Coal Mine Stability Concerns Due to Slurry Backfilling

Surface disposal is used for fine coal refuse by many mining industries as a traditional method. However, the surface disposing causes many environmental issues including contamination of groundwater and fields. One of the effective alternatives for surface disposal would be slurry backfilling which allows injection of fine coal refuse into void area of abandoned room-and-pillar coal mines. Due to the vast mined area in the US, slurry backfilling method has become more common approach for coal waste disposal than surface disposal. Decreasing environmental and health issues and increasing sustainability is one of the most important advantages of slurry backfilling. On the other hand, due to the existence of underground coal in the levels deeper than groundwater aquifers, the disposal material would not jeopardize environmental aspects of water resources. Furthermore, as coal seams in most of the underground mines are nearly flat strata, slurry backfilling would not cause leakage problem for mining industries in most cases.
One of the typical lithologies of coal seams consists of weak underclay stratum beneath the coal seam. The ‎immediate weak underclay causes frequent floor instability problems in many underground mines specifically in Illinois underground coal mine regions. ‎There are two different states in assessing floor stability of the mines where slurry backfilling is planned. The first state is during the mining operation when weak underclay layer squeezes under pillar stress and results in punching or bearing capacity failure. The second state is after slurry backfilling. After slurry injection, the watery ‎disposed material soaks into non-durable floor layer and causes a considerable reduction of floor strength. Due to the decrease of the strength of non-durable layer, the bearing capacity of the floor would be reduced and as a consequent phenomenon pillar punching or bearing failure would be happened.
Pillar stability ‎conditions should also be taken into account during both the operational period and after the slurry backfilling procedure. In most cases, the pillars were designed for a stable condition by mine companies. However, the effects of slurry backfill on reducing pillar strength have not been considered by mine industries and investigators. Therefore, comprehensive investigations are crucial to evaluate the effects of slurry backfilling on pillar stability as well as on floor stability in coal mines with weak underclay immediate floor.