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By Jesse L. Craft

Field investigation of surface subsidence events associated with both active and abandoned underground coal mines in the United States has established criteria that enable the prediction and classification of mine-related subsidence. The key was establishing the role of geology in all types of subsidence and the recognition of beam subsidence in specific geologic settings. The classification is based on overburden thickness, variations in stratigraphy, topography and mine plan. Types of subsidence are Pit, Room, Sag, and Beam. Pit subsidence is associated with three geologic settings: shale overburden, seasonally wet unconsolidated overburden, and perched water table in unconsolidated overburden. Room subsidence occurs where the roof across the width of the room fails to the surface and a shallow depression develops. Sag subsidence develops where the pillars are failing in conjunction with roof failure. Overburden thickness is generally greater than 150 feet. Beam subsidence can occur in all terrains. It is associated with retreat mining and room and pillar mining where pillar crushing or pillar compression has taken place and barriers are left inplace. They occur as beam arch or one-sided beam subsidence. Beam arch failure occurs where mining has taken place on both sides of a coal barrier. One-sided beam failure is associated with retreat mining moving from a barrier towards the entry in hilly topography. This type of subsidence occurs where a hard, thick, rock unit is acting as a bridge supporting the overburden and outcrops on the hill side.

Jan 1, 1992

By R. Fraher

Hazard analysis utilizes interaction mechanisms in combination with geologic, structural and mining conditions to delineate areas of potential damage in seams subject to interaction. Using characterizations of conditions peculiar to multi-seam mining, such as innerburden thickness, geology and structures, initial interaction contour maps can be constructed. Geologic hazard analysis is then carried out on roof, pillar and floor conditions in the affected seam using lower seam rock properties and layout geometries in the overlying seam. Either safety factors or damage ratings can then be developed representing degree of hazard in the lower seam. The final hazard map shows both the aerial extent and magnitude of potential damage from interaction.

Jan 1, 1992

By Satyendra K. Singh

If the extraction is beneath massive roof strata, the roof tends to 'bridge' across large spans without failing, the caving behaviour is complex and irregular. Understandably, these give rise to many geotechnical problems with safety implications and involving sometime huge direct and indirect costs. Only two important problems, 'feather-edging' in pillar mining and `severe periodic weighting' in longwalling, are discussed in this paper in the background of geomechanics investigations in Australia at Blue Mountain Colliery and in South Bulga Colliery respectively. `Feather edging'- a major source of injury and fatal accidents in Australia and South Africa in such situations, refers to a phenomenon where the roof, at the goat edge, falls as a thin wafer of rock often suddenly, over-riding conventional timber breakerlines. The results of the first comprehensive, monitored trial in an Australian pillar extraction coal mine, suggests how the roof falls into feather-edging caving (generally) because of multiple shear-planes (interpreted) and how it may be alleviated. Because of complex fracturing behaviour of massive roof strata, periodic weighting at a longwall face is certain to manifest itself as 'yielding supports', heavy conditions', rapid convergence and in the worst case, as a cause of face stoppage. After briefly discussing the programme of instrumentation (in a chain-pillar and over the would-be goal) and results, this paper attempts to improve mechanistic understanding of the related geotechnical issues associated with the weighting behaviour and aims at establishing a 'threshold' value of weighting severity index (suggesting intensity of periodic weighting) with an objective to optimise operational controls before mining through potential (future) weighting areas.

Jan 1, 2000

By Christopher Mark

The Bureau of Mines is conducting research to assess the effectiveness of different chain pillar designs in maintaining gate entry stability. The study described in this paper was performed in a 1200 ft long, experimental headgate section which contained three pillar designs: - a three-entry, yield-abutment pillar' system, - a four-entry, yield-yield-abutment pillar system, and - a five-entry, all-yield pillar system. As the longwall mined by the experimental section, Bureau engineers monitored entry convergence, roof sag, and changes in roof quality. The results of the study indicate that the all-yield system performed at least as well as, and probably better than, the two abutment pillar systems.

