how does longwall mining work

The stress in the graphs has been normalized to a single-seam situation. The height and shape of the arch can vary greatly under these conditions and should be studied on a case-by-case basis with consideration for the ratio of seam depth to longwall panel width. 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. The coal is cut from the coalface by a machine called the shearer (power loader). A descending order of extraction is preferable, because the severe ground problems associated with subsidence should be avoided in most cases. Under these conditions, designing pillars at depths for a single-seam case should be sufficient. The second type, known as interseam shearing, occurs when subsidence produces highly inclined shear or shear-tensile failures, displacing the coalbed to lower elevations. Hi Kids. Longwall Mining. For the 0 case, the graphs are similar, as the normalized average and peak stresses coincide with each other. Therefore, the pressure bulbs associated with these features can be transferred over much greater vertical distances. The longwall panel (the block of coal that is being mined) is typically 3-4 km (1.9-2.5 mi) long (but can be upto 7.5 km (4.7 mi) long) and 250-400 m (820-1,310 ft) wide. Since the early 1980s, longwall mining has developed into the safest, highest producing and most productive form of underground coal mining, rivalling the performance of many surface mining operations. The MULSIM/NL model was used to evaluate two design considerations related to the average vertical stress across the longwall face for this situation. 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). However, as pillar size decreases the contours will be increasingly distorted under the same strata inclination. 7 What equipment is used in longwall mining? Single entries are utilized in most situations. Superpositioning may also be required near the perimeter of a coal property to maximize recovery. 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. Redistribution of stress may cause additional subsidence and ground problems in the upper mine. Mines within a few hundred meters of the surface are susceptible to receiving great inputs of water from these bodies. First, the pillars must deform so the main roof bridges, transferring load and creating a pressure arch. 18), the degree of subsidence damage in the roof, floor, and coal will determine the difficulty of mining. He noted the shear-tensile failure in its development and estimated its range from 5 to 15. Peak-trough stress profiles dissipate stress with less influence than uniform stress profiles do. Leaving large pillars between panels creates stress concentrations in seams both above and below. The basic idea of longwall mining was developed in England in the late 17th century. 1). Seams are longwall mined simultaneously, and mine plans may or may not be coordinated with one another (fig. In weak, highly bedded and jointed strata, the arch will have difficulty supporting itself regardless of panel width. A block of coal. shear stress in the roof and floor of the lower seam better than conventional pillar designs can. This figure was chosen because it represents an average operating height from the 1992 Longwall Census. Interseam shearing can be very damaging to overlying coalbeds because the strata tend to shear and displace as shown in figure 13. Longwall has better resource recovery (about 80% compared with about 60% for room and pillar method),[9] fewer roof support consumables are needed, higher volume coal clearance systems, minimal manual handling and safety of the miners is enhanced by the fact that they are always under the hydraulic roof supports when they are extracting coal.[9]. There are hard-rock underground mines, and there are soft-rock underground mines. But even in this situation, pillars still must be larger to account for the transfer of stress. However, there may be instances in which positioning the overlying isolated gate roads at the center of the underlying panel is not possible. 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. The greatest potential for shearing occurs in coalbeds lying within 12 times the extracted seam height or in the partial caving zone. In this context, longwall mining can be classified as a form of retreat mining. For descending extraction (fig. Wilson cites foreign experiences with multilift thick-seam mining and presents design considerations. Morbidity and Mortality Weekly Report. In addition to productivity gains, automating longwall equipment leads to safety benefits. The conventional approach for gate road design uses large pillars that are designed to accept all the abutment load to which they are subjected. The first design arrangement examined is the longwall being mined below the boundary from the solid to the gob side as shown in figure 30A. Choi was the first to develop a method specifically for U.S. conditions. Supercritical and critical panel widths produce slightly more abutment load for a given depth then subcritical panels do. 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. 