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By Weijia Guo

With the exploitation of coal resources, a large area of land had been affected by surface subsidence, causing conflicts between man and land to become more conspicuous by the day, affecting the life of general public, and constraining further development of the land. This paper predicts the subsidence areas according to the 10-year mining plan of the Jibei mining district as an example. A plan of environmental management was developed based on sciences and practical economics.

Jan 1, 2006

By Kazem Oraee, Mehran Gholincjad, Navid Hosseini

"Chain coal pillars are pans of the structure of longwall mining system that play a significant role in the stability of the entries. With mechanization and developments in the various aspects of the method, higher efficiency in optimization of the design of chain coal pillars seems appropriate. In this paper the three main methods of chain pillar design, namely the empirical analytical and numerical methods arc compared. Real data from the Tabaslran coal mines have been used in order to make the comparison process reliable. It is concluded that the most apparent advantage of the empirical method is the reliability of the results while the use of numerical methods enjoys the advantage of flexibility. On the other hand the analytical methods are complex unless simplifying assumptions arc made that can substantially decrease the accuracy of result which is thought to be the main advantage of the design method. A new method is therefore introduced here that combines all of the three presently used methods and by doing so the new method has the advantages of the three while minimizing complexities and inaccuracies associated with the use of the individual methods.INTRODUCTIONLongwall mining is a high production method that can use mechanization to its full extent. Its initial development can be traced to 17th century European collieries but it was not until the second half of the 20th century with the development of self advancing support systems that the method gained popularity in the USA (Oraec. 2001). During the last decade, the production of longwall faces has increased significantly (Peng. 2006). The ability for mechanization along with good safety condition is the main reasons for such development. In the longwall mining method a good design for panel and entries on both sides, are two essential parameters. In this method, the thickness of overburden (depth of mine) nom1ally ranges from 60 to 20 m (197 to 2.7 9 ft) (Hartman, 2000) and with increasing mine depth designing for stabilizing the entries on both sides of the panel become more difficult. Thus, for safety and stability the size of chain pillar must increase. This causes the overall coal recovery to decrease and increase mining costs per ton. As such wide pillars hinder the air flow and hence ventilation costs increases too. Therefore with accurate identification of loading conditions and stress analysis. pillar that are a minimum size and sufficient safety factor are carefully designed."

Jan 1, 2010

By Yingda Zhai

The range of interburden thickness between coal seams in Datong Mining District of China is large. From mining operation point of view, the closer is between the two seams, the greater is the impact from the upper seam when the lower seam is mined. This impact is most notable when the two seams are ultra-close. In this paper the definition of ultra-close seams and its practical method of determination are presented. Based on the difference in strata thickness in the interburden and the concept of "yield depth ratio" which is defined as the yield zone depth in the interburden to the total thickness of interburden, the roofs of the ultra-close multiple seams in the Datong District of China were divided into 3 types: (1) false roof or rock partings in coal seam. This type of roof does not require special handling except mining them simultaneously as one single seam. The roof thickness is slightly less than 0.5 m; (2) completely broken roof. In this type of roof, the yield depth ratio is larger than (or equal to) 1. When the roof thickness is larger than 0.5 m but less than 2.0 m, the yield depth ratio must be smaller than 1; and (3) fractured roof. In this type of roof, the yield depth ratio is less than 1 while the roof thickness is larger than 2.0 m. In retreat longwall mining, if the gateroads of the lower seam are located under the gob of upper seam, roof bolting can be used in the 3rd type of roof. But if the 2nd type roof exists, powered supports must be used.

