SUBSIDENCE OF BEDROCK ABOVE ABANDONED COAL MINES IN ILLINOIS PRODUCES FEW FRACTURES.

In areas that have surface subsidence of . 3 to 1 m (1. 0 to 3. 25 ft), examination of cores from exploration drilling and investigations of boreholes by closed circuit television have shown that fractures are spaced at least . 9 to 3. 6 m (3 to 12 ft) apart and are located within a zone 6 to 15 m (20 to 50 ft) above the collapsed area in mines 42. 6 to 112. 7 m (140 to 370 ft) deep. Typical core recovery is 80 to 95 percent within the subsided areas and 100 percent outside.