Estimating Highway Subsidence due to Longwall Mining

Longwall mining is a common underground coal extraction technique in Appalachia. The extraction takes the form of panels whose width and length can reach approximately 450 m and 4000 m, with a thickness of about 2.0 m. Typical depth ranges from 180 m to 280 m. Longwall panels were mined underneath highway I-79 in the Cumberland and Emerald mines in southwestern Pennsylvania, causing large subsidence that affects traffic safety and can potentially damage highway structures such as pavements, culverts, and bridge abutments. Mining under the highway prompted the close monitoring by the Pennsylvania Department of Transportation of the impact of mining on the highway sections above the mines. A substantial amount of data was collected that formed the basis of this work. The data included time series of surveying data and inclinometer data in selected points. With the aid of a genetic algorithm, a three dimensional subsidence model was developed. The model gives the spatial and temporal distribution of surface subsidence in terms of the depth of mining, the panel width, the thickness of extraction, and the location relative to the face of the panels. Although the prediction of vertical deformations through the empirical model is feasible, the lateral deformation behavior of highway foundations did not always follow the premises adopted in existing subsidence prediction tools, often based on flat conditions. The complex topography of highway foundations, dominated by embankments with irregular cross sections, a sloped grade, and different orientations with respect to the direction of mining, gives each case a unique character that deems it very difficult to develop comprehensive empirical models to predict the location and magnitude of lateral deformations and strain/stress concentrations. The lateral component of subsidence prediction is very important as it is directly related to damage of the highway structures. A FEM model was developed in order to better understand the mechanisms of subsidence. The results of both empirical and numerical modeling are presented. The findings of this study have a broader scope than highway deformations, with potential applications on any type of earthen structures impacted by underground mining