ABSTRACT

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This paper describes an engineering study that was made to assess the stability of a site that is underlain by abandoned coal mining operations. Several procedures and techniques that are customarily associated with different types of engineering problems were employed in the analysis and interpretation of the site and subsurface conditions to obtain a rational solution to this difficult problem. The first stage of this study consisted of research to determine the past history of the site and the mining conditions underlying the site. Field reconnaissance and subsurface exploration were performed to supplement and confirm the findings from the research phase. Preliminary analyses of the mine conditions then led to an identification of the vital aspects of the site evaluation--evaluating the safety factor of the mine pillars and determining the probable subsidence behavior that is reasonable to expect in the future due to collapse of mine pillars having low safety factors. This determination proceded first by an application of empirical methods for predicting mine subsidence behavior and then by a finite element analysis of the site and mine conditions. The results obtained were used as a guide to engineering judgment in arriving at conclusions about the suitability of the site for development.

In the region around Wilkes-Barre, Pennsylvania, there are numerous abandoned underground workings in the anthracite coal veins and ground surface subsidence is not an uncommon occurrence. Most of these workings are presently flooded such that the mine pool elevation is, in places, above the ground surface elevation. As a result, artesian springs of this highly acidic mine water flow out upon the ground surface polluting the regional streams and rivers and ground water resources. As a temporizing measure to limit the maximum mine pool elevation, the Department of Environmental Resources opened the side of the abandoned Buttonwood air shaft and obtained a controlled drainage of this mine pool. About the time this drain was constructed, plans were advanced for the design and construction of a plant to purify the acid mine drainage water before it was released to the surface waters. A site for the proposed Acid Mine Drainage (AMD) treatment plant was selected adjacent to the Susquehanna River and to the west of Wilkes-Barre. This paper describes the engineering evaluation of the mine subsidence potential that was made for this proposed site for the AMD plant.

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The proposed AMD plant site is located within the Wyoming-Lackawanna Valley--a long narrow canoe-shaped depression in the Pocono Plateau that extends for about 56 miles in a southwest-northeast direction, Figure 1. This valley is also known as the Northern Anthracite Field of Pennsylvania with the site being located in the Wyoming Basin of the field. Upon first inspection, this valley appears to be a broad synclinal structure resulting from the folding movements that created the Appalachian Mountains and left this basin as a relic of one of the downwarps. Geologic investigations (9)* have shown a rather complex history to the valley evolution involving cycles of deposition and erosion, tectonic activity, regional drainage changes, etc., that is incidental to the problem at hand.

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