Horizontal Regional Stress
Horizontal stress fractures and related falls will occur in entries perpendicular to the maximum stress direction where the horizontal stress field is strongly directional. Maps of regional horizontal stress-field data are provided in Zoback and Zoback (1980) and for the Illinois Basin in Nelson and Bauer (1991). Online data, which can be converted into map view, are updated and available at the World Stress Map Project.
In western Kentucky the stress field is oriented at near east-west orientations. In southern Illinois and Indiana, maximum horizontal stress strikes from N73°E to east-west (Nelson and Bauer, 1982; Molinda and others, 2008). In western Kentucky, the stress field is oriented at near east-west orientations. In southern Illinois and Indiana, maximum horizontal stress strikes from N73°E to east-west (Nelson and Bauer, 1982; Molinda and others, 2008). Nelson and Bauer (1982) reported that joints in hard black shales above mined coals in southern Illinois commonly are parallel to the main stress axis (N80°E to east-west). Ault and others (1985) thought that many north-south-oriented faults, including small, in-mine thrust faults, and fractures in the Illinois Basin were the result of the contemporary horizontal stress field. Even where headings are reoriented to intersect the horizontal stress direction at oblique angles, cutters may form along ribs or cutting across northwest-southeast- and northeast-southwest-oriented entries. Although these cutters may appear to be randomly oriented, when plotted on a mine map, they commonly group into an average north-south direction.
In eastern Kentucky, the current horizontal stress field strikes along northeast-southwest orientations, approximately parallel to the trend of thrust faults formed during Appalachian mountain building.