. However, decreases in the effective stresses also result in the relaxation or expansion of the rock, which induces an upward movement (Fjar et al., 2008). All of these aspects, including the long-term residual subsidence due to compaction under dry conditions, result in a net upward movement. The phenomenon discussed here is clearly different from the upsidence, sometimes observed on other continents (e.g., in Australia, Galvin, 2016). When upsidence occurs, the rock near the surface bends and buckles upwards due to the overstressing of the floors of the valleys."

Jan 1, 2017

By William G. Pariseau, Michael K. McCarter

"The challenge of estimating mine-wide subsidence and linkages to seismicity over tabular deposits is addressed by a special finite element technique (dual node - dual mesh). Subsidence and mine-induced seismicity begins near the face when caving occurs and propagates to the surface as extraction reaches a critical extent. Thus, the challenge is to obtain details at the face at the meter scale and also at the surface over the whole mine at the kilometer scale. Interactions between old and new sections of a mine are automatically taken into account with this technique. The finite element method is well established technology based on fundamentals of physical laws, kinematics and material laws. With this technique, no empirical ""scaling"" or fitting computer output by input data ""adjustment"" to mine measurements is necessary. Capability is demonstrated for doing practical whole-mine subsidence analysis from first principles. Mine-induced seismicity is shown to correlate well with face advance and element failure.INTRODUCTIONSeismicity associated with coal mining has been under study for several years by a team at the University of Utah. Team members include mining faculty, research geophysicists and students from the Departments of Mining Engineering and Geology & Geophysics. Mines in central Utah that have been studied include the Trail Mountain Mine (Pariseau, 2015; Pariseau and McCarter, 2015), the Beehive Mine, also known as the Des-BeeDove (Pariseau, 2014), and the Crandall Canyon Mine (Pariseau, 2015). These mines were developed from outcrops in the Wasatch Plateau coal field west of Price, Utah, where the topography is characterized by . steeply incised canyon drainages and high plateaus with relief of a thousand meters or so.Seismicity associated with coal mining in central Utah has been of interest for many years and includes studies in the Book Cliffs to the east of Price, Utah, where mining is still active (Smith et al, 1974; Agapito, Goodrich and Moon, 1997; Arabasz et al, 2001; Arabasz et al, 2002; Fletcher and McGarr, 2005; McGarr and Fletcher 2005; Maleki, 2005) and to the northwest (Ellenberger et al, 2001). Results of many micro-seismic studies are summarized perhaps best by lannacchione, et al (2005) in stating, ""Deviations from normal strata response can provide useful stability information.""This study concerns a fourth underground coal mine in the southern portion of the Wasatch Plateau coalfield, hereafter referred to as the MINE. Focus is on mining from 2004-2008 with the objective of examining mining in relation to seismicity."

Jan 1, 2016

By Pengfei Wang, Guorui Feng

With increase of cover depth, dynamic incidents, such as large gateroad convergences and coal bursts, are frequently encountered when mining deep longwall panels. It would be more adverse due to large inclination of the coal bed. Because of the shearer and armored face conveyor (AFC) sliding downhill, toppling of shields, disconnection of AFC to the stage loader are related to it. Tangshan coal mine is a typical deep, inclined coal mine situated in the east of China. The split-level longwall panel layout is being adopted for mitigating problems above, where tailgate entries of longwall panels are driven below gob edge and along the floor, while headgate entries are driven along the roof. Wire mesh and steel sets have been used empirically for support for tailgates for about a decade, which reduces the tunneling speed. In view of this fact, the paper serves to investigate the proper adjustment of the tailgate locations and to analyze the resulting stress and yield conditions according to support strategy. First, field measurements and observations were carried out. Stress data distributed within surrounding rock mass were obtained, especially within the roof since the roof consists of only a coal sheet with a thickness of 2–4 m. Roof pressure of the tailgate represents the pressure at the gob edge since it is below the gob edge. The convergence data were also collected. Numerical modelling using FLAC3D™ was conducted and validated by side abutment stress data. The double-yield constitutive model was used to simulate the gob. The finalized tailgate layout and design were determined through comparison of three scenarios. The research in this paper can help engineers choose the proper tailgate layout and support strategies for coal mines that are going to adopt a split-level longwall panel layout for mining deep, inclined longwall panels.

