how does longwall mining work

Furthermore, the stoppings, which are generally hung from the roof, are not secured at the bottom, in case machinery and coal must be transported from one side to the other. The coal is cut from the coalface by a machine called the shearer (power loader). Dinsdale refers to the formation of minor and major pressure arches in underground excavations. Injuries by location A dot map showing the approximate location of nonfatal lost-time injuries by sector. In superpositioning, the longwall face has the advantage of operating in the destressed area but the gate road pillars are subjected to potential stress concentrations. One should be cognizant of the fact that this analysis only considers stress and does not take into account the strata displacements and fracturing that subsidence will create. Coal-mining technology in China - ScienceDirect Studies indicate that physical properties of the strata, namely high elastic modulus, are a prominent feature in shear failure. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle on to the collapsed goaf. For very wide openings, such as those created by longwall mining, major pressure arch formation is likely to create points of excessive pressure in seams above or below. In both these cases, the operators were able to incorporate previous knowledge in the design process and planned accordingly when room-and-pillar workings were encountered. As a result, the overlying isolated gate roads may be positioned at various locations across the panel width, either at center or toward the tailgate or headgate sides as shown in figures 31A through 31C. Holland was the first to propose the concept of yielding pillars for ground control purposes. Although the yield pillar approach seems to be the optimum method for controlling longwall interactions, designing a gate road system that utilizes all yielding pillars is a complex problem. Face haulage systems must be designed to handle large, instantaneous production from the cutting machines, whereas the outer haulage systems must be designed to accommodate such surges from several operating faces. In the conventional system, the unit operations of undercutting, drilling, blasting, and loading are performed by separate machines and work crews. (May 6, 2013) http://epcmworld.com/news/feature-interview-safety-in-underground-mining, Mine Safety and Health Administration. Coal haulage, the transport of mined coal from working faces to the surface, is a major factor in underground-mine efficiency. (May 6, 2013) http://www.ilcoalassn.com/underground.html, Kentucky Coal Education. Superpositioning may also be required near the perimeter of a coal property to maximize recovery. In some cases, complete collapse of the overlying strata during extraction eventually travels to the surface, resulting in surface depressions. Compliance Guide for High-Voltage Longwall Regulations 30 C.F.R. Parts Under this condition, the magnitude of stress transfer to the longwall face would be maximized, thereby making it easier to distinguish stress trends. Also, displacements due to shearing are less likely to occur. The physical characteristics of the interburden are equally important, as the magnitude and distance of load transfer is largely dependent upon the thickness, stratification, and degree of fracturing. 33A and 33B) with the normalized peak stress (figs. Longwall mining of multiple seams has dominated European coal production for decades. Panel belts have widths ranging from 90 to 150 centimetres, the wider belts being used with longwall panels. However, when a seam exceeds these thicknesses, its extraction usually involves dividing the seam into a number of slices and mining each slice with longwall, continuous, or conventional mining methods. Surface mines excavate from the top down, a method that can become inefficient at depths greater than about 200 feet (60 meters) [sources: Illinois Coal Association, De Beers]. Final Report - Effects of Longwall Mining - Department of Environmental In this case, interactions in underlying or overlying operations may experience high stress in the compressive zone of the arch as depicted in figure 9. The primary concern is the different geologic settings in the United States and the United Kingdom and the behavior of overburden and interburden strata. The side abutment load is represented as the wedges of strata defined by the abutment angle as described by Mark and shown in figure 16. History [ edit] This phase of an underground mining operation is termed mine development. Often the extraction of coal from the seam during mine development is called first mining; the extraction of the remaining seam is called second mining.. Longwall Shearers Selection Guide: Types, Features, Applications Maleki concluded that a two-entry system that utilized one yield pillar may provide stability for both mines. The normalized peak stress for the headgate and tailgate locations is. Haycocks used photoelastic models to further investigate the pressure bulb concept under anisotropic conditions representative of various geological conditions. The report (IC 9305) is available from the USBM upon request. For recovering coal from the pillars themselves, many methods are practiced, depending on the roof and floor conditions. "Longwall Mining." This method was empirically developed from room- and-pillar and longwall workings for categories 2 and 3 above. For instance, the panel width in each seam may differ, as the current trend for longwalls is increasingly wider panels. Design and geologic factors will be further addressed as they relate to these two types of subsidence failure. Gravity then takes over, as the ore above the blast site fractures and collapses in succession as