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December 2002 Senna Coal Company * Seneca II W Mine Sedimentation Pond 017 Stability Analytic ~ 1 • 1.0 INTRODUCTION 1.1 PURPOSE The purpose of this stability analysis is to calculate the factor of safety (FOS) under static conditions for the as-built conditions of Sedimentation Pond 017 embankment in order to document design compliance with applicable regulatory standazds under Rule 4.05.6(11)(j) or (k). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 017 was calculated using a basic, two-dimensional equilibrium analysis program called SLOPE/W (Version 5). The calculated FOS is the ratio of those resisting forces that would prevent embankment movement or failure to the corresponding driving forces. Resisting forces include the internal strength pazameters for both the soil materials used in the construction of the embankment and the underlying base materials on which the facility was constructed. Potential driving forces include gravitational forces on [he embankment and foundation materials and the gravitational and lateral forces associated with the water volume that could be impounded behind the embankment. For this analysis, a circulaz failure mode was assumed, and the FOS was calculated using the Bishop's method. 1.3 PROCEDURE The as-built embankment cross-section shown on Exhibit 13-9B of the Seneca II W Permit (Leidich, 2002) was used to evaluate embankment stability. A water surface elevation behind the embankment of 7,391.9 feet above mean sea level (amsl) was used for the analysis. This water surface elevation is equivalent to the elevation of the invert to the principal spillway. • The embankment geometry analyzed is based on the as-built survey. The phrea[ic surface for the stability analysis is based on a water surface elevation at the invert elevation of the principal spillway. The phreatic surface through the embankment was estimated based on Dazcy's law and observation of similaz homogenous pond embankments at the mine site. At these locations seepage does not occur above the embankment toe. The resulting surface is shown on the SLOPE/W computer printouts contained in Appendix B. A foundation soil sample was collected from the foundation azea of the Pond 17 embankment. Additionally, an embankment soil sample was collected from the Pond 17 embankment fill. Grain size and Atterberg limits tests were run on [he samples and the results can be found in Appendix A, and aze summarized on Table I. Soil sheaz strength properties for the pond foundation and embankment were determined by comparing the results of index classification tests (particle size data and Atterburg limits) for soils in the azea. The index test results and shear strength for the soils tested for ponds at the site are shown in Table 1. The Pond 14 results were selected to represent the strength properties of the Pond 17 foundation soils due to the similarity in index properties. The sample of [he Pond 14 foundation soil was collected by excavating a test pit and sampling with a Shelby tube. An effective stress friction angle of 24.5 degrees with no cohesion was used in the stability analysis. The strength properties for the embankment fill were obtained from tests on a sample from Pond 010. The Pond 10 embankment strength results were selected to represent the strength properties of the Pond 17 embankment fill due to the similazity in index and sieve properties between [he sample from the Pond 17 embankment fill and the Pond 10 embankment fill (see Table 1). The Pond 10 embankment sheaz strength test was performed on a remolded sample at 95% of the maximum dry density from the s[andazd Proctor test (ASTM D698). An effective stress friction angle of 27.5 degrees with a cohesion of 338 psf was used • in the stability analysis for the Pond 17 embankment soils. The consolidated undrained triaxial shear MIY7H, Inc. 165 South Unian Baukvard, Suite 470 * I.akewvvd, Cvkrado 80728 * (303) 763-5140 J:\1470108 Senero Coal~PondrAr67t\Worx7~POMJ 17 ArbuiU STA911J7Y.ko 08/05/03 djin