Jan 1, 1988

By B. Tulu, D. Gearhart

A comprehensive monitoring program was conducted to measure the rock mass displacements, support response, and stress changes at a longwall tailgate entry in West Virginia. Monitoring was initiated a few days after development of the gateroad entries and continued during passage of the longwall panels on both sides of the entry. Monitoring included overcore stress measurements of the initial stress within the rock mass, changes in cable bolt loading, standing support pressure, roof deformation, rib deformation, stress changes in the coal pillar, and changes in the full three-dimensional stress tensor within the rock mass at six locations around the monitoring site. During the passage of the first longwall, stress measurements in the rock and coal detected minor changes in loading, while minor changes were detected in roof deformation. As a result of the relatively favorable stress and geological conditions, the support systems did not experience severe loading or rock deformation until the second panel approached within 30–50 ft of the instrumented locations. After reaching the peak loading at about two to three inches of roof sag, the cable bolts started to unload, and the load was transferred to the standing supports. The standing support system was able to maintain an adequate opening inby the shields to provide ventilation to the first crosscut inby the face, as designed. The results were used to calibrate modeled cable bolt response to field data and to validate numerical modeling procedures that have been developed to evaluate entry support systems. It is concluded that the support system was more than adequate to control the roof of the tailgate up to the longwall face location. The monitoring results have provided valuable data for the development and validation of support design strategies for longwall tailgate entries.

Jan 1, 2017

By Jonathan Duncan, Tim Burgess, William Kendall, Jens Lange

"Steel wire mesh is the accepted standard for roof and rib skin control at most coal mines in the United States. Wire mesh is delivered to the mine in bundles that are heavy and awkward to handle. The large size, stiffness, and weight of the wire mesh can present a safety hazard in transporting underground and storing on the roof bolter. Installing wire mesh on the roof and ribs with the roof bolter can be a challenge as well.Synthetic mesh is lighter than wire mesh and can be folded and rolled into a safe, manageable size. A single synthetic mesh roll large enough to cover the roof and ribs in a deep cut can be safely handled by two men. The roof bolter is designed to carry and dispense the roll as it advances and installs primary roof support, resulting in a single pass roof and rib ground control solution. This paper focuses on the design and implementation of the roof bolter synthetic mesh handling system developed by J. H. Fletcher & Co., Huesker, Inc., and Oxbow Mining.INTRODUCTIONRoof and rib skin control in some coal mines has been an issue since mining first began. Roof and rib skin problems vary from mine to mine. The type of roof rock material exposed after mining; the mine atmosphere; and stresses from depth of cover, over mining, and/or undermining are some of the variables that can contribute to roof skin problems from mine to mine.Potential roof and rib skin problems can be time dependent as well. Some mines have roof skin rock, sometimes called draw rock, that falls when drilling the hole for the roof bolt as the initial roof support is installed. Some roof skins are more stable but can fail over time. The failure of roof skin can even be between properly installed roof bolts with roof plates (Figure 1).Mines have most commonly used a wire mesh that comes in individual panels anywhere from 5 ft x 5 ft (1.5 m x 1.5 m) all the way up to 5 ft x 16 ft (1.5 m x 4.9 m) in size. They are typically constructed of 8- or 9-gauge wire welded on a 4 in (102 mm) square pattern. A 5 ft x 16 ft (1.5 m x 4.9 m) panel weighs approximately 35 lbs (16 kg). These steel wire mesh panels can make it a difficult and sometimes dangerous task to deliver them from the surface to the roof bolter and then store on the machine. Manipulating the mesh panels from the storage area on the machine to the final installation position on the roof or rib can also be a daunting task (Figure 2)."