Jan 1, 2018

By William J. Conrad, Ryan J. Molka, Erik C. Westman

"In order to study pillar and overburden response to retreat mining, a ground control program was conducted at a Central Appalachian mine. The program consisted of several monitoring methods including a seismic monitoring system, borehole pressure cells in the pillars, and time-lapse photogrammetry of the pillar ribs. Two parallel geophone arrays were installed, one on each side of the panel with the sensors mounted 3m (IO ft) into the roof. A total of fourteen geophones recorded more than 5,000 events during the panel retreat. A MIDAS datalogger was used to record pressure from borehole pressure cells (BPCs) located in two adjacent pillars that were not mined during retreat. A series of photographs were taken of the pillars that had the BPCs as the face approached so that deformation of the entire rib could be monitored using photogrammetry.Results showed that pillar stability and cave development were as expected. The BPCs showed an increase in loading when the face was 115 m (380 ft) inby and a clear onset of the forward abutment at 30 m (100 ft). The photogrammetry results displayed pillar deformation corresponding to the increased loading. The microseismic monitoring results showed the overburden caving inby the face, again as expected. The significance of these results lies in two points, I) we can quantify the safe manner in which this mine is conducting retreating operations, and 2) we can use volumetric technologies (photogrammetry and microseismic) to monitor entire volumes of the mine in addition to the traditional point-location geotechnical measurements (BPCs).INTRODUCTION AND BACKGROUNDExcavation of underground openings is a difficult job in a challenging environment. The rigor of the process is compounded by the lack of a method that allows quantification of changes within the rock mass as excavation progresses. Statistics kept by the Department of Labor, Mine Safety and Health Administration show that 16% of fatalities and lost time incidents in the underground mining industry are due to unexpected rock mass failure (MSHA, 2015). A recent example is the Crandall Canyon, Utah, coal mine accident caused by the vertical collapse of a longwall coal mine on August 6, 2007 (Dreger, Ford, Walter, 2008)."

Jan 1, 2016

By David A. Newman

Severe roof control problems have plagued a West Virginia underground mine since its initial development in the late 1970's. Adverse roof conditions in the Eastern portion of the reserve result from a combination of mining: - perpendicular to the major principal horizontal stress direction, - near lineament intersections, - in areas where the ratio of horizontal stress to vertical stress exceeds 2.00 to 4.00, and - where laminated strata are present in the immediate roof. The focus of this study is to identify the engineering (mine orientation, roof safety factor) and geological parameters (lineament and horizontal stress orientation, horizontal to vertical stress ratio, and immediate roof lithology) causing poor roof conditions and to quantify their influence on projected mining conditions in the Western portion of the reserve Rose diagrams were used to illustrate the correlation between the orientation of eight hundred thirty-six (836) roof falls with that of forty-four (44) lineaments and the principal horizontal stress Similarly a composite map was used to demonstrate the relationship between roof falls and the ratio of horizontal stress to vertical stress The immediate roof strata across the property were categorized as either laminated or non-laminated based upon a thorough examination of one hundred sixty-two core logs. The presence or absence of fireclay, a rider coal, and carbonaceous shale was also noted because of the historic correlation with poor roof conditions. Rock strength and physical property tests were conducted on selected immediate roof strata recovered from coreholes drilled along the mains projected to the Western portion of the reserve Potential roof control problems in the Western reserve were quantified based upon the correlations developed by analyzing engineering and geological data The main conclusion is that laminated roof strata, the rider seam, and its associated fireclay will have the greatest influence on mining conditions. Roof problems attributable to horizontal stresses will not be as severe in the West due to an increase in overburden depth and corresponding decrease in the stress ratio. Recommendations are provided for mining orientation, the layout of mains and submains, panel location, and roof support based on intended service life.