support is withdrawn. Cohesion between the individual beds is also an important consideration for it determines if slippage will occur along bed interfaces, leading to differential movement and bed separation. The angle of failure has been observed to range from nearly vertical for thick sandstone units to 25 for highly bedded strata, and it appears to coincide with the shear or caving angle discussed previously. The primary function of the gate road pillars is to maintain the stability of the headgate and tailgate entries as the longwall panels are mined. The mechanics of stress transfer between workings has been analyzed extensively through case study documentation and the use of mathematical and photoelastic models. lower mine to withstand this additional load is the primary concern. The coalbeds were also assumed to be flat-lying deposits with no dip. Earth: Scientists Seek Ban on Mountaintop Mining, Britannica Kids. Peng estimated the height of the arch from 30 to 50 times the seam thickness. This analysis was developed for perfectly elastic, isotropic, and homogeneous materials. Do it fast enough, and the reward is a coin. [18], Many ecosystems rely on the annual consistency of water inputs and outputs, and disturbing these patterns can result in unsustainable conditions for species reliant on water for species reproduction. shear stress in the roof and floor of the lower seam better than conventional pillar designs can. Longwall Mining is currently the main method of underground coal mining in Australia. Access to the coal seam is gained by suitable openings from the surface, and a network of roadways driven in the seam then facilitates the installation of service facilities for such essential activities as human and material transport, ventilation, water handling and drainage, and power. Where the coal seam does not outcrop but is not far below the surface, it is accessed by driving sloping tunnels through the intervening ground. Simultaneous extraction can lead to obvious production and scheduling problems. Studies indicate that this zone is generally three to six times the mining height depending on the expansion ratio or bulking factor of the rock. The degree of fracturing or natural jointing of the rock also affects its elastic modulus. Two mine design factors may be responsible for these large interactive distances. Eight of the seventeen mines report experiencing ground problems because of this condition. "Underground Mining." Peng implicitly called it a caving angle and based on observation estimated its range from 15 to 35 with an average of 25 applicable for most conditions. Those activities which are essential to maintain safe and productive operating conditions both at the working faces and in all parts of the mine are known as auxiliary operations. Rather than drilling through the ore deposit, a longwall machine cuts across it, shaving off slices up to 600 feet (182 meters) long. A conservative method for estimating entry location is to determine the outer limits of the subsidence trough based on the angle of draw. In the U.S., the mining industry saw thousands of deaths from accidents each year in the early 1900s. The results of this analysis are shown in figure 33A. He noted the shear-tensile failure in its development and estimated its range from 5 to 15. entire unit. Factors of primary importance are the number and thickness of the individual beds, the relative stiffness of each stratum, the presence of natural fractures or joint systems, and the cohesion between individual beds. Oct. 14, 2010. There are two obvious drawbacks to this arrangement related to recovery. At either end of the main body are fitted the ranging arms which can be ranged vertically up down by means of hydraulic rams, and on to which are mounted the shearer cutting drums which are fitted with 4060 cutting picks. As shown in figure 10, the subsidence trough is identified by distinct zones of tension and compression. 18), the degree of subsidence damage in the roof, floor, and coal will determine the difficulty of mining. Forrest also studied the use of yield pillars for controlling multiple-seam longwall interactions. The gate entries of the longwall face are formed as the coal is mined. The elastic modulus indicates the stiffness of the rock and its ability to deform. If the gate roads are offset, load from overlying pillars is then transferred to the longwall panel and face supports and they must contend with the additional load. These entries facilitate the movement of workers, coal, and equipment as well as provide ventilation and escape in emergencies. There are two basic options to consider for conventional pillar designs. Advancing Longwalls | Introduction | underground COAL In this method, a number of parallel entries are driven into the coal seam. This research showed that: Haycocks concluded that pressure bulb theory is useful in analyzing pillar load transfer when a passive interaction occurs. These factors are as follows: first, the sequence or order in which the seams will be mined, which will determine the type of interaction; second, the design of the gate road pillars, which will define the magnitude of the interaction; and third, the layout of the gate roads and longwall panels, which will define the location of the interaction. This page is archived for historical purposes and is no longer being maintained or updated. Real-time, intelligent connectivity. Successful demonstration of pillar recovery for single-seam mining has been documented in several case studies. Improvements in mine planning and design that would control or eliminate multiple-seam interactions present a major challenge to the mining industry. In situ horizontal stresses also have a negligible effect on the