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Apn! 2000 Seneca Coa! Company * Seneca II W Mine Sedimentation Pond 016 Srabi(tyAna~tit ~ ] • 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 016 embankment in order to document design compliance with applicable regulatory standards under Rule 4.05.6(11)(j) or (k). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 016 was calculated using a basic, two-dimensional equilibrium analysis progam called SLOPE/W (Version 3). The calculated FOS is the ratio of those resisting forces that would prevent embankment movement or failwe to the corresponding driving forces. Resisting forces include the internal svength parameters for both the soil materials used in the consvuction of the embankment and the underlying base materials on which the facility will be constructed. Potential driving forces include gravitational forces on the embankment and foundation materials and the gravitational and lateral forces associated with the water volume that could be impounded behind [he embankment. For this analysis, a circular failure mode was assumed, and the FOS was calculated using [he Spenser method. 1.3 PROCEDURE The as-built embankment cross-section shown on Exhibit 13-9A of [he Seneca II W Permit (Leidich, 2000) was used [o evaluate embankment stability. A water surface elevation behind the embankment of 7,462.8 feet above mean sea level (amslj was used for [he 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 potential phreatic swface for [he stability analysis is based on a water surface elevation of 7,462.8 ft-amsl corresponding to the invert elevation of the principal spillway. The phreavc swface through the embankment was estimated based on Darcy's law and observation of similar 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 SLOPFJW computer printouts contained in Appendix B. An undisturbed sample was collected from [he foundation area of the proposed Pond 16. The foundation soil sample was obtained from a Shelby tube sample taken from a test pit at [he location of the proposed embankment. Triaxial (with pour pressure), grain size and Atterberg limits tests were run on the sample and the results can be found in Appendix A, and are summarized on Table 1. During consvuction the embankment 511 material was sampled and subjected to sieve analysis and Atterberg limit testing. The strength properties for the embankment fill were obtained from tests on a sample from Pond 010. The Pond 10 embankment svength results were selected [o represent [he svength properties of the Pond 16 embankment fill due to the similarity in index and sieve properties between the sample from the Pond 16 embankment fill and the Pond 10 embankment fill (see Table 1). The Pond 10 embankment shear strength test was performed on a remolded sample at 95% of the maximum dry density from the standazd Proctor test (ASTM D698). The results can be found in Appendix A, and are summarized on Table 1. • s ~ r ~TABL ° ~ ~ ' R D ~~ _ _ _ ,~~a`~ND_ _E7C REN C i PROP R C,~ O P $,: II:5FO : ON S_, _ ' SAMPLE ATTERBERG PARTICLE SIZE EFFECTIVE EFFECTIVE LIMITS DATA FRICTION ANGLE, COHESION, (LL - PI) {°/ passing No. 200) ~~ c' (pst) Pond 16 Foundation 49 - 20 96 29.9 0 Pond 76 Embankment Fitt 37 - 22 83 - Pond 10 Embankment Fill 38 - 22 86 27.5 338 Momrgomery IY/auon Amenrat, L¢ 1 GS Sonrh Uaion Boulevard, Suite 410 * Lakewood, Colorado 80228 * (303J 763-5140 f:\2AOt08 Srnnv CoaATr-35\POND 7d AtbuilrSTABIL.rfY.dot r r/28/P9 mb