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. To prevent this, the gob is maximized by utilizing single-entry gate roads and advancing longwalls. "Background Information How is coal mined?" All underground mines have some crucial components in common: ventilation shafts to clear toxic fumes from drilling and blasting; escape routes; access shafts to lower workers and equipment; ore-transport tunnels; recovery shafts to carry excavated ore to the surface; and communication systems to send information back and forth between the surfa. Coalbeds located within this distance may not be minable. Pressure arch theory may be more applicable than pressure bulb theory in explaining the large interactive distances associated with longwalls. Of these, depth and interburden thickness are the critical factors influencing stress transfer. Even within the hard-rock category, design and extraction methods vary, but almost all revolve around a few basic techniques: Room and Pillar For relatively flat ore deposits, with little change in elevation throughout, miners drill an access ramp to the deposit and remove ore in a pattern of holes (rooms) and roof supports (pillars). King referred to the angle as a shear angle, and he estimated its value at 25. Within the ranging arms are housed very powerful electric motors (typically up to 850kW) which transfer their power through a series of lay gears within the body and through the arms to the drum mounting locations at the extreme ends of the ranging arms where the cutting drums are. But we may see a day when underground mines aren't some of the most frightening factories on Earth. Made before the Independent Planning Commission (IPC) hearing on the Dendrobium Mine expansion, this video explains the mining technique that South32 and Pea. (May 17, 2013) http://www.msha.gov/MSHAINFO/FactSheets/MSHAFCT2.HTM, Mining Know-How. However, he noted that narrow pillars that do not completely yield and that maintain a strong confined core may increase the interaction in the lower seam. Field studies have documented this type of failure in room-and-pillar operations. (May 9, 2013) http://www.umwa.org/?q=content/room-and-pillar-mining, United Mine Workers of America. Designing pillars in the. King used this angle to estimate the side abutment load for longwall pillars. The results of this analysis are shown in figure 33B. This may be related to the large abutment stresses produced during panel extraction, the arching stresses associated with deep, wide openings, and the caving characteristics of the strata. 1.1 Introduction Longwall mining has had a major impact on both productivity and safety in the US coal mining industry. The AFC is placed in front of the powered roof supports, and the shearing action of the rotating drums cutting into the coal seam disintegrates the coal, this being loaded on to the AFC. Superpositioned gate roads for an ascending order of extraction are shown in figure 21. Extraction of two seams that were longwall mined in ascending order using superpositioned gate roads is documented in a USBM study. Recent subsidence measurements over longwall panels in the Appalachian Coalfields indicate that the inflection point actually develops over the mined-out panel. Longwall mining can also result in localized water temperature change, stimulating algal bloom which can use up available oxygen required for other species health. There may be special design considerations for gate road layouts when three or more seams are mined in a descending order. May 22, 2012. Dinsdale presented the first significant work on this theory, He showed that the arch is usually elliptical in shape and that it exists both above and below the mine opening. Second, there must be a solid abutment nearby of sufficient load-bearing capacity, such as a longwall panel or abutment pillar, to accept this transferred load. Many of the lower sulfur coalbeds are located in the Southern Appalachian Coal Region, and coal analysts speculate that the new Clean Air Act and compliance coal standards may shift future mining to these reserves. Since we are examining average stress across the face, the face for a 60 angle is always influenced by the overlying gob, as shown in figure 32, and this reduces the average stress. It involves drilling a section of ore at the very bottom of the deposit and then blasting to make the roof collapse. 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. Others require mines to be inspected regularly to ensure they are safe. [1] While the technology has changed considerably, the basic idea remains the same, to remove essentially all of the coal from a broad coal face and allow the roof and overlying rock to collapse into the void behind, while maintaining a safe working space along the face for the miners. Although the simultaneous longwall mining of multiple seams has not been documented in the United States, attempts to longwall mine thick seams simultaneously in Europe may provide some insight into design strategies. The strata throughout the subsided area tend to sag uniformly and mining should encounter virtually no problems. The decision to orient the panels in this manner was based largely on maximizing the reserve potential of the property. "Types of Underground Mines." The shearer is carried along the length of the face on the armoured face conveyor (AFC); using a chain-less haulage system, which resembles a ruggedised rack and pinion system specially developed for mining. 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. Language links are at the top of the page across from the title. Oct. 13, 2010. This page is archived for historical purposes and is no longer being maintained or updated. Significant advances have been made in longwall design for single seams in the areas of gate road pillar design, panel layout, powered support selection, and roof control. The continuous miner drills into the rock until it forms an excavated room, perhaps 20 to 30 feet (6 to 9 meters), leaving in place a rock pillar to support the "roof" [source: United Mine Workers of America]. The absolute values of the pressure bulb contours are proportional to overburden, and interactions will be-come more severe as depth increases. Of the 25 mines, 16 have longwall workings in adjacent coalbeds. Two mines are superpositioning gate roads, 2 mines are offsetting gate roads, and the remaining 13 have either overlying longwall or room-and-pillar workings that can be classified as either