Use of waste materials for control of acid mine drainage and subsidence

This paper presents the results of a study of the use of fluidized-bed combustion ash in an underground coal mine for control of acid mine drainage and subsidence. Fluidized-bed combustion ash (FBC ash) is a waste by-product of advanced coal combustion. Presently, most of the FBC ash produced in power generation is disposed in landfills. Subsidence and acid mine drainage are two major problems found in coal mining areas. The problems of disposal of FBC ash, mine subsidence, and acid mine drainage can be reduced by placing the ash in abandoned coal mines. The study included laboratory experiments, computer modeling, and a field application of FBC ash in an abandoned underground mine. Potential subsidence values under different backfill configurations were computed by using the finite element method. Strength requirements for the grout were determined on the basis of site-specific geologic information and a stress analysis. The FBC ash was placed in a mine cavity by pumping it in the form of a grout to explore the viability of hydraulic backfilling. Field observations were made on the field strength and flow characteristics of the FBC ash grout. The paper contains information on strength requirements of the grout, flow characteristics of the grout, expected reductions in potential subsidence, and acid mine drainage around the grouted mine. The results show that a grout made of fluidized-bed combustion ash can be successfully placed in a mine cavity by hydraulic backfilling in order to reduce the problems caused by subsidence and acid mine drainage.