Overview of coal mine ground control issues in the Illinois Basin

Some of the most difficult coal mine roof in the United States can be found in the Illinois Basin. Factors contributing to the high roof fall rate include weak, moisture-sensitive roof rock; high horizontal stress; and limited longwall mining. The depth of cover ranges from 27 to 300 m (90 to 1,000 ft), and roof damage from horizontal stress can be severe. Moisture-sensitive roof rock, which contributes to roof skin deterioration and roof fall, is common above the Springfield-Harrisburg Herrin #5 and #6 seams in the Illinois Basin. The roof fall rate increases significantly in the humid summer months. Using laboratory and field studies, the National Institute for Occupational Safety and Health (NIOSH) has shown that highly moisture-sensitive roof rock can be directly correlated to poor roof conditions. Controlling the skin is the key to reducing rock fall injuries, and roof screening is, by far, the best remedy. Illinois Basin coal operators have been successful in reducing the number of rock fall injuries in recent years. NIOSH has documented best practices for screen installation, which has resulted in safe, efficient operations. Other solutions to skin failure include the use of denser five bolts/row patterns to reduce spans between bolts; systematic supplemental support in intersections; straps and large pans to protect operators; and air conditioning to remove moisture from the intake air. Introduction Rock fall injuries continue to present a significant hazard to U.S. coal miners. In addressing the problem, NIOSH has reviewed ground control issues contributing to this hazard. A number of these geotechnical issues are present in the Illinois Basin. The following overview documents the current state of coal mine ground control in the Illinois Basin and the efforts operators have made to prevent rock fall injuries. The Illinois Basin is a major coal-producing basin in the United States, with more than 86 Mt (95 million st) of coal mined in 2006 (MSHA, 2006). Of this total, 9 Mt (10 million st) of coal were mined by longwall methods, 47 Mt (52 million st) were mined by room and pillar methods and 30 Mt (33 million st) were surface mined. The basin includes Illinois, southwestern Indiana and western Kentucky (Fig. 1). More than 75 individual coal seams have been identified in the basin, of which 20 have been mined (Archer, 1975). The primary producing coal seams in the basin, the Herrin #6 and the Springfield-Harrisburg #5, are middle Pennsylvanian in age. There are now 30 underground coal mines operating in the basin (Fig. 1). Twenty-seven of the mines are room and pillar operations and three are longwalls. (Two additional longwall mines are permitted but are not yet operating.) Annual production of the active mines ranges from 58 kt (64,000 st) to more than 6.5 Mt (7.2 million st). In 2005-2006, the Illinois Basin had a roof fall rate that was significantly higher than other coal-producing regions in the United States (Fig. 2). One of the reasons for this is that the Illinois Basin has few longwall mines. Longwall mining has fewer roof falls than room and pillar mining, because there is far less entry development per ton of coal mined. The Illinois Basin has only two producing longwall mines (a third mine has not yet begun its first panel) and had only 15.6% of its production from longwall mining in 2005-2006 (MSHA, 2006). However, the lack of longwall mining cannot explain all of the increase in roof fall rate. The southern Appalachian Basin has a similar proportion of longwall mining (15.0%), and yet its roof fall rate is 35% lower than that of the Illinois Basin. There may be two other reasons for the high roof fall rate in the Illinois Basin: A strong biaxial horizontal stress field and weak, highly moisture-sensitive roof rocks. High regional horizontal stresses in Illinois, and the damage resulting from them, have been both measured and documented by underground observation and mapping (Nelson and Bauer, 1987; Ingram and Molinda, 1988; Mark and Mucho, 1994; Mark et al., 2004). The Wabash mine in southeastern Illinois (now closed) had hundreds of long, running roof falls. Entries oriented north-south had severe damage because the regional stress field is approximately N80°E. To minimize the damage caused to entries oriented perpendicular to the regional stress direction, the mine turned its development 45° to the nearly E-W stress field. Roof conditions improved as a result of the reorientation. Figure 1 — Underground mines in the Illinois Basin