Jan 1, 2010

By Kazem Orace, Navid Hosseini, Mehran Gholinejad

"Selection of the optimum support system for underground openings such as tunnels is a complex process. In this paper a new approach, based on a combination of the Analytical Hierarchy Process (AHP) the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and the Preference Ranking Organization METHod for Enrichment Evaluations (PROMETHEE), is introduced. For this purpose, the selection process is assumed to be a multi criteria decision-making problem. First different support systems using FLAC3D numerical code, based on technical safety and stability parameters of the tunnel are specified. Then taking economic and performance parameters as the decision criteria by using the combination of Al-IP, TOPSIS, and PROMETHEE the optimum support system is selected. As a real mine case study this approach is used in the main access entry to the C, coal scam of Tabas collieries. Results clearly demonstrate that the proposed support system selection method is more advantageous than other alternatives.INTROD CTIONGenerally, the support system for a tunnel is selected primarily with the aid of design engineers experience. In other words personal judgment instead of intellectual and scientific criteria is mostly the main basis (Oracc. 2005). However since various support systems can be applied in any particular situation, accurate selection of the optimum support method depends on the integration of many technical, economical and performance parameters, and also on the analysis of the intensity of influence by each of the criteria.In this paper an applicable approach for se lection of tunnel support systems based on Multi Attribute Decision Making (MADM) techniques including: Al IP (Orace et al.. 2009a; Saaty, 1980), TOPSIS (Hwang and Yoon, 1981: Yoon and Hwang, 1995), and PROMETHEE (Brans and Vincke. 1985; Brans and Mareschal. 1992) is introduced. As a field study, this approach was applied to tunnel C1 as one of the main entries in the Tabas coal mine which is the major collieries in the central pan of Iran (Hosseini, 2008).Due to the coal seam conditions coal is mined by mechanized longwall mining (Hosseini 2008). This method requires the 58 excavation of several entries some of which will be used for many years and even for the entire life of the mine (Peng, 2006). Therefore the selection of the support system in these entries is·very important in mining design."

Jan 1, 2010

By Peng Zhang

In a recent research effort, a laminated overburden loading model (as implemented in LaModel3.0 was compared with the results of ARMPS 2010, and the laminated overburden model was found to more accurately classify a ARMPS database of 52 deep cover case histories by a factor of 60% versus 54%. However, the LaModel analysis of each case study required several days to complete, as compared to a requirement of only several minutes for each ARMPS analysis. In the research presented in this paper, a computer code (ARMPS-LAM) has been developed to effectively implement the laminated overburden model into the ARMPS program. This program takes the basic ARMPS geometric input for defining the mining plan and loading condition and then automatically develops, runs, and analyzes a full LaModel analysis of the mining geometry to output the stability factor on the Active Mining Zone(AMZ), all without further user input. The present laboratory version of ARMPS-LAM can be run in batch mode; therefore, the ARMPS 2010 database of 645case histories can be analyzed very quickly. In an initial logistic regression analysis of this database, the ARMPS-LAM program was seen to be slightly more accurate than ARMPS 2010 by a factor of 71% versus 63%.

Jan 1, 2013

By Dakota D. Faulkner

Cable bolts anchored with traditional two component polyester resin cartridges have successfully supported difficult roof conditions in underground mines. Resin cartridges provide a costeffective cable bolt anchoring system with consistent anchorage performance characteristics when properly installed. The most commonly used cable bolts in the U.S. mining industry are 270 ksi (1,862 MPa), 0.7 and 0.6 inch (18 and 15 mm), seven-strand smooth cable with resin mixing bulbs (also known as bird cages), and 40- and 30-ton nominal capacity, respectively. However, there are other types of cable bolt anchoring systems aside from bird cages. Recently, many questions have been raised regarding the anchorage characteristics of various types of cable bolts. Keystone Mining Services, LLC (KMS), the affiliated engineering company of Jennmar Corporation, conducted a variety of controlled tests developed to closely model actual underground installations. These tests compared the performance of three commonly used cable bolt varieties, including plain, bird caged and indented seven-strand cable. Details of the laboratory testing and comparative analysis of the results are provided.

