how does longwall mining work

1). Minor pressure arches can form independently from pillar to pillar as shown in figure 7 provided the strength of the pillar in situ exceeds that of the abutment pressure. Most coal is extracted using the room-and-pillar approach described previously [source: United Mine Workers of America]. In the hardest rock, no roof-support is needed; in softer rock, bolts may be placed in the roof as the drill progresses [source: Mining Know-How]. 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. There are two basic options to consider for conventional pillar designs. 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]. In this case, outer entries for the upper mine gate roads were located over lower mine pillars as in figure 25. First, the two mines are planned individually and the upper mine pillars are sized initially for a single-seam case. December 2012. Transfer of stress to developments mining beneath the abutment, can occur through the pressure bulb effect, The abutment is a high-stress zone usually occurring where the gob and solid coal meet. Underground mining is a method of mining in which minerals and ores are extracted from the earth through tunnels or shafts. Made before the Independent Planning Commission (IPC) hearing on the Dendrobium Mine expansion, this video explains the mining technique that South32 and Pea. There may be special design considerations for gate road layouts when three or more seams are mined in a descending order. Second, individual strata that characterize the overburden and interburden cannot be represented, so a generic modulus is chosen to depict the overall lithology. An operator then drives a drill through the ore, creating a drift, or a horizontal cut, from one side of the deposit to the other. However, there may be instances in which positioning the overlying isolated gate roads at the center of the underlying panel is not possible. Of the 25 mines, 16 have longwall workings in adjacent coalbeds. [citation needed] Subsidence is minimised by increasing the block's adjacent chain pillar widths, decreasing extracted block widths and heights, and by giving consideration to the depth of cover as well as competency and thickness of overlying strata. Record productions exceeding 20,000 tons per day, 400,000 tons per month, and 3.5 million tons per year have been reported from a single U.S. longwall shearer face. The normalized peak stress for the headgate and tailgate locations is. 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. "Longwall Mining." Simultaneous mining implies that both mines will be worked at the same time in the same area so that active interactions may occur between the two operations. A final pass drills through the pillars to recover the ore there, allowing the roofs to collapse behind the machine as it leaves each room. When seams are mined in ascending order and subsidence is the consideration, the total width of the gate roads defines the resulting subsidence profile in overlying coalbeds. Several design factors should be considered when positioning the overlying gate roads. MULSIM/NL is a boundary element model, developed by the USBM, for calculating stresses and displacements in tabular deposits. Several theories explain how roof bolts hold the roof. Reversing the headgate and tailgate entries had little affect on the average stress across the longwall face but did increase the stress on the tailgate side of the panel considerably, Since the tailgate usually experiences the most adverse conditions, it is recommended that the tailgate be placed under the overlying gob to reduce stress. 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. 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. This paper contains the results of an examination of accident, injury, employment, and . Also, displacements due to shearing are less likely to occur. It is then drilled and blasted, and the broken coal is loaded onto a conveyor at the face. Superpositioned gate roads for an ascending order of extraction are shown in figure 21. A conservative method for estimating entry location is to determine the outer limits of the subsidence trough based on the angle of draw. For instance, the panel width in each seam may differ, as the current trend for longwalls is increasingly wider panels. 3, and Blue Creek. At this point, the stress decreased to less than the single-seam datum for all angles of approach. Currently, there are no proven methods for estimating the quantity of stress transferred and safe pillar sizes for superpositioned gate roads. First, the model considers the direction of mining in relation to the gob-solid coal boundary to evaluate whether it would be more advantageous for the panel to be mined from the solid to the gob side or the gob to the. Interactions were likely, but difficult to predict because of the complex geometry of the workings as shown in figure 14. The closer the overlying coalbed is to the mined-out panel, the lesser the extent of the subsidence trough, but the greater the bending in the strata. Two mine design factors may be responsible for these large interactive distances. The mines also cooperate in sequencing seams and maintain a rigid phasing of panels to control interactions. Once past the boundary and under the gob, face conditions improved. The models simulated stress transfer and dissipation as a function of three major variables: pillar geometry and loading, interburden layering (stratification), and interburden elastic modulus. Ground control problems will mainly be encountered in the subsidence. The inner limits of this zone and the location of entries can be estimated by using the caving angle. This interaction is more likely to occur when both operations are active as described by Zhou. As shown in table 1, about half of these mines report some type of interaction problem with the adjacent workings. In many cases, the surface facilities near a portal include bathhouses and a lamp room; coal handling, storage, preparation, and load-out facilities; a fan house; water- and waste-handling systems; maintenance warehouses; office buildings; and parking lots. These studies show that load-carrying capacity of the gob depends upon the depth, panel width, and the composition of the overlying strata. A common denominator in successful applications of yield pillars is using increased support density in entries, usually in the form of wood cribs. Once past the face the air is no longer fresh air, but return air carrying away coal dust and mine gases such as methane, carbon dioxide, depending on the geology of the