downward transfer of stress. While safety has definitely increased in developed countries, it still has a long way to go in some developing countries. Modern Longwall Mining. Seams are longwall mined in an ascending order, with mining completed in the lower seams before any mining is initiated in the upper seams (fig. The roof is supported by mechanized, self-advancing supports called longwall shields, which form a protective steel canopy under which the face conveyor, workers, and shearer operate. The only other machinery used was an electric cutter to undercut the coal face and electric drills for blasting to drop the face. This method can be used in wider deposits, as well, by drilling two adjacent access ramps and cutting two adjacent drifts, often called drift and fill [sources: Mining Know-How]. As the angle increases, the graphs begin to vary, and the 60 angle of approach best illustrates the difference. Studies involving subsidence measurements over longwall panels have shown that additional surface subsidence may occur over a mined-out panel as an adjacent panel is being mined, King referred to this as a triggered subsidence. This concept can also be applied to multiple seams when mining above a mined-out panel. The term is used in longwall mining where a panel is a rectangular block, or pillar, of coal, the size of which is determined by the surrounding workings which is ultimately completely mined. The longwall retreating mining method has been popularized, and production from longwall faces account for about 92% of the total. the operator must decide how to distribute the stress across the face: Either encounter the peak stress all at once, the 0 case, or distribute the peak stress across the face, as in the 60 case. [6], Typically, intake (fresh) air travels up the main gate, across the face, and then down the tail gate, known as 'U' type ventilation. Improved conditions should result if the center supports, rather than the supports at the panel edge, contend with the additional stress. The vertical. King referred to the angle as a shear angle, and he estimated its value at 25. But even in this situation, pillars still must be larger to account for the transfer of stress. Two- or three-entry gate road systems that utilize yield pillars that tend to crush-out when both panels are mined may reduce interactions. Main ventilation stoppings and air crossings, on the other hand, are constructed of brick or blocks and coated with mortar; the fans, regulators, and doors are also of substantial construction. Since the load is not uniformly distributed, the pressure bulb becomes distorted and can transfer stress to greater depths than in the uniformly loaded case. Many variations exist in the manner in which the complete seam is extracted. At one site in northern West Virginia, the USBM moni- tored a series of longwall panels in the Pittsburgh Coalbed, remote from previous high-extraction mining. Since very little empirical information exists to assist operators with multiple-seam longwall design, numerical models provide a tool for initial design. He assumed that this angle defined a failure plane where the weight of the overburden was evenly distributed between the pillars and the gob. Therefore, the pressure bulbs associated with these features can be transferred over much greater vertical distances. In comparing these two sets of graphs (mining above versus mining below the boundary), there appears to be little difference in the average face stress produced. The first design consideration has been recently addressed in USBM IC 9305, in which procedures were established for properly designing lower seam gate road pillars. There were 93 operating longwall faces in 1991, with the average width being approximately 715 ft. Based on this figure, a panel width of 700 ft was selected for analysis in the model. Empirical studies that document interactions involving superpositioned gate roads and recommendations for safe pillars sizes are lacking, but designs based on numerical methods have provided insight into the problem. Second, individual strata that characterize the overburden and interburden cannot be represented, so a generic modulus is chosen to depict the overall lithology. The MULSIM/NL model was used to evaluate two design considerations related to the average vertical stress across the longwall face for this situation. Joy longwall systems incorporate shearers, roof supports, face conveyors and more into one system that can be tightly orchestrated by advanced electronics including cutting-edge automation. [13], There have been cases of surface subsidence altering the landscape above the mines. For example, mainline belts are laid for the life of the mine and are much wider and faster than intermediate belts. Updates? However, as pillar size decreases the contours will be increasingly distorted under the same strata inclination. If the abutment pressure exceeds the pillar strength, the arch will fail, but another arch may form if the excavation is sufficiently deep. In this region, strata may separate along bedding planes and fracture or joints may open, but the individual beds tend to remain intact. Mine planning and coordination should be a primary consideration to minimize interactions between multiple-seam longwall layouts. This figure was chosen because it represents an average operating height from the 1992 Longwall Census. Several case studies document the effects of room-and-pillar workings on longwall operations. [23][24] This mine expected to have carbon dioxide emissions of 17 megatons per year; however, a carbon cap was placed on it by the Canadian federal government to keep the emissions at 500,000 tonnes per year.