Jan 1, 2005

By Leslie Carver

The effects of longwall mine subsidence on water resources were studied at a West Virginia coal mine (mine Z) by Carver (1994). Data were obtained for 137 domestic supply wells and springs and 12 baseflow streams. Subsidence caused short-term dewatering (of a few days to weeks) of wells located on lower hillslopes and valleys, and long-term dewatering of hillslope and hilltop springs (with no recovery yet). New or increased spring discharge was observed at lower elevations. Unsubsided water supplies experienced no dewatering beyond a nearby panel's 27°-38° angle of dewatering influence zone. Subsidence from longwall mining beneath streams commonly reduced discharge when panel ages were less than two to three years. However, "normal" discharge was typically observed over panels greater than three years in age; such streams exhibited lower baseflow recession constants, moderated winter baseflow, and increased summer baseflow compared to unsubsided streams.

Jan 1, 1994

By Thomas L. Hutchinson

The mechanical design of a roof support is basically a matter of a statics and dynamics problem, assuming of course, that the imposed loads and mining conditions are known. Here in the Appalachian coal fields the general conditions are known well enough to allow the majority of the proposed new faces of longwall to be furnished with standard commercially available supports equipped with suitable variants and options. While not trying to negate the primary importance of thoroughly investigating a potential face and the selection of the correct equipment for that face, it still becomes clear that an equally important aspect of the design of an adequate ground control program has in the past too of ten been overlooked, having been overshadowed by the ?endless? selection process of the roof supports proper. The equally important aspect which this paper discusses is the overall design of a mining system, which design incorporates not only the size and capacity of the equipment but the following points as well, namely: Equipment adaptability to mining scheme, Equipment va. human constraints, Operation at the designed levels, and Coordination of operation, maintenance and support design. Unless the coordination of these factors is considered in the design and selection of the equipment, a lopsided and inefficient operation is possible. How can we afford to put an 8-10 million dollar investment up to chance as to whether it will yield optimum results or not?

Jan 1, 1981

By R. Karl Zipf

Highwall mining is an important coal mining method. Upwards of 60 highwall miners are presently in operation, and they may account for approximately 4% of total U.S. coal production. A review of the Mine Safety and Health Administration (MSHA) accident data over the 20 year period from 1983 to 2002 identified 9 fatalities attributable to auger and highwall mining of which inadequate ground control accounted for 1/3. In the past 5 years, 1 fatality occurred in highwall mining. Estimates of the manpower requirements in highwall mining suggest that its fatality rate is essentially the same as for surface coal mining, thus highwall mining appears to be a very safe modern mining method. Highwall stability is the major ground control related safety concern, and operators are required to develop and follow an appropriate highwall mining ground control plan. The plans usually specify the following geotechnical parameters: hole width, maximum hole depth, maximum overburden depth, seam thickness, web pillar width, barrier pillar width and number of holes between barriers. Calculated web pillar stability factor exceeded 1.3 for most designs evaluated. This study examined records from 5,289 highwail miner holes with a total completed footage of about 2,560,000 feet to understand the reasons for early pull out. Average lost footage is typically about 20% of planned footage. Only 35% of the holes reached planned depth, and 20% were short due to rockfalls. Water and adverse geology accounted for 15% of the losses. Mechanical/electrical problems, guidance, and slope stability problems accounted for the remaining 30%. Web pillar stability factor for these holes also exceeded 1.3 in 95% of cases. Trapped highwall miners present a safety hazard to workers during their recovery. Best practices to avoid trapped highwall miners include: avoid mining in stream valleys, avoid mining near outside corners, careful alignment of each hole and the use of an onboard guidance system, Several issues in highwall mining ground control require further investigation including highwall mining through old auger workings, highwall mining near old underground mines, multiple-seam and multiple lift highwall mining and finally the size and frequency of barrier pillars.