Jan 1, 1999

By Hongwei Wang, Hao Zhang, Panshi Xie, Xicai Gao, Youfu Zeng, Yongping Wu, Juan Dou

"The reserves and production of steep coal seams account for approximately 15–20% of the total coal reserves in China and 5–10% of the total annual coal production. More than 50% of the coal is high-quality coking coal and anthracite, based on reviews of steeply dipping seam mining at home and abroad. Fully mechanized mining is considered to be the only safe and efficient way to mine steeply dipping seams. To achieve fully mechanized longwall mining, four issues must be addressed: ground control, equipment stability control, development of complete sets of equipment, and safety. Engineers must focus on many factors of steeply dipping seams, such as the following: stope, roof asymmetrical structure, stability control, “roof-support-floor” system dynamics control, multi-region big-range ground control technology, adjustment false-inclined method, nonlinear layout method, control technology, safety, and development of complete sets of equipment. Engineering practices in China introduce these concepts systematically: fully mechanized mining of medium-thickness seams, fully mechanized high mining of thick seams, fully mechanized caving of extremely thick coal seams, fully mechanized mining of coal seams, roadway support, and so on. A steeply dipping seam is one with a dipping angle in the range of 35°–55°. This accounts for about 15%–20% of the total coal reserves in China. Because of the environment, more than 50% of the coal is high-quality coking coal and anthracite. Mining steeply dipping coal seams is known to be difficult in the mining industry. After the study conducted by Germany, Poland, and the former Soviet Union in the1970s–1980s, the fully mechanized mining basic theory, core technology, and key equipment research have not been a breakthrough. Being fully mechanized is a technological “restricted area” at home and abroad. A lot of steeply dipping seams are excavated by non-mechanized mining, leading to harsh job environments, labor-intensive work, low productivity, frequent accidents, and significant security hazards. The death rate per million tons of coal reaches above 4.0 and is far higher than slow inclined coal seam mining. Making steeply dipping seam mining safe and efficient is a global problem that urgently needs to be solved. Fully mechanized mining is the only way to overcome these problems (Wu, 2003; Wu, Yun, and Zhou, 2000). The realization of fully mechanized coal seam mining must solve four problems. (1) Examine the disaster mechanisms induced by overburden “key strata” mutation under steeply dipping seams as a basis for ground control methods and technical problems. (2) Work to solve the dynamic stability control technical problems of thousands of tons of complete sets of equipment under the steeply dipping seams. (3) Complete sets of equipment must be developed to adapt to the conditions of steeply dipping seams. (4) Overcome the steeply dipping face safety protection technical problems. In the last two decades, a number of universities, research institutes, and enterprises have realized the importance of safety and efficiency of steeply dipping seams in China. This is especially true of Professor Wu Yongping’s “hard coal layer safe and efficient mining” team at Xi’an University of Science and Technology. After several generations’ research and technical work, the team puts forward China’s longwall mechanized coal mining of steeply dipping seams strata control basic theory. This paper puts forth a series of key technologies of fully mechanized mining, develops and manufactures complete sets of equipment by independent innovation, and establishes a perfect face security system, forming a theory and technology system for fully mechanized longwall mining of steeply dipping seams."

Jan 1, 2017

By Erdal Unal

In underground mining today, safety and economical aspects demand a better understanding of the rock-mass conditions, particularly for design of underground mine openings excavated in weak and stratified rock mass. The rock mass classification systems, in essence, are empirical approaches utilized during preliminary design stage. These systems have been developed for specific purposes and rock mass types, therefore, direct utilization of the classification systems in their original form, for characterization of complex rock-mass conditions is not always possible. This is probably one of the main reasons why designers continue to originate new systems, or modify and extend the ones already existing. RMR and Q systems, for example, although widely used in mining and tunnelling, can not fully describe the specifications of the weak, stratified and flay bearing rock mass. Consequently, the engineering applications that would be carried out based on original RMR and Q ratings could be inadequate for making design decisions even during the preliminary design stage.

Jan 1, 1990

By Yi Luo

Subsurface strata movements and deformations associated with underground mining activities could cause problems to subsurface structures and water bodies. By incorporating the methods for surface subsidence prediction with the subsidence parameters derived from collected subsurface subsidence data, methods have been proposed for the prediction of final and dynamic subsurface movements and deformations induced by underground longwall mining. Two new' types of strata deformations in the field of subsidence research, vertical and total strains that could he useful in assessing subsidence influences to the subsurface structures and water bodies, have peen introduced.

