The use of slurry impoundments has traditionally been the primary method of disposal of fine waste slurry resulting from the recovery of fine coal from coal preparation plants. However, disposal of coal waste has become challenging with stringent regulations for surface disposal facilities. The process of backfilling of mine void space is called slurry backfilling. This method would eliminate environmental, health, and safety risks associated with the surface disposal facilities such as impoundment failure due to ground motion, subsidence, etc. Although slurry backfilling has significant advantages, it may raise some mine stability concerns. As the floor or pillars of the mine are exposed to moisture, they may soften and lose strength. In this study, the slurry backfilling effect on stability of two Illinois underground coal mines is investigated. The mine floor consists of non-durable claystone material with various thicknesses. The geotechnical and geological information of the mine will be analyzed. The stability of these mines will be evaluated using empirical and numerical methods considering the softening of floor beds and pillars with slurry.
Based on the estimation of the Illinois State Geological Survey, about 330,000 unit houses are exposed to the dangers of surface subsidence . Consequently, stability of mines has been one of the main concerns in Illinois which should be professionally and practically addressed. The stability of mines deteriorates when exposed to moisture. On the other hand, due to more stringent Environmental Protection Agency (EPA) regulations slurry backfilling of mines as one of alternatives for disposing fine coal waste has attracted more attention. Slurry backfilling consists of pumping coal fine refuse to abandoned space. Slurry backfilling of coal waste can remove environmental, social, and health problems . In light of many case studies by different researchers and committees such as Griffith and Connor , on different mines, hydraulic backfilling has been recommended. However, some studies show that slurry backfilling does not work well for all types of mines. For example, in the Donovan study  this method has been recommended only for thin-seam coal mines.