[24]. Capital investment for coal haulage in a shaft or a slope is somewhat similar, but operating costs are generally higher in a shaft, owing to the noncontinuous nature of shaft coal-handling facilities. If conditions are such that the roof will not cave or subsidence to the surface is not allowable, it will be necessary to backfill the void with materials such as sand, waste from coal-preparation plants, or fly ash. For example, the self-contained self-rescuer (SCSR) represents a significant development in raising a miners chances of survival and escape after an explosion, fire, or similar emergency contaminates the mine atmosphere with toxic gases. Corrections? These studies indicate that longwall interactions can extend over greater vertical distances than room-and-pillar interactions do. The fifth factor, stratigraphy, can influence the other four factors that determine subsidence damage. Of these, depth and interburden thickness are the critical factors influencing stress transfer. Proven methods for designing conventional gate road pillars have been presented by several researchers. "[15]:2 A 2005 geotechnical report commissioned by the Roads & Traffic Authority warned that "subsidence could happen suddenly and occur over many years".[15]. The strata throughout the subsided area tend to sag uniformly and mining should encounter virtually no problems. For instance, downtime on the longwall face of the upper mine will delay mining of the lower face until these problems are corrected. The entries are connected at intervals by wider entries, called rooms, that are cut through the seam at right angles to the entries. Oklahoma advancing longwall mine c.1917; arrows show airflow, West Virginia retreating longwall mine c.1917, "Longwall Mining, Underground Coal Mining, Mining Techniques, Mining Companies", "Study measures the environmental impacts of longwall mining subsidence", "Final Report - Effects of Longwall Mining", "Longwall mine plan a threat to water supply", "Risk to life, but more mining under bridge", "IMPACTS OF LONGWALL COAL MINING ON THE ENVIRONMENT IN NEW SOUTH WALES", "Fact sheet - Subsidence from longwall coal mining | Independent Expert Scientific Committee on Coal Seam Gas and Large Coal Mining Development", "Potential Management and Rehabilitation Requirements of Environmental Effects From Longwall Subsidence on Streams, Lakes and Groundwater Systems", "Assessing Longwall Mining Impacts on the Forests Above Landsat Science", "Prediction of Longwall Methane Emissions: An Evaluation of the Influence of Mining Practices on Gas Emissions and Methane Control Systems", "Countries with the biggest coal reserves", "Initial pitch for Murray River did not use longwall mining", "B.C. Interactions between longwall operations that use conventional pillar designs depend primarily on the depth, the interburden thickness and composition, the panel width, and the pillar size. Two theories have been developed to explain interactions due to load transfer from overlying workings: pressure bulb theory and arching theory. This relationship is best shown by comparing figures 33A and 33B using the 0 angle graphs. Longwall Mining - an overview | ScienceDirect Topics Additional mining in concentrated areas continuously move these water flow paths, which take years to return to their original states. Longwall Mining Method and Design - 911 Metallurgist The high productivity being achieved by longwall mining demonstrates its potential for being a substantial segment of underground coal production. Does longwall mining impact the value of properties scheduled to be undermined? [8], Longwall and room and pillar methods of mining can both be used for mining suitable underground coal seams. Hsuing considered the properties of the roof and floor in his designs. Geologic discontinuities, such as clay veins, are more likely to cause localized instability in the workings than to have an effect on the overall transfer of stress. The research recommends that stability factors range between 1.3 and 1.5 after tailgate loading to maintain stability. The longwall panel (the block of coal that is being mined) is typically 34km (1.92.5mi) long (but can be upto 7.5km (4.7mi) long) and 250400m (8201,310ft) wide. As illustrated in this figure, if similar-sized pillars are used in both mines the outer entries in the upper mine will be positioned over the lower mine gob or in the tension zone. In the past, coalbeds in the United States were mined in no particular order with regard to controlling interactions between operations and reducing associated ground control problems. The region from 6 to 12 times the lower seam mining height is an area of partial cave. Breathing in welding fumes, radon or mercury (often found in mines) also causes respiratory diseases. Oct. 13, 2010. Many changes have occurred as longwall mining matured from its infancy in the 1970s. Finally, on Day 69, rescuers lifted each of the miners out alive [source: Boston]. While most longwall mines use 42 mm chain or above on their armoured face conveyors (AFCs), many mines are running 48-mm and some are. There are hard-rock underground mines, and there are soft-rock underground mines. One major reason is that the post-yield and stress transfer mechanics of yield pillar systems are not yet fully understood. [citation needed]. As the shearer removes the coal, the AFC is snaked over behind the shearer and the powered roof supports move forward into the newly created cavity. This method has been successfully practiced in coalfields in the United Kingdom to increase percent extraction and avoid overlying gate road pillars in three-seam scenarios. After passing through the workings, this air (now termed return air) is conducted back to the surface through another set of entries (called