Analysis of a coal refuse dam subject to potential mining-induced subsidence

One of the most economically feasible methods of disposing coal mine waste is the construction of slurry impoundment structures. However, these structures are potentially hazardous if not properly designed. Because many potential impoundment sites are subject to abandoned or future mining activity, surface subsidence must be accounted for in design. This is the case for the proposed Harper Branch impoundment in Buchanan County, Virginia which is undermined at both abutments by two coal seams, the Hagy and Splashdam. Potential subsidence was investigated as to type, magnitude, and time of occurrence. This was accomplished using various methods with empirically derived functions determined for the region. Subsidence was predicted for both actual and worst-case conditions. The predicted subsidence was then put into a two-dimensional finite element program as displacement boundary conditions to calculate deformations and potentially stressed areas in the dam. The results of this analysis show that the strength and magnitude of the remnant coal pillars in the mines are sufficient to prevent subsidence under the impoundment. Sinkholes may potentially occur in areas of shallow mine overburden, however. Assuming a worst-case subsidence scenario in which the remnant pillars effects are largely ignored, dam crest settlement is a maximum of 3.27 ft. (1.0 m) and 2.49 ft. (0.76 m) at the north and south abutments respectively. Stressed areas are also predicted at both abutments and in the top center of the dam. The analysis provides a guide of which portions of the dam to monitor or instrument. Based on these results, geotextile could be placed in the foundation and in potentially critically stressed areas of the dam to stop sinkhole propagation and reduce differential settlement. The area could also be grouted for stabilization. Also, the dam material could be compacted wet of optimum in the potentially stressed areas to prevent cracking. Finally, internal filters could be provided in designated areas to prevent potential piping.