Shale facies and mine roof stability: A case study from the Illinois basin
The Hornsby area in south-central Illinois contains one of the largest deposits of low-sulfur coal in the Illinois basin coal field. Mining of the resource has not proceeded apace with the demand for low-sulfur coal, mainly because of fear of unstable roof conditions beneath the Energy Shale (Desmoinesian). Three different facies of the Energy Shale overlie the Herrin Coal in the Hornsby area. All three facies are fine-grained, argillaceous rock, but their response to mining is different. The laminated shale facies has a pervasive lamination and in many places is cut by small faults and joints which make the rock prone to slabbing and potting prior to bolting. The gray shale facies that is interbedded with the laminated shale facies lacks pervasive lamination, and fractures are not well developed in it. The gray shale facies generally provides good roof. The carbonaceous shale facies had not yet been encountered in mine workings in the Hornsby area when this study was completed. Lamination in the carbonaceous shale facies is better developed than that in the gray shale facies, but it is not as well developed as that in the laminated shale facies. When the carbonaceous shale facies is the roof rock, the extensive roof control measures that were implemented for mining beneath the laminated shale facies will probably not be required.