Longwall mining is a common underground coal extraction technique in Northern Appalachia. The coal extraction takes the form of large rectangular areas whose width and length can reach up to 450 m and 4000 m, respectively, and have a thickness of around 2.0 m; mining depths range from 180 m to 280 m. A number of longwall panels have been mined underneath interstate highway I-79 in southwestern Pennsylvania causing large subsidence that raises concern for traffic safety. The Pennsylvania Department of Transportation monitored the impact of mining on the highway and collected the data on which this study was based. Specifically, field data obtained from eight longwall panels included time series of surveying data collected as each mine advanced underneath the highway, and inclinometer data obtained at selected points where stability of embankment slopes was a concern. With the aid of a genetic algorithm, a three dimensional subsidence model was developed that described the data well. The model gives the spatial and temporal distribution of surface subsidence in terms of the depth of the extracted coal panel, the width of panels, the thickness of extraction, and the location relative to the face of an advancing panel.
When underground mining is performed, the overburden, or earth portion from the mine to the surface, experiences a loss of equilibrium due to the extraction of material. For a new equilibrium condition to be achieved, the overburden moves towards the created cavity. If the extracted area is large enough, disturbances in the overburden can reach the surface . Surface deformations have both vertical and horizontal components and form a basin or trough [1,2,3]. Subsidence and surface strains induced by mining can affect the safety and integrity of buildings, and reservoirs . The correct prediction of the magnitude, location, and defining characteristics of the subsidence trough is important in aiding the monitoring and protection of structures located at the surface and near subsurface. In Northern Appalachia, which includes southwestern Pennsylvania, northern West Virginia and Ohio, extensive data gathered in the last few decades helped researchers develop empirical subsidence models based on profile and influence function methods [5,6]. The applicability of the existing predictive models in the case of highways has not been investigated, and little has been done to develop models to predict highway deformations associated with underground mining. In highways, structures that need protection or repairs due to the negative impact of mining are mainly pavements, culverts, and bridges . This paper describes the development of an empirical highway subsidence model based on data collected by the Pennsylvania Department of Transportation along interstate highway I-79 in Greene County, Pennsylvania. The surface deformations were a consequence of undermining the Pittsburgh Coal seam underneath the highway in the Cumberland and Emerald mines.
Figure 1 gives the approximate location of the Emerald and Cumberland mines in southwestern Pennsylvania. Figure 2 and Figure 3 give an overview of the longwall panels in the Emerald and Cumberland mines.