Jan 1, 2013

By Srisharan Shreedharan, Gang Huang, Hongqing Song, Sijing Cai, Pinnaduwa H. S. W. Kulatilake

"Subsidence prediction due to ore extraction and backfilling during open stoping operations for an underground iron mine in China is reported in this paper. Geologic complexities including faults and interfaces between different lithologies, and the stoping and backfilling sequence adopted from the mine plans are incorporated in building a 3-D discontinuum numerical model. The stoping was simulated in two vertically stacked horizontal layers (Fig. 6), with a total of 16 stopes. Large displacements of up to 50 cm were predicted by the model along the roof of the stopes, and a maximum surface subsidence of 22.5 cm was predicted by the model . Backfilling was found to control subsequent deformation and subsidence. The extraction of the upper orebody led to relatively higher deformations in extracting the lower orebody. Finally, the predicted subsidences at all locations along a subsidence profile on the surface are shown. INTRODUCTIONThe Luohe iron (pyrite) mine, located in Hefei city, Anhui, is an underground mine in China, being mined through the open stoping and backfilling method. The orebody lies at a depth of about 400 m and has an average thickness of 250 m (Magang Steel Company, 2012a). Discovered in the 1960s, the location has a reserve estimate of 300 million tonnes, which have been mined since 2007. The orebody has been divided by the mining company into an east and a west section to facilitate planning for ore extraction.Figure 1 shows the surface above the underground mine, which is seen to contain many human settlements including villages, pastures and other infrastructure. These locations are sensitive in that they need to be protected from potential adversities due to the underground mining, such as surface subsidence. While it may be near impossible to completely eliminate the subsidence due to ore extraction, operations such as backfilling have been adopted by the mine in an effort to substantially reduce it. Hence, this paper focuses on describing the stability of stopes in the mine and estimates future subsidence in a part of the mining region through numerical modeling."

Jan 1, 2016

By Ismet Canbulat

"In order to ensure the stability of roadways, Anglo American Metallurgical Coal (AAMC) has developed a roof support design methodology that integrates analytical, numerical and empirical modelling. This methodology currently is based on a deterministic approach (a single factor of safety against failure is calculated). However, an improved methodology, based on stochastic modelling technique, has also been developed and currently being evaluated. The main advantage of this methodology is that the design is based on the probability distributions of input parameters; thus the outcome is based on a distribution of factors of safety rather than a single factor of safety. An evaluation of factors of safety may also be used to determine the likelihood of failure, which in turn may be utilized to quantitatively determine and evaluate the associated risks in decision making process. This methodology has been evaluated at Grasstree and Moranbah North Coal Mines in the designs of roof support in various critical areas and has been proven to be successful and a better way of determining the roof support requirements.This paper demonstrates the application of this methodology in a design of an install road roof support at Moranbah North Mine, where the 1,750 tonne capacity ''world's highest rated longwall"" has recently been installed.INTRODUCTIONAnglo American Metallurgical Coal Australia (AAMC) operates three longwall (LW) mines located in Central Queensland. There is an increasing emphasis on the reliability at these operations, as the longwalls are getting deeper and facing more geologically challenging conditions. In addition, the Anglo American Vision is to achieve ""Zero Harm"" through the effective management of safety at all businesses and operations. In order to accomplish this vision, AAMC has developed a proactive ground control management system for a safe and efficient production of underground reserves.Proactive ground control management involves understanding the impacts of the geotechnical environment on likely ground behavior, and it consists of approximately 15 major elements. One of the most essential fundamentals of proactive ground control management is to utilize a roof support design methodology that considers different failure mechanisms and also takes into account all important elements. The design methodology becomes even more important when the area in question is a critical area, which is defined as a high risk roadway where any failure may cause increased levels of safety and financial risks to underground workforce and operations."

