Sustainable Land Development Today - July/August 2008 - (Page 30) GEOTECHNICAL Undermined: Land Development Over Coal Mines By Gennaro G. Marino, Ph.D., P.E. Developers are faced with an unusual ground. This method is called the room- those which are more recent. Extraction problem when they discover a property and-pillar mining. The mine void areas rates of underground coal mines typithey are considering for construction is are usually called rooms, entries or cally range from 50 to 75%. undermined. This is particularly prob- mains and the columns of coal are called lematic in some areas of Mine subsidence the country where prime defined land is in short supply. In Ground surface subsicertain areas, finding such dence resulting from the collocations which are underlapse of an underground developed and not undermine are of two fundamental mined are becoming more types: pit or sag. Pit subsidifficult. dence expresses itself on the Until they are economground surface in a sinkhole ically viable, undermined configuration. A pit can be as areas sit vacant for a consmall as a pothole or as big as siderable amount of time a large crater typically 30 feet because of the potential or so in diameter and on the risks commonly associorder of up to about 20-feet ated with underground deep (see Figure 3). Some coal mines. In fact, lending pits can be more trough institutions can be hesitant shaped with similar dimento finance such developFIGURE 1 MAJOR COAL FIELDS IN UNITED STATES (FROM SUB BITUMINOUS TO ANTHRACITE QUALITY) sions. Land with shallow unments because of the poderground mines up to tential risk of significant future damage pillars. Figure 2 is a conceptual depiction 80-feet deep are more susceptible to pit to the proposed structures. Therefore, of an older layout of room-and-pillar banks can require assurance that this risk workings. In a subsidence engineering subsidence than deeper mines. In general, sag subsidence can vary investigation it is important to have the will be nominal. from fairly abrupt to a gradual depresbest available mining data in order to assess subsidence potential. Mineable coal sion of the ground surface which can be Coal mining defined bowl to almost Mine subsidence of the ground sur- seams are typically 3 ft trough-like. The fairly face can unexpectedly result when the thick and can reach 20 abrupt cases are usuland is undermined with abandoned ft or more thick, but ally the smaller sags coal mines. Abandoned coal mines are are more commonly up to about 300-feet usually no less than 30 ft to typically no less than 8 ft. When wide. These depresmore than 600 ft deep. The organization the coal is extracted sions typically occur of the remnant coal workings depends this results in correfrom the collapse of sponding mine void upon the age of the mining. an area of a mine Abandoned underground coal mines heights with mine room. A more gradexist in all the major coal fields in the opening widths typiual sag results from U.S. (see Figure 1). Each coal field can cally 15 to 30 ft. Older the yielding of multimine have more than one mineable coal seam. abandoned ple pillars. Even the Therefore, in some areas there can be workings are much more gradual subsimore than one level of coal workings. less organized and dence depressions Coal extraction by underground mining maps depicting them can be visually seen consisted of leaving mine voids with (if available) are much in relatively flat tocolumns of coal to support the overlying less accurate than pography, but this FIGURE 2 CONCEPTUAL DEPICTION OF AN OLDER LAYOUT OF ROOM-AND-PILLAR WORKINGS 30 July/August 2008 Sustainable Land Development Today
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