Summertime rashing of coal mine roof shales can be deleterious to mine operation by restricting ventilation, impeding access, and posing other safety hazards. A two-year field monitoring program, coupled with laboratory investigations of shale behavior under various humidities, was aimed at defining causes and methods of prediction of shale roof deterioration.
Both strong and weak shales may exhibit deterioration in mines. It appears, on the basis of both laboratory and field investigations, that some weak shales, such as the “soapstone” above the Pittsburgh seam, may undergo very large swelling strains under high humidities. Some of this is recovered during wintertime drying, but the result is general loosening and disaggregation of the rock mass, which falls in small or medium-size chunks. By contrast, a more competent shale that exhibits less swelling strain (such as the roof of the Harrisburg [No. 5] seam in the Illinois basin) may still develop high compressive stresses and fail, by buckling or shear, into medium or large slabs. Further monitoring, both in the lab and underground, is needed to enable confident predictions of roof deterioration for all shales.