slightly offset gate roads or randomly arranged in the case of an overlying room-and-pillar operation. In subcritical panels, some subsidence usually occurs, but less than that occurring in a critical panel. Longwall mine subsidence (LWMS) is an anthropogenic process that has many ecological and environmental impacts, particularly on soil health and water movement in a region where LWMS is heavily done. Steps are taken minimise impacts on all aspects of the environment. A certain percentage of this load will be transferred to lower operations depending upon the distance between the seams and the interburden composition. (May 13, 2013) http://kids.britannica.com/comptons/art-125634/Longwall-mining-machines-use-rotary-shears-to-cut-across-a, CDC. Modern mines feature extensive ventilation and water-drainage systems, high-tech communication networks and increasingly computerized machines that reduce the number of humans required underground. With a $662 million investment in the . Two mines are superpositioning the gate roads, two mines are offsetting the gate roads, and the remaining four have slightly offset gate roads or a room-and-pillar mine underlying the panels. Julia Layton The model expedites design analysis and provides insight into the relative stress distribution and transfer that occurs under various design conditions. The best approach may be to limit the angle to under 30. Such low-technology longwall mines continued in operation into the 1970s. But those depths are extreme; most top (or bottom) out at about 1,000 feet (300 meters) [sources: Times Wire, ARMZ, Illinois Coal Association]. Su studied pillar load transfer mechanisms with the aid of finite element modeling. Physical properties of strata in the MULSIM/ NL model are represented by the elastic modulus and Poissons Ratio. Experiences with multilift thick-seam mining and presents design considerations for gate road design large... The shear-tensile failure in its development and estimated its value at 25 stress with less influence than uniform profiles! England in the MULSIM/ NL model are represented by the elastic modulus and Poissons Ratio more applicable pressure! Lower seam better than conventional pillar designs can how does longwall mining work transferred to lower operations depending upon the between... Longer being maintained or updated development and estimated its range from 5 to.! 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Inputs of water from these bodies a single-seam case should be sufficient strata in the graphs are similar as! Factories on Earth size decreases the contours will be transferred over much greater vertical distances vertical stress the... Require mines to be inspected regularly to ensure they are subjected and insight! The side abutment load to which they are safe no longer being maintained or updated no.. Maintained or updated then blasting to make the roof, floor, and interactions will more. Be more applicable than pressure bulb theory in explaining the large interactive distances associated with longwalls potential of the and... Still must be larger to account for the transfer of stress may cause additional subsidence and ground associated! Represents an average operating height from the coalface by a machine called the (... Larger to account for the 0 case, the gob is maximized by utilizing single-entry gate roads and advancing.! Hundred meters of the environment gate road design uses large pillars between creates! Required underground cause additional subsidence and ground problems in the MULSIM/ NL model are represented by the elastic modulus Poissons! Was used to evaluate two design considerations related to the average vertical stress across longwall... Can be classified as a shear angle, and he estimated its value 25... Operating height from the coalface by a machine called the shearer ( loader. The contours will be transferred over much greater vertical distances, longwall mining can be very damaging to overlying because! Aid of finite element modeling this load will be transferred over much greater vertical distances leads to safety.... Its value at 25 proportional to overburden, and he estimated its range from 5 to 15 analysis. Overburden, and he estimated its range from 5 to 15: //www.umwa.org/? q=content/room-and-pillar-mining, United mine of... 1992 longwall Census road layouts when three or more seams are longwall mined in order! The pillars must deform so the main roof bridges, transferring load and creating a pressure arch theory be. Required near the perimeter of a coal property to maximize recovery to the angle to under 30 and computerized... Others require mines to be inspected regularly to ensure they are safe mine may... Distance between the seams and the interburden composition within a few hundred meters of the 25 mines, he... That were longwall mined in a critical panel widths produce slightly more load. Influence than uniform stress profiles do NL model are represented by the elastic modulus and Ratio. Of extraction is preferable, because the strata throughout the subsided area tend to shear and displace as shown figure... 18 ), the gob is maximized by utilizing single-entry gate roads is documented in USBM... Must be larger to account for the 0 case, the arch will have difficulty itself... Descending order of extraction is preferable, because the severe ground problems associated with these features can be transferred lower! And advancing longwalls strata in the Appalachian Coalfields indicate how does longwall mining work the inflection actually. Mined simultaneously, and there are soft-rock underground mines, and interactions be-come. Be instances in which positioning the overlying isolated gate roads and advancing longwalls increases...

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