Jan 1, 2010

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013

By Bob J. MacDonald

The Cape Breton Development Corporation is a federally owned Crown corporation in Canada that operates two coal mines the Prince and Phalen Colliery in the Province of Nova Scotia. Retreat longwall mining was first introduced to the shallower workings (I 80m) of Prince Colliery in the early 1980's. Phalen Colliery started longwall retreating in 1987 and since that time has extracted twelve panels. In 1993 there had been a total of eight longwalls varying in widths from 140-230m mined in Phalen Colliery that is on the Phalen Seam. Up to this point there were only minor ground control problems on the faces except for one of the earlier longwalls that was mining a split seam. There is some interaction on Phalen workings from remnant structures in the overlying Lingan and No. 26 Collieries which worked the Harbour Seam. Induced abutments loads along pillar edges have been observed and resulted in poor face conditions when mining the Phalen Seam. The ninth panel, 6 East was the first panel to suffer a major roof fall from the First Major Weighting. This was also the first panel to undermine a paleo sandstone river channel named the Lower Sandstone Unit (LSU). For the first time in Phalen's history subsequent longwalls began to experience periodic weighting events. The 6 East panel suffered a number of periodic weightings and on two occasions had to use foam cement to fill the cavities that resulted. On the next east side panel, 7 East, there were numerous periodic weighting events, but interestingly there was no significant First Major Weighting. 7 East was the first 260m wide panel and all others were 230m or less. This longwall was 3,400m long and began producing in December of 1994 and was still cutting coal in May of 1997. There were five significant roof fall events on this panel, three of which required foam cement to fill the cavities. Of these three the interruption to production varied from four days to seven months. There were definite precursors to each of the weightings including: 1) an increase in audible noise (termed bumping) on the face, 2) supports went rapidly into yield, 3) roof convergence increased, 4) water percolated from the face roof strata, and 5) severe spalling of the coal occurred resulting in increased tip to face distance. Longwall face weightings are a result of geomechanical factors and cannot be avoided However, the loss of roof control is a different matter and highly subjective to operational procedures. The causes of weightings, their magnitude and impact on the mining at Phalen have been investigated. The following factors have been identified as relevant to these occurrences. 1. The influence of pillars in the overlying Harbour Seam workings. 2. Location of the LSU relative to the Phalen Seam. 3. Thickness of the LSU. 4. Pressure arch effect from the adjacent gobs. 5. Poor operational practices on the longwall including: a) slow rate of retreat; b) inadequate supply of pump pressure to the hydraulic supports; c) premature yielding of the yield valves; and d) poor face alignment practices. This paper will review the events that took place on the 7 East longwall and show that the Lower Sandstone Unit is the primary cause of the weightings. However, other factors contributed to the loss of roof control and the formation of the roof cavities.