returns). Mining has changed a lot from the images we have of the 19th century when men with shovels toted canaries to make sure the air underground was not toxic. "Underground Mining." The old-school technique of blast mining, that uses explosives like TNT to break up ore, is still in use, but just barely less than 5 percent of U.S. production [source: Great Mining]. However, longwall mining is exploding in popularity [source: Great Mining]. Third, a more balanced subsidence profile is produced where consideration for surface strains is a design constraint. The machine moves through the ore, creating rooms and pillars, until the full deposit is covered. A longwall miner shearing coal at the face of a coal seam; from an underground mine in southern Ohio, U.S. Inserting steel bolts to support the roof of an underground mine in West Virginia, U.S. This machine can weigh 75120tonnes typically and comprises a main body, housing the electrical functions, the tractive motive units to move the shearer along the coalface and pumping units (to power both hydraulic and water functions). Optimization of the mine design factors is arguably the primary means for controlling interactions between operations. Soft floor strata create floor heave and difficulties in advancing supports. When the ore deposit in relatively narrow, shorter cuts are made. Mine and Mine Worker Charts | NIOSH | CDC What Is A Longwall Move? - CLJ Hearing loss from noisy equipment and back injuries from lifting heavy loads are also common [source: Live Science]. The model provides the capability to analyze many coal raining situations and to determine the effects of the three-dimensional stress redistribution caused by mining in either single or multiple seams. The stress in the graphs has been normalized to a single-seam situation. In general, this will include all longwall equipment that is supplied by the longwall power center. The absolute values of the pressure bulb contours are proportional to overburden, and interactions will be-come more severe as depth increases. Beyond the subsidence trough lies the zone of maximum subsidence. 33B) shows that the maximum stress occurs about 50 ft past the boundary under the solid coal and is approximately 1.25 times the single-seam stress. In the case of mining from gob to solid, the normalized average stress (fig. The decision to offset or superimpose gate roads when mining multiple seams has been a topic of much concern in longwall design. Zhou classified subsidence interactions into three possible categories based on a time factor: (1) Lower seam mining is currently active with upper seam mining, an active condition; (2) mining in the lower seam is complete but the ground is still in the process of settling, both an active and passive condition; and (3) subsidence is complete and the ground has settled reaching a new state of equilibrium, a passive condition. The shearer moves at a speed of 1030m/min (3398ft/min) depending on cutting conditions. Longwall mining, because of its extraction efficiency and increased productivity, is rapidly gaining a larger portion of underground coal production. The modern longwall operator requires many skills and the ones discussed are only during . 3, and Blue Creek. [3] This longwall mine did not use a conveyor belt, instead relying on ponies to haul coal tubs from the face to the slope where a hoist hauled the tubs to the surface.[4][5]. Maleki investigated the use of yield pillars in longwall gate entries for controlling interactions between two seams at a western mine. (May 6, 2013) http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5751a3.htm, De Beers. This goaf collapses under the weight of the overlying strata. These problems were usually experienced when developing across a gob-solid coal boundary in the lower mine where the subsidence trough is located. Stemple noted in his study. (May 9, 2013) http://www.nma.org/pdf/c_most_requested.pdf, Oresome Resources. The number of individual beds that characterize the interburden can also influence the magnitude and distance of stress transfer. In another case, a longwall panel encountered overlying room-and-pillar workings as shown in figure 15. Others require mines to be inspected regularly to ensure they are safe. First, geologic discontinuities cannot be represented in the model. The extradosal ground forms the zone of high compressive stress that can cause ground control problems in the roof, floor, and pillars. The assumed density of the overburden was 162.5 lb/ft. Within these panels longwall machines have the potential for removing 100% of the coal. Longwall Mining - an overview | ScienceDirect Topics The arch is more likely to form for subcritical panel widths. The center of the longwall panel must then contend with the transfer of stress from the overlying gate roads. Hard roof strata may cause coal bumps or less violent spalling on the longwall face. In manual or semimechanized operations, the coal is undercut along the width of the panel to the depth of the intended slice. In modern mechanized mines, roof bolting is the most common method employed. In combination with shields and conveyors, longwall shearers or plows create a truly continuous mining system with a huge production capacity. In addition to carrying coal from the face, the AFC serves as the guide for the longwall shearer, which rides on it (see above, Mining methods: Longwall mining). Holland observed that in 38 cases of coalbeds affected by subsidence, about 75 pct had a shear-tensile failure. In longwall mining, as in the room-and-pillar system, the safe transfer of roof pressures to the solid coal ahead of the face and to the caved roof behind the face is necessary.

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