Jan 1, 2004

By Simon H. Prassetyo

Violent failures of coal pillars, known in practice as coal mine bumps, have long been a subject of investigation. Many field investigations have considered geological conditions that create high stress in the pillar to be the main causative factor leading to bumps. In recent years, constraint at interfaces has been observed to be associated with the occurrence of coal bursts. This paper shows how interface friction and width-to-height (W/H) ratio affect the degree of violence of coal specimen failure. A series of UCS tests on 178 coal samples from bump-prone (Utah, called UT coal) and non-bump-prone (West Virginia, called WV coal) mines were conducted. Three violent failure parameters? peak sound pressure level, core zone failure, and ultimate stress? were used to assess the violence of failure. The degree of violence were investigated at varying interface frictions (high: µ = 0.40, medium: µ = 0.22, and low: µ = 0.13) and W/H ratios (W/H = 2, 3, 4, 5, 6, 7, 8, 10, and 12). A simulation series of modeled coal specimen under uniaxial loading was also conducted using the finite element method to observe the equivalent plastic strain distribution experienced by the modeled coal specimens. It was found that the violence of coal specimen failure depends on W/H ratio and interface friction. Interface friction has more significant influence on the violence of failure at high W/H ratios than it does at low W/H ratios.

Jan 1, 2011

By John Shepherd

Pillar extraction carried out using various methods (split and lift, Wongawilli and old Ben) involves the formation of narrow fenders (commonly 7 m of coal) formed by driving "splits" for lifting off with a continuous miner. The strata above a split and fender adjacent to the goaf consists of a cantilever beam (usually >3 m thick) that extends from the solid coal and bridges across the fender. Lifting off the fender from the split in safe conditions depends upon the bridging capability of the cantilever and the fender strength. The nature and competence of the immediate and near roof, the support capability of the coal fender and the presence of faults and or joints can adversely affect the bridging behaviour of the cantilever. Insufficient knowledge of these parameters can lead to extraction under dangerous conditions. Remnants of fenders ("stooks") also support the cantilever and their dimensions are critical to obtain regular caving and goaf falls. Stook area, goaf roof span and roof stand-up time after lifting can be related and only a relatively small increase in Btook area of 10-15 m2 can lead to a significantly increased stand-up time, especially under a massive roof. Such conditions strongly influence the redistribution of abutment stresses and can create increased outbye pillar loading. Based on detailed underground geotechnical investigations, it is concluded that strata caveability, roof cantilever behaviour, fender H:W ratio and remnant stook size are all critical parameters for successful and safe pillar extraction.

Jan 1, 1992

By Gift Makusha

Surface subsidence associated with coal mining activities in the Mpumalanga Province of South Africa changes the natural environment in several ways, and current challenges for mining companies include rehabilitation of the natural environment and the prevention of further degradation. To monitor the spatial and temporal evolution of surface subsidence, traditional fieldbased monitoring approaches such as GPS and spirit leveling are employed at a number of locations. However, the resulting measurements are point-based, and frequent revisitations are necessary to map the evolution of surface subsidence basins over time. To address these limitations, differential interferograms derived from repeat-pass satellite-borne synthetic aperture radar (SAR) systems were tested for their ability to measure and monitor surface deformation. The SAR data was captured by the European ERS-1 and ERS-2 satellite and covered a timeframe between 2008- 09-15 and 2008-09-15. The resulting interferograms revealed several features indicative of surface subsidence. Ground truth data confirmed the presence of a subsidence basin, detected using differential interferometry techniques during the 35-day period between April 12, 2008 and May 17, 2008; a maximum vertical deformation of 3.2 cm (1.3 in) was recorded. Interferometric monitoring revealed an eastward migration of the subsidence basin between June 2, 2008 and September 15, 2008 with an additional 4.7 cm (1.9 in) of subsidence observed. This migration coincides with the advance of the working face of the mine during this period. Finite element modeling will be performed in order to model the rates and evolution of surface subsidence haloes as underground mining advances. These presented results demonstrate the ability of interferometric synthetic aperture radar techniques to measure surface subsidence as well as the monitoring of the evolution of subsidence basins over time. This implies that the technique could be included, together with traditional field-based surveying techniques, in an operational monitoring system. With knowledge on deformation rates and subsidence basin evolution, informed decisions on current and future infrastructure development can be made and remedial actions and prevention strategies can be formulated for the problems associated with environmental degradation.

