Grouting to control coal mine subsidence

Abandoned underground coal mines are located throughout the coal fields in the United States. The mines are anywhere from a few meters to several hundred meters below the ground surface. In many of these mines coal pillars and/or wooden supports were left in the mine to keep the roof from collapsing during operations. After abandonment these roof supports deteriorate and eventually collapse causing subsidence. To protect the public from subsidence and other adverse effects of coal mining an Abandoned Mine Lands Reclamation Fund was established under Public Law 95-87. This fund is used to finance subsidence control projects. Grout is frequently used to stabilize areas in the mines by filling the mine voids and providing the support needed to minimize future subsidence. However, the placement of the grout may require unique equipment or customized grout materials since abandoned mines are generally inaccessible, the areas requiring stabilization are beneath surface structures, the voids may be flooded, and methane gas may be present. In addition to special considerations in grout placement the areas requiring roof support may be extensive and filling the voids in the entire mine are cost prohibitive. Special grout placement techniques have been evaluated including controlled grout columns or synthetic bags filled with grout and chemical additives to control grout set-up.