Multiple-seam mining in the United States: background.
Proceedings: New Technology for Ground Control in Multiple-Seam Mining. Mark C; Tuchman RJ, eds., Pittsburgh, PA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, 2007 May; :3-14
Hazards resulting from multiple-seam interactions are a serious issue at many U.S. coal mines, particularly in the central Appalachian and western mining regions. The four types of interaction are: 1. Undermining, where stress concentrations caused by previous full extraction in an overlying seam is the main concern; 2. Overmining, where previous full extraction in an underlying seam can result in stress concentrations and rock damage from subsidence; 3. Dynamic interactions, caused when full extraction takes place above or below open entries that are in use (the most extreme dynamic interactions involve mining beneath open entries in an upper seam); and 4. Ultraclose mining, where room-and-pillar development of two seams within 25-30 ft of each other can result in interburden failure. Undermining and overmining are by far the most common types of interaction. Nearly a century of research has identified a number of factors that can affect the intensity of a multiple-seam interaction. These include: 1. Depth of cover: The deeper the overburden, the greater the potential stress concentration caused by multiple-seam mining. 2. Mining sequence: Overmining is more difficult than undermining because of the potential for rock damage caused by subsidence. Dynamic interactions (particularly retreating beneath open works) should be avoided at all costs. 3. Interburden thickness: The smaller the distance between the seams, the greater the intensity of the potential interaction. 4. Type of remnant structure: Isolated remnant pillars that are surrounded by gob cause more intense interactions than gob-solid boundaries. First workings are generally not a concern unless the seams are ultraclose. 5. Interburden geology: Stronger, less bedded interburden tend to distribute multiple-seam stress concentrations more rapidly, resulting in less intense interactions. 6. Immediate roof geology: Weak roof (and floor) are more likely to be damaged by multiple-seam interactions. 7. Angle of approach to remnant structure: Retreat mining should proceed from the gob toward the solid side of a gob-solid boundary, and a longwall should not be brought broadside into long remnant structure. The large number of geologic and mining variables involved in multiple-seam interactions has made them very difficult to analyze. Empirical studies have foundered because the databases were too small for the number of variables and because bivariate analyses are inappropriate when there are so many variables involved. Numerical models have been helpful, but to be most useful they have required site-specific calibration to underground conditions. A hybrid approach, employing multivariate statistical analysis of a large database combined with numerical modeling, could provide the mining community with a valuable tool for predicting, avoiding, or controlling multiple-seam hazards.
Mining-industry; Underground-mining; Coal-mining; Computer-models; Computer-software; Models; Mathematical-models; Ground-control; Ground-stability; Rock-mechanics; Rock-bursts; Rock-falls
Proceedings: New Technology for Ground Control in Multiple-Seam Mining