Jan 1, 1997

By Yuanping Cheng, Jingyu Jiang, Qiang Zhang

"With the increase of mining depths, deep-seated coal seams face threats from high ground stresses, high gas pressures, high gas content, and low seam permeability, which significantly increase the likelihood of coal and gas outbursts. Challenges continue to grow in coalbed methane (CBM) extraction fields, especially for short-distance gassy multiple-seams. Research has proved that the most effective way to increase coal seam permeability is to decrease the ground stress. Protective seam mining technology is a surrounding rock control technology, which can effectively reduce the ground stress and increase the permeability of the protected coal seam. In this paper, we try to use upper protective seam mining technology to simultaneously extract coal and gas. Findings are based on a FLAC3D simulation of stress variation during the mining of the No. 12 coal seam in the Daxing coal mine, Tiefa Coalfield, China. Results indicate that, after mining the upper protective seam, the pressure-relief effects of the rock mass generated three belts and five zones below the protective seam, providing the space and time for gas pre-extraction engineering. Within the protected area, the numerical simulation indicates that the stress of the No. 13/14 coal seam reduces 95.84% and 88.25% respectively, while the expansion deformation of the protective seams reach 6.87 ‰ and 6.70 ‰ respectively, which demonstrate that the protective mining technology can provide a significant pressure-relief effect to the outburst-prone coal seam. Combining stress relief in this manner with pressure-relief from engineered gas drainage (performed by crossing boreholes from the floor rock roadway) applied before the mining of the protective seam, it would not only be beneficial to CBM drainage but also can reduce the gas outburst risk of the protected seams.INTRODUCTION Internationally, since France used protective seam mining technology to control coal and gas outburst in 1939, it has been promoted and applied in many countries (?????, 1966). Since 1958, China has taken the mining test and engineering practice of protective seam mining technology in the Beipiao, Tianfu, Nantong, Zhongliangshan, Songzao, Huainan, Huaibei, and other coalfields (Yu, 1992). Long-term theoretical research and mining practice of outbursts and dangerous coal seams show that protective seam mining technology is the most effective, safest, and most economical outburst prevention measure (Cheng, Fu, and Yu, 2009; Hu et al., 2008; Cheng and Yu, 2007). Under the condition of high gas content coal seams, the safety and efficiency of simultaneous extraction of coal and gas can be realized through a reasonable mining sequence and an effective gas extraction method (Cheng and Yu, 2003). The in situ stress distribution of the protective seam (No. 1 coal seam) and protected seam (No. 2 coal seam) during protective seam mining in the Haishiwan coalfield formation (Figure 1) (Li et al., 2014). A comparison of the stress and displacement (five zones and three belts) of the rock mass below the protective seam, and in a multiple-seam context in the Dalong Mine, is shown in Figure 2 (Jiang et al., 2015). After mining protection coal in a close-distance seam group, the small spacing between the coal seams causes the protective coal and the protected coal between part of the rock to crack through each other until the protective layer of gob is through the area. This provides a desorption of gas to the protective layer of the mining space channel, making the coal seam permeability increase to several hundred times and improving the gas discharge capacity (Zhai, 2008)."

Jan 1, 2017

By Anand Bhagwat

Surface control can prevent the falls of ground between primary roof support systems and rib support. Using existing industrial products, and adopting them for mining applications, has historically attained surface control in underground mining. However, the latest innovative solution, a revolutionary wire mesh product, is designed specifically for underground mining applications jointly by Bekaert and Orica. This knotted wire mesh, FlexKnot? 2500 and FlexKnot? 3500, uses smaller gauge, higher tensile strength wires, than a chain link fence or welded wire mesh. The higher tensile wire enables the mesh to achieve greater ultimate load-bearing capacity, with significantly lower overall weight. Test results, obtained at the National Institute for Occupational Safety & Health (NIOSH) Mine Roof Simulator facility, illustrate the comparisons of the new FlexKnot with traditional 8- and 10-gauge welded steel mesh. The FlexKnot is superior in deflection, load-bearing capacity, and stiffness?categories that are critical in achieving safer surface control. It also offers enhanced corrosion resistance because of the galvanized coating applied on the individual wires prior to the knotting process. Case studies from underground coal mine installations confirm that FlexKnot has the ability to significantly reduce installation cycle time and to minimize use and repetitive manual handling of individual mesh panels. An added advantage is the ability to supply FlexKnot in rolls for quick and easy handling for either rib or roof support application. When using rolls, the bolter can install the entire roll with one continuous forward motion, unlike the wasteful back and forth travel for mesh panel.

Jan 1, 2014

By Keith Heasley

This paper is a retrospective on the development, application, and enhancement of the LaModel program from a personal perspective. The narrative starts with an explanation of the objectives behind the original development of the program and then introduces some of the unique and very practical features of the program. Next, a demonstration of how LaModel was advantageously used to model stresses and displacements in a multiple-seam mining situation with highly variable topography is presented. This is followed by the details of a number of the major enhancements and expansions of the program over the years. A back-analysis of the Crandall Canyon Mine collapse is then briefly presented, along with the associated development of a recommended calibration procedure for the critical input parameters to LaModel. Finally, some of the present research areas are briefly described, and future enhancements are introduced.