Jan 1, 2011

By G. Skybey

The Directional Drill Monitor (DDM) is an advanced instrument for real time in hole surveying. It was developed by Du Pont (Australia) to suit Australian gassy coal mining conditions. The instrument consists of three units: i. Down hole transmitter ii. Acoustic transducer iii. Data receiver processor unit (i.e. up hole unit). This paper describes the function of each unit in detail and highlights the importance of the DDM in long hole drilling efficiency. Surveying is carried out independent of hole depth. Each survey takes 2-3 minutes to complete, the frequency of surveying to control borehole trajectory is determined by the accuracy required for a particular hole. The DDM is currently being used successfully for long hole drilling in a number of collieries. A high degree of directional control has been achieved. Borehole trajectories were controlled to within +l¬2m of the planned path with intersurvey distances varying between 6 to 18 metres over distances up to 700 metres.

Jan 1, 1992

By Joao Felipe Costa, Jair C. Koppe, Andre Zingano, Cleber Gomes

"Run-of-mine Brazilian coal has high ash content and low calorific power. Therefore, the coal must be washed to achieve the power plant specifications for burning coal. A large amount of waste results from the coal preparation, and it must be disposed of in waste piles or waste dams, depending on the material size. These waste disposals are the cause of serious environmental problems, not only in the short and medium terms, but mainly in the long term, due to the risk of provoking acid drainage or spontaneous combustion. Waste disposal requirements also demand large areas. Other problems include the pillar and roof stability in abandoned and mined out areas, building of edifices on the old and active mines, and mining under urban areas. To solve these problems simultaneously, backfilling the entries in mined out areas began with two objectives: to understand the mechanical behavior of the filling material and to model the backfilling of room-and-pillar coal mine entries through numerical simulation. Laboratory tests were performed for different filling material (with and without cement) to determine its mechanical properties and failure criteria. Based on this study, it was possible to conclude that the backfill method is a good strategy to improve the ground stability in room-and-pillar coal mines for the long term, especially for weak coal seams, and to reduce the quantity of material on waste piles on the surface.INTRODUCTIONRun-of-mine (ROM) Brazilian coal has high ash content and low calorific power. Therefore, the coal must be washed to achieve the power plant specifications for burning coal. Two types of waste are associated with power plants that bum coal for energy generation: 1) the waste from the coal preparation plant and 2) the ash from the power plant after coal burning.The Brazilian coal ash content in the ROM coal is between 45 and 55%. The clean coal is 20 to 30% ash for 4,500 kcal/kg (18,000 BTU). The coal preparation plant recovery is only about 35%. Santa Catarina State (SC) produces about 7 million tons/year of ROM coal, which results in a total of 4.5 million tons of waste. This is a large amount of waste that must be disposed of in waste piles or waste dams, depending on the material size. The waste and ash content is represented by silicate materials, such as clay and sand, and by sulfide materials, like pyrite and organic sulfur (between 1and5%)."

Jan 1, 2010

By Ashok Kumar, Rajendra Singh, Petr Waclawik

High-grade coal is locked in the shaft protective pillars at 900 m below surface in the Czech part of Upper Silesian Coal Basin. To optimise recovery of coal from the larger size shaft protective pillar, room and pillar (called bord and pillar in India) is trialled to further develop these on smaller pillars. This development followed a constant gallery width of 5.2 m and 3.5 m height. The monitored pillar size in the first developed panel varied from 860 m2 to 1225 m2 of area. The stability of these pillars (i.e., natural supports) at this depth of cover is observed to be a vital issue to control ground movement and safety of the surface structures. A simple calculation of safety factor revealed that application of an empirical method for estimating size and shape of the pillar is inappropriate due to complex geo-mining conditions of the site. Accordingly, geo-technical instruments are installed in the panel for monitoring the stability of the diamond shaped pillar at different stages of the development. Extensive monitoring of the pillar behaviour by these instruments provided an important set of data for such optimum recovery. An established process of simulation-based pillar strength estimation is often used by the mining industry, but calibration is generally done with the help of empirical formulation. However, for the deep-seated pillar, the strength suggested by the existing empirical formulations (except the CMRI formula) is found to be much different from the instrument observations. Using a study conducted at Czech hard coal mines, an attempt is made to develop a strain-softening-based numerical modelling approach to assess pillar performance at deeper cover. The recorded value and nature of the pillar spalling in the field along with the CMRI formula are used to calibrate the developed simulation approach. An assessment of the stability of the developed pillars using simulated models did not indicate any collapse of the natural supports. This paper briefly presents difficulties in pillar design at great depth of cover and field and simulation results of pillar strength estimation in Czech hard coal mines.

