A field geotechnical study of the effects of weak floor strata on underground coal mining in Illinois

In Illinois, the commonly mined coal seams (Springfield No 5 and Herrin No 6) are generally underlain by relatively weak (300-1000 psi compressive strength) and thick (2-7 ft) underclays. Weak floor strata in a partial extraction room-and-pillar mining system may lead to problems of a) pillar settlement and instability, b) floor heave and squeeze, c) roof falls due to differential settlement of pillars, and d) surface subsidence movement. Unfortunately, even small subsidence movements may have adverse effects on surface land use. To address these problems, a detailed geotechnical study was undertaken in September, 1983 to investigate the effects of weak floor strata on room-and-pillar coal mines with an emphasis on mining conditions in central Illinois.