coal. The magnitude of the side abutment load on the gate road pillars after the longwall panel(s) are mined determines the intensity of the interaction in lower operations. It involves drilling a section of ore at the very bottom of the deposit and then blasting to make the roof collapse. Pillar shape influences the transfer of stress: A rectangular pillar will transfer less load, and also, the interactive distance will be less than would be the case for a square pillar of equal load-bearing capacity. The conventional approach for gate road design uses large pillars that are designed to accept all the abutment load to which they are subjected. (May 9, 2013) http://www.umwa.org/?q=content/room-and-pillar-mining, United Mine Workers of America. These entries facilitate the movement of workers, coal, and equipment as well as provide ventilation and escape in emergencies. First, the benefits of developing beneath the gob are maintained and a buildup of stresses in gate roads is avoided. 18), the degree of subsidence damage in the roof, floor, and coal will determine the difficulty of mining. (May 9, 2013) http://www.umwa.org/index.php?q=content/longwall-mining, United Mine Workers of America. In the United Kingdom, the intent is to maximize the gob and to avoid leaving pillars between panels; leaving pillars between panels is known to create stress problems in subsequent workings both above and below. The extent of each zone is a function of the subsidence process and the geologic composition of the overlying strata. (May 6, 2013) http://epcmworld.com/news/feature-interview-safety-in-underground-mining, Mine Safety and Health Administration. Here the water is also used to transport the coal from the working faces to a common point through open channels and from the common point to the surface through high-pressure hydraulic transportation systems. The minimum stress occurs at 100 ft approaching the boundary and is approximately 0.30 times the single-seam stress. Within these panels longwall machines have the potential for removing 100% of the coal. Located in southwestern Pennsylvania, CONSOL Energy's Bailey Complex has been a world leader for many years. Those activities which are conducted sequentially in a production cyclei.e., cutting and hauling the coal and supporting the immediate exposed roof after coal removalare called unit operations. In all-belt mines, it is not unusual to have trolley rail haulage for carrying workers and materials to and from the working face. A surface profile for supercritical and critical panels consists of a subsidence trough, the outer limits defined by the angle of draw, and an area of maximum subsidence. Second, the gate roads can be offset along the longwall panel as shown in figure 17; in this case, the primary design consideration is the magnitude of the resultant stress concentration on the longwall face. Mainline haulage is also provided by belt or railcar. In simple words, longwall mining is a method of underground mining, in which coal is extracted from 60m to 200m long faces, moving continuously in one direction over large distances, leaving behind the goaf. 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. Moreover, after mining interference disturbing the natural landscape near the mines, the natural water flow paths can be redirected which results in additional erosion across a stream or river bank. Mark gives some preliminary formulas for determining the width of yield pillars, but concludes that design is based mostly on experimentation and that no real quantitative guidelines exist. Seams are longwall mined simultaneously, and mine plans may or may not be coordinated with one another (fig. A descending order of extraction is preferable, because the severe ground problems associated with subsidence should be avoided in most cases. The task of bringing fresh air near the production faces is an important auxiliary operation, while the task of carrying this air up to the working facesthe locations of which may change several times in a shiftis the unit operation known as face ventilation. In this region, both room- and-pillar and longwall mining have contributed to coal production, but seam sequencing is still based mostly on availability and ownership. The USBMs MULSIM/NL model was used for this purpose. Gravity then takes over, as the ore above the blast site fractures and collapses in succession as support is withdrawn. As with many mining techniques, it is important to consider the local and regional environmental impacts from longwall mining. In general, shales have a low elastic modulus, and the beds tend to bend and fracture as an. No two mines are alike, though. Field studies show that longwall interactions can extend over much greater distances, suggesting that some other load transfer mechanism may be responsible. Strata that have a high elastic modulus, such as sandstone, tend to dampen stress transfer but are more prone to shear failure. In situ stresses, such as a high horizontal stress, may affect overall stress magnitude, but the transfer of stress is more directly related to the normal cover load, which is a function of depth. Interseam shearing can be very damaging to overlying coalbeds because the strata tend to shear and displace as shown in figure 13. In European investigations, researchers have found that interburden distances for which interaction effects are no longer encountered ranged from 210 ft to as much as 750 ft. U.S. studies have shown similar interactive distances involving interactions between room-and-pillar and longwall operations. For ascending extraction (fig. (May 6, 2013) http://www.ilo.org/oshenc/part-xi/mining-and-quarrying/item/597-techniques-in-underground-mining, Health24. Extraction of two seams that were longwall mined in ascending order using superpositioned gate roads is documented in a USBM study. In some semimechanized or manual longwall operations, chain haulage is used, while the face haulage equipment of choice in modern mechanized longwall systems is an armoured face conveyor (AFC). First, the ratio of the seam depth to longwall panel width must be at some critical value so that the arch can support itself. A common factor in mines encountering load transfer problems is an isolated barrier pillar or a gob-solid coal boundary left in the upper mine. First, long gob-solid coal boundaries were left in the overlying mine. Similar experiences have been documented in the United Kingdom in mining beneath gob-solid coal boundaries in coalfields. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle on to the collapsed goaf. A yield and conventional pillar design resulted in the most stable conditions for the upper mine gate roads, but this design created interaction problems in the lower mine. In longwall mining, the roof supports are built into the machine, sitting between the top of the longwall miner and the roof of the room. The major differences are only in the size, scope, and permanence of installations. Interactive distance associated with these types of openings may be increased because of arching stresses. To generate additional value, all devices must collect and share relevant information gathered through sensors. Once the drift is complete, backfill, or waste material, is spread into the open drift, creating a platform for the next pass. The subsidence over a longwall face is generally more uniform than it is over room-and-pillar workings. "How Underground Mining Works" This paper is intended to summarize, and cite findings and current concepts about the complex groundwater impact of longwall mining, for the benefit of hydrogeologists and engineers who may need to be consultants to either side, but who have little background in longwall hydrogeology. 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. If the roof conditions or spontaneous-combustion liability of the seam requires that there be no caving, the void created by mining will be backfilled. For this reason, coal mining worldwide is heavily regulated through health and safety laws. Deep beneath the surface of the Earth lie some of the most frightening factories in the world: underground mines. Referred to as pillar load transfer, this interaction can usually be predicted with reasonable accuracy, and design changes can be implemented to minimize damage to underlying operations. Haycocks also found that the arches can have various shapes, from a dome to a modified parabolic shape, depending on geology. As the machine progresses into the ore, the supports move with it, allowing the area behind it to collapse and fill in the excavated area. In the solid to gob case (fig. History [ edit] This produced 11 stress values, which were then normalized to a single-seam situation. Longwall shearers and shields can be remotely operated, and continuous miners have also been equipped with automatic controls. Figure 12 illustrates a simplified example of this zones influence using an angle of draw of 25 and a caving angle of 25. In modern mechanized longwall operations, the coal is cut and loaded onto a face conveyor by continuous longwall miners called shearers or plows (see photograph). Superpositioning may also be required near the perimeter of a coal property to maximize recovery. 2). Various model runs were made with the longwall face oriented at 0, 15, 30, 45, and 60 to the boundary. Second, the gate road pillars must be of sufficient in situ strength to support the abutment pressure. In most instances, the pillars in the lower mine will be slightly larger in width. This analysis was developed for perfectly elastic, isotropic, and homogeneous materials. Longwall mining is an efficient mining method and using the right equipment can make it even more profitable. 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. (May 13, 2013) http://kids.britannica.com/comptons/art-125634/Longwall-mining-machines-use-rotary-shears-to-cut-across-a, CDC. In some cases, complete collapse of the overlying strata during extraction eventually travels to the surface, resulting in surface depressions. Using numerical methods, combined with case study results and theoretical and statistical analyses, engineers can attempt to develop optimum mining plans for different multiple-seam conditions. 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 environmental toll of underground mining is significant. [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. These include the beam theory (roof bolts tie together several weak strata into one), the suspension theory (weak members of the strata are suspended from a strong anchor horizon), and the keying-effect theory (roof bolts act much like the keystone in an arch). 3). Although this research was directed at room-and-pillar workings, it may provide insight into the pressure bulb mechanisms for longwall gate roads. The extradosal ground forms the zone of high compressive stress that can cause ground control problems in the roof, floor, and pillars. In strong, monolithic strata with little jointing, the arch formation may be low or the strata may bridge for narrow panel widths. For long-wall mining, there are four possible mining sequences involving multiple seams : The mining sequence determines the type of interaction the longwall developments will experience. Studies involving full extraction mining show that coalbeds lying within 20 times the lower seam mining height are most susceptible to subsidence-related damage, as they usually fall within the caving zone. In some of the gassiest mines, more than six tons of air are circulated through the mine for every ton of coal mined. The coalbeds were also assumed to be flat-lying deposits with no dip. In Australia, for example, advances in technology, geotechnical engineering and work practices over that period have resulted . Morbidity and Mortality Weekly Report. These conditions become less intense progressively higher in the zone. Cut and Fill For relatively narrow ore deposits, miners drill an access ramp adjacent to the ore deposit, from the surface down to the lowest point of the deposit. The coalface is a hazardous area because methane and carbon monoxide are present, while the area is hot and humid since water is sprayed over the face to minimize the likelihood of sparks occurring when the shearer picks strike rock. Still, underground mines are not the safest places to be. Second, the longwall panels were relatively narrow in width in relation to their depth. In situ horizontal stresses also have a negligible effect on the downward transfer of stress. Caving is the preferred practice, however. Ground conditions in the zone of maximum subsidence may even be less than ideal depending on the caving characteristics of the strata. Using photo-elastic models, he showed that yield pillars can reduce. The sequence in which the seams will be mined is the first fundamental decision confronting mine planners. In all but one case, the interburden between the mines is 200 ft or less and the overburden above the upper mine ranges from 800 to 2,000 ft. Three mines report severe interaction problems, but in the remaining cases, problems are moderate and manageable.

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