Jan 1, 2000

By Claude A. Goode

Large quantities of high grade coal exist in thick seams in the United States. Many of these thick seams are too deep to be surface mined and do not lend themselves readily to underground extraction. To develop the new mining technology required to effectively mine thick seams, the Department of Energy has entered into a Cooperative Agreement with Mid- Continent Resources, Inc. to demonstrate the multilift longwall mining technique in a 28 foot-thick seam at their L.S. Wood No. 3 mine near Redatone, Colorado. The seam will be extracted in two lifts, with a 5-foot natural partition of coal left between the upper and lower lifts. Proper selection of face supports is critical to the success of this project. Specifications were defined for the roof supports, and it was concluded that the supports selected must provide full coverage, yield at the legs, and have a 60-inch push- out cantilever that can be set at any position under full load. A Hemscheidt two-leg shield was judged as the best qualified candidate and was selected for the multi-lift trial.

Jan 1, 1981

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

Coal mass strength properties must be estimated to obtain realistic estimates of coal pillar strength. The limitations of existing techniques for approximating these properties are discussed. The coal mass is anisotropic. The Hoek and Brown approach to rock mass property estimation is modified to allow for this. This approach requires the Rock Mass Rating (RMR) for the strata. In the normal case where bedding is sub-horizontal, these major planes of weakness have little effect on the strength of pillars. As a result of this the RMR must be measured parallel to bedding. This means that a significantly higher rating than that usually obtained for coal is observed. The use of mass strength property estimates in extant empirical relationships is outlined. At high width to height ratios, an acceleration in the rate of increase in pillar strength with increasing width to height ratio has been observed. The empirical relationship developed by Salamon (the so called squat pillar formula) takes account of this. This relationship was found to have significant limitations with respect to its universal application. Modifications arc made to Bieniawski's empirical formula to extend its use to large width to height ratio (squat) pillars. This new squat pillar formula has none of the disadvantages associated with Salamon's relationship. Strain softening was incorporated into non-linear finite difference numerical modelling to estimate pillar strengths. The technique outlined above is used to obtain peak coal mass properties. Modelled pillar strength was found to be sensitive to both peak and post peak mass strength properties. Post peak properties were found to be difficult to obtain and required some back analysis. Modelling results were used to estimate both the strength of squat pillars and parameters for use in a now squat pillar empirical strength relationship.

Jan 1, 1992

By Frank E. Chase

Sacrificial entries, roof slotting, and other tactics have been used to combat high horizontal stresses during roadway development in U.S. coal mines. In Australia, the "pillar extraction on the advance" mining method has been successfully employed as a horizontal stress control technique. The concept of advancing and relieving mine workings through pillar extraction developed from observations that some mines experience difficult ground conditions when advancing and retreating the first panel in a new reserve. However, conditions dramatically improve in subsequent panels. Apparently, the creation of a gob area stress relieves the adjacent panel workings. After much debate and close scrutiny, the first U.S. roof control plan to advance and relieve was approved in 1998, for the Sargent Hollow Mine in Wise County, VA. The major concern was that this mining practice places section personnel inby of the gob. The tentative approval permitted barrier pillar slabbing and removal of the adjacent development pillar during panel development. The intent of the plan was to provide a "stress shadow" for the advancing faces and outby workings to reduce the excessive number of roof falls. Sargent Hollow successfully extracted their first panel by pillaring on advance and retreat mining using posts. Mobile roof supports were employed to extract pillars while advancing the second panel. This paper summarizes Sargent Hollow Mine's experiences with the advance and relieve mining method.

Jan 1, 1999

By X. L. Yao

This paper analyses the effects of mining extraction geometry (width/depth ratio) and dip of seam on subsidence parameters based on a large Database, containing 120 longwall mining cases all within the UK coalfields. The subsidence parameters evaluated in this paper include the following: the maximum subsidence value with different goaf-treatments, the position of maximum subsidence, the position of half-subsidence, subsidence value over the rib-side, positions of maximum tensile strains, positions of inflection points and the magnitude of maximum tensile strains on both rise and dip sides. Additionally, the maximum compressive strain and two limit angles are also studied in this paper. Based on a comprehensive Database analysis, several equations are derived relating these parameters to the width/depth ratio and the dip of seam. The results should assist engineers to design new structures, or take measures to protect existing structures from strain effects. The findings of the research assist in extension of the SEH to inclined seams with greater certainty.

Jan 1, 1990