July 11, 2017
Steeply sloped, high relief landforms with fractured sedimentary geology in proximity to underground mine voids have the potential to produce far-field ground strains which initiate strain failure modes for cross valley dams. In Greene County, Pennsylvania, the Ryerson Station Dam was breached in 2005 due to substantial structural and seepage issues resulting from unpredicted ground movement caused by nearby longwall coal mining.
A three-dimensional Finite Element Method (FEM) stress/deformation model was constructed in SVSOLID™ to study and predict the effects of ground movement which may lead to excessive strains in dam components (Figures 1 and 2). The numerical model was calibrated from displacement data obtained from three field survey stations for ground movements based on field site-specific material testing values.
The analysis compared results from increasing the subsurface strata precision from 3 layers to 14 layers. The analysis indicated that the number of rock layers had little influence on the precision of the model to predict actual field site ground displacements.
The sensitivity study also investigated resultant stress magnitudes in the subsurface strata based on the ground displacement of geo-referenced control station survey data. It was concluded that the underground mine encroachment on the ridge hillside landform adjoining the dam and reservoir had a greater influence on resultant stresses than the offset distance to the mine.
The modeling investigated fourteen survey stations, totaling 140 calculated strains at eleven time dates from April 26, 2006 through July 27, 2006 to determine the failure mode occurrence potential. Table 1 presents the legend and frequency of occurrence for each strain threshold.
Approximately 86% of the identified failure modes exist after the ridge hillside landform is influenced by mining. Foundation bearing capacity strain thresholds at the fourteen survey stations were exceeded up to 1005 m (3,298 ft.) away, while dam joint stress thresholds were surpassed at offsets up to 566 m (1,857 ft.). Table 2 shows that approximately 10% of the 140 failure scenarios modeled resulted in strain magnitudes which may result in seepage and structural failure modes at distances up to 566 m (1,857 ft.). Approximately 86% of all identified dam failure modes occurred in extraction regions R2 to R7.