That Sinking Feeling
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By Wyona Coleman, Chair, Pennsylvania Chapter’s Mining Committee The increasing use of longwall and other full-extraction underground mining techniques is creating widespread damage to homes, water, and land resources in the bituminous coalfields in this country. Unlike room-and-pillar underground mining, longwall removes 90 percent or more of the coal and creates immediate and dramatic subsidence.That subsidence is destroying homes, creating flooding and erosion problems on farmland, destroying or contaminating groundwater supplies, and altering drainage patterns in entire watersheds. Why are deep mine operators granted carte blanche in how and where they mine? The answer may be in the title of the federal law, which regulates deep mining: The Surface Mining Control and Reclamation Act of 1977.The main goal of Congress in 1977 was to curb the blatant, uncontrolled abuses of strip mining. Although the surface effects of underground mining were included in the Act, those issues were given less attention. The result is that the current abuses of deep mining have few legal remedies. Extraction of coal by underground mining results in unsupported voids in the coal seam(s). In time the rock strata and earth above these voids collapse. On the land’s surface, the collapse results in subsidence. Surface subsidence can show up as a large sinkhole, small pothole, a pit, or large trough. The surface area affected is larger than the associated collapse area in the mine due to the cratering effect as land is pulled down toward the mine void. The thinner and weaker the overburden, the more coal removed, the sooner the subsidence is likely to occur and the more serious it is likely to be. Underground mining techniques are divided into two broad categories: low-extraction mining with “unplanned” subsidence and high-extraction mining with “planned subsidence.” Lowextraction mining uses the traditional “room-and-pillar” technique, extracting up to 65% of the coal within each panel. The barriers of coal between panels and the “pillars” or columns of coal left in place to help support the roof and overlying strata above the “rooms” from which coal has been removed. High-extraction mining includes both longwall (up to 90% recovery) and room-and-pillar retreat mining where pillars are pulled as the end process as the operation backs out (about 80% recovery). Longwalling is a technique requiring specialized equipment. Straight
panels are lain out—varying from 800 ft. to 1,000 ft wide
and 700 ft. to 10,000 ft, long. A series of entryways are developed
along both sides of the panel. They are in the form of a chain of
low-extraction pillars and are known as the “gateway “
or “rib.” They provide access to the panel for ventilation,
power, and personnel movement as well as conveyor belts to remove
the coal. Longwall equipment is set up at the end of the panel furthest
away from the main tunnel. The longwall- cutting machine then moves
back and forth along the coal face. As the machine advances, a series
of hydraulic pillars and shields support the mine roof to provide
a safe working area behind the coal face. As the mining progresses
toward the front of the panel, the supporting shields follow along
and the roof behind them falls.The only surface support is along
the gateway and at either end of the panel. History of LongwallingModern longwall mining was first introduced in the United States in the early 1960s but was not used in more than a few mines until the late 1970s. It has been practiced in Europe much longer. The coal industry portrays longwall mining and other fullextraction techniques as creating planned and controlled subsidence. They point to longwalling in Europe under towns, rivers and industries without major damage (they don’t point out that coal lies there at a much greater depth).They describe longwall mining as “a gentle lowering of the earth,” allowing the surface and structures on it to gently settle down to a lower elevation intact and relatively undamaged. Such images are illusions at best. For people living above a longwall panel, reality more closely resembles an earthquake nightmare. The reason for this disparity is that longwalling as practiced in this country and in Europe is very different. Longwalling is heavily regulated in Europe and precise subsidence engineering is required. Surface features actually determine how a mine is laid out and the amount of subsidence which will be allowed. In fact, currently there is very little, if any, longwall mining being done in Europe. But then German conglomerates have found that buying into U.S. longwalling is cheaper by far. The European experience stands in stark contrast to this country’s.
U.S. longwall mines are laid out in straight lines for the convenience
of production and minimization of costs with little or no consideration
of surface features. Subsidence engineering is conspicuously absent
from the “drop ’em and try to fix ’em” approach
of most U.S. coal companies. ImpactsSubsidence creates damage in a variety of ways. It can damage homes and buildings ranging from cosmetic cracks to collapse. A building can tilt; be pulled off its foundation; have wall, floor and ceiling joints separate; and basement walls and foundations can be cracked.The building frame can be stressed and pulled to the point it tears apart. Doors and windows can fail to close, floors can tilt, windows shatter, and chimneys fall.Water and gas pipes can rupture and electric and phone lines can be broken. Ground shifting has cracked wells, cisterns, and septic tanks. It has burst water and oil pipelines, torn down utility lines, and dropped and cracked roads, streets, and rail lines.Trees have died when their root systems were pulled apart. Orchards and woodlots have been damaged. Subsidence damage to farmland ranges from small sinkholes to more than two-acre water traps. Large widespread troughs over mined out panels have severely disrupted surface drainage patterns making fields too wet to farm. Farm ponds and major impoundments have had banks and shorelines disrupted and have even been drained. Cracks and deep fissures pose hazards to livestock, farm equipment, and vehicles on damaged roadways. Subsidence is also creating the loss of ground and surface water. Fracturing and cracking of the rock strata is draining aquifers, springs, and wells. Streams are being drained or are pooled. In areas such as Greene and Washington Counties in Pennsylvania where longwall mines affect thousands of acres, entire watersheds are being disrupted. The coal industry portrays longwall subsidence impacts as being a short-term problem. Once subsidence begins, the majority of the ground movement does usually occur within the first three to nine months. However, experience has shown that sufficient ground movement to damage structures and thwart repair efforts often continues for four or more years. In the case of disrupted water tables and aquifers, no one can accurately forecast how long it will be, if ever, before usable water will once again be available. Under the current regulatory programs and coal company policies, mitigation and repair efforts can best be described as too little, too late. Click here
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