Jan 1, 2018

By B. N. Whittaker

The paper briefly reviews the type of support and stability problems encountered in mining tunnels in U.K. Carboniferous rock conditions, particularly the Coal Measures where almost all such tunnels are located. Stability prediction of mining tunnels is discussed and a computer-based model which deals with stratified rock conditions is described. Various schemes for monitoring the stability of mining tunnels are examined and the application of the results discussed. Comparisons between predicted and measured Closure values are presented for a major Coal mining tunnelling project at Selby in the North Yorkshire Coalfield. Conclusions are drawn concerning the application of the present prediction technique together with the importance of underground monitoring. Examples of different types of mine tunnel stability problems encountered in the U.K. Coal Measures are discussed and comments made on the secondary effects of tunnel peripheral rock weathering and corrosion of the associated support systems.

Jan 1, 1987

By WenFeng Li

Comparing to the coal seams in Eastern China, the coal seams in Western China usually have better geological conditions. Therefore, changes in the conventional gateroads layout are required for the Western longwalls. A case study based on a Western coal mine (LiMin) was presented to discuss the feasibility of an innovative gateroads layout. Based on the geological conditions, a 5m wide pillar between the adjacent two longwall panels, 09112 and 09113, was proposed in order to increase the recovery rate. More importantly, the entry supporting strategy of panel 09113 headgate needed to be determined and verified carefully since only 5m wide pillar was left to separate the entry from the panel 09112 gob. Three strategies with different supporting intensities in the panel 09113 headgate were evaluated by numerical modeling. The model outputs indicated that roof and rib boltings plus single hydraulic props were essential to maintain the gateroads stable. Field observation showed that under the proposed entry supporting strategy, the total roof-tofloor and rib-to-rib convergences were only 360mm and 460mm, respectively, during the service life of the panel 09113 headgate, which indicated that the innovative gateroad layout proposed in this paper is suitable for longwalls with similar geological conditions in Western China.

Jan 1, 2013

By Weidong Pan, Yinchao Yang, Baodong Zuo

Fully mechanized mining technology, with a large cutting height in thick coal seam, has been developed to be large-scale, mechanized, and intelligent. Furthermore, it is also superior to other mining methods because it operates more efficiently and has higher recovery rates of coal resources. Dongpang coal mine (located in north China) is one of the largest coal mining productions in Hebei province. It is significant because it is the first set of coal mines in China to adopt fully mechanized mining technology that has large cutting height potential. When looking at the coal seams of deep mining, specifically in the mine No. 2612 where the working face is 4.5–6.5 m cutting height, the coal walls have low stability due to the way the coal seam fluctuates in a large working face. This paper adopts mechanical analysis, numerical simulation, and field observation research methods to control the rib fall of coal walls in a coal mine. By adopting research methods that analyze the mechanical characteristics of face roof, we are able to conclude the main roof structure of the masonry seam stability criterion in overhead mining with large cutting height. This is done through employing the numerical simulation method, which helped to determine the main factors that influence the rib fall of a coal wall and weighting factor. Through field observations, we discovered the technology method to relieve, control, and mitigate rib falls of coal seam in a coal mine. We also discovered methods that would ensure the smooth mining of the working face of the mine by strengthening the bolting and grouting reinforcement methods used to improve the working face of stope surrounding rock conditions. This paper discusses research results that enrich the stope wall rock control method by expressing how fully mechanized mining technology, with a large cutting height, is meaningful to solve the complex conditions of rock fall caused by thick coal seam.