Jan 1, 2011

By Bowler Jonathan, Lorraine Kent, Tereza Crisu, Stuart Walker, Nicholas Lightfoot

"It has been the practice in Europe to extract the most economic seams first: taking the seams sequentially, top down, to avoid operating within the fractured strata formed by the subsidence trough above previously mined coal seams. In the 1990s, the UK coal industry found itself at a number of sites having to operate over one or more previously mined seams. This led to specific geotechnical challenges with respect to maintaining economic, safe and stable rectangular roadways with rock-bolted support.Within this paper, the current challenges and mitigation strategies of overmining in the UK are described following the completion of a -three-year European Commission-funded project in 2015. The challenges and mitigation strategies are related to:• A case study with overmining of older workings of variable geometry;• Numerical modeling for overmining layout and support design; and• A case study site planning mine layout when considering future overmining.INTRODUCTIONAt the first case study site, case histories were examined for a mine operating at a depth of approximately 700 meters where five longwall panels were extracted above workings 35-40 m below. The overmining of various extraction geometries below led to challenging conditions in areas of high vertical stress concentration. It was found that results from one longwall panel could be suitably used to predict behaviour in a subsequent panel. The experience was used to update previously existing guidelines on the positioning of roadways in relation to subsidence troughs. Existing guidelines recommended that only as a last resort should longwalls be planned to retreat over narrow pillars, or from solid ground into a subsidence trough. Existing guidelines regarding the layout of overmining panels are conservative, and new work indicates that such multi-seam geometries are possible with appropriate support management strategies."

Jan 1, 2016

By Khaled Morsy

Longwall mining is one of the most widely practiced underground coal mining method. The behavior of the longwall gob is very critical in the understanding of the complex ground response to longwall mining. In the limited data available, the values of gob moduli used in numerical modeling ranged from 1,000 psi to over 300,000 psi. Such a wide variation in moduli would greatly affect the results of the numerical analysis. A better understanding of the behavior of the gob will allow numerical models to be more accurate for simulating longwall mining conditions. In this study, a gob model based on the Terzaghi's model was incorporated into the comprehensive finite element package, ABAQUS, to simulate the loading behavior of longwali gob material. A case study was used to verify the proposed gob model.

Jan 1, 2002

By P. Forrest

Based on the results of mathematical analysis, photoelastic modelling, and case studies, the mechanisms of longwall under- mining interaction has been thoroughly investigated. The effects of using yield pillars in combination with rigid pillars in lower seam mine design have been found to provide significant improvement in ground conditions during undermining. Off- setting of solemnized longwall panel entries relative to upper-seam remnant structures has proven to be effective in alleviating adverse stress conditions.

Jan 1, 1988

By J. Marc Coolen

Marrowbone Development Company operates a large drift mining complex in the central Appalachian coal field. In 1997, five continuous miner supersections produced close to 9 million tons of raw plant feed. All production is from the Coalburg seam, a 5 to 10 ft thick coal seam with multiple coal benches and shale binders. The immediate roof lithologies consist of laminated shales, sandstones, or a mixture of sandstones and shales. The mine area is bisected by the regional Warfield anticline and associated Warfield fault. Mining conditions vary considerably due to frequent changes in seam and roof geology. The following geological categories can be distinguished: 1) presence of coal riders in the immediate shale roof, 2) excessive seam slopes around the Warfield fault, 3) splay faults associated with the Warfield fault, 4) zones of abundant slickensides, 5) abrupt roof lithology changes (sandstone - shale), 6) splitting and merging of different benches of the Coalburg seam, 7) sandstone washouts, 8) excessive roof dilution, 9) pillar sizing problems due to seam anomalies (soft fireclay binders). Several examples of these features are discussed in detail. Also, their relationship to roof fall occurrences is evaluated. Early recognition of potentially adverse geological conditions is of prime importance for the economic viability of the mining plans. Detailed core drilling in advance of the mining faces and coordination of the core data with underground mapping are the main forecasting tools.

Jan 1, 1999