Settlement of mine spoil fill from water infiltration: Case study in Eastern Kentucky

Mine spoil valley fills are a by-product of mountaintop removal mining in the Appalachian coal mining region of the United States. These fills often result in large expanses of relatively flat land covering thousands of acres, which can be used for commercial or industrial development. However, this material is susceptible to damaging settlement, and highly publicized failures of structures built on mine spoil fills have led to reluctance on the part of investors to develop these areas. A key settlement mechanism in mine spoil is water infiltration. Percolating water slakes the shaly, angular spoil material at interparticle stress points, leading to excessive deformation and settlement. A lumber processing facility in Hazard, Ky., is an example of a structure that sustained serious damage as a result of settlement caused by water infiltration. A forensic site investigation of the facility revealed that excavation of existing surface mine spoil beneath the building footprint removed the low-permeability crust that forms on the top of mature mine spoil fill deposits. The removal of the crust allowed the infiltration of surface water. This, coupled with the unique configuration of the storm water drainage system at the facility and surface water drainage toward the building, led to differential settlement up to 1:120 (vertical:horizontal) and angular distortion up to 1:150 over a period of several months. Foundation underpinning was performed to remedy the situation. For future development on mine spoil sites, recommended mitigating measures include presaturation of the mine spoil, design of drainage systems to adequately convey surface water away from the building, and use of geosynthetic barrier layers to prevent infiltration of surface water into the mine spoil beneath the structure.