Jan 1, 2017

By L. Munsamy

The bulk of primary mining at the Anglo Coal's Bank Colliery was carried out by drill and blast method, in a bord and pillar layout exploiting the No. 2 Seam. Secondary extraction, top/bottom coaling, also took place using both the drill and blast (D&B) method and continuous miners (CM). Extensive mining in this seam resulted in the risk of sterilising the No. 4 Seam reserves. Therefore, multiseam design guidelines established by Salamon and Oravecz (8) were re-evaluated in light of the low safety factors and the low pillar width to mining height ratios of pillars in the No. 2 Seam. Two and three dimensional (2D & 3D) numerical modelling was carried out to quantify and verify the parameters that influence the multiseam mining. The modelling was conducted primarily in two dimensions (plane strain), with 3D models to confirm the validity of the 2D results. A panel risk assessment flow chart was developed to assess the panels with respect to pillar failure and multiseam mining. This flow chart was extended into the evaluation of risk of surface subsidence using the current rock engineering knowledge. The aim of this risk assessment tool is to assess all underground workings at Anglo Coal collieries simply and rapidly. The identification of high-risk areas would warrant further investigation and possible backfilling to reduce the risk of pillar failures ensuring that reserves in the No. 4 and 5 Seams above are intact.

Jan 1, 2004

By N. C. Saxena

Subsidence investigations in Indian coalfields have led to development of relationships between subsidence movement parameters and defining of safe limits of subsidence movements for surface properties. The maximum subsidence over caved workings was generally not more than 60% of extraction thickness underground while in case of hydraulically sand stowed workings it was generally not more than 5%. The relationships have been useful in extraction of more than 7-million tonne of coal from underneath and in the vicinity of surface properties.

Jan 1, 1988

By Kot F. v. Unrug

The depletion of thicker coal reserves has resulted in the thinner seams being economically attractive. Surface protection against subsidence limits the extraction ratio to 50% for the room and pillar method as a "no subsidence condition". That rule was developed for pillars with smaller width/height ratio because seams mined at the time were commonly 6 feet thick. Over time pillar ribs deteriorate around the pillar perimeter and thus decrease the area of the pillar. Using 50% extraction as the no subsidence condition in the squat pillar category (below 42 in with w/h > 7) causes unnecessary losses of reserves because squat pillars are stronger. In this paper a method is presented for design of long-term pillars. Based on the analysis of pillar deterioration and field observations, an increase of the "no subsidence" extraction rate to 60% for squat pillars is proposed.

Jan 1, 2001

By Ernest A. Curth

An estimated 49 billion tons or 29 percent, of the coal reserve base to a depth of 1,000 feet in the eastern United States fall in the 28- to 42-inch range. Often left out as a consequence of selective mining, it is a paramarginal source that will become increasingly important. At present, all active thin seam longwalls are located in the Eastern Coal Province, and a number of operations have been suspended due to the erosion of the market for metallurgical coal. Extensive premining studies, appropriate mine planning, reliable equipment that is designed with ergonomics in mind, adequate roof support, intensive training, and strong face discipline are the ingredients that provided a measure of consistency in production for the last 10 to 16 years, particularly in the Pennsylvania bituminous coalfield where single drum shearers dominate. Plows, which were found to be very productive in suitable strata in Europe, are used in the Pocahontas field. Low seam four- and six-legged shields have been introduced recently. The future holds automation at progressive levels. The first step is batch control with the benefits of quickly applied roof support and reduction of workmen exposure to environmental hazards.

Jan 1, 1981

By Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001