2008-12-04_PERMIT FILE - C1994082 (34)

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Orto6tr700r Senera C°a/Compatry • Scntra II IY/ tllinr SrGmtntati°n Pond 014 Stability Ana/ytit ~ 7 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 014 embankment in order to document design compliance with applicable regulatory standards under Rule 4.OS.ti(11)(j) or (k). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 014 was calculated using a basic, two-dimensional equilibrium analysis program called SLOPE/W (Version 3). The calculated FOS is the ratio of those resisting forces [hat would prevent embankment movement or failure to the corresponding driving forces. Resisting forces include the internal strength parameters for both the soil materials used in the construction 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 [he gravitational and lateral forces associated with the water volume that could be impounded behind the embankment. For this analysis, a circular failure mode was assumed, and the FOS was calculated using the Morgenstern-Price method. 1.3 PROCEDURE The as-built embankment cross-section shown on Exhibit 13-7 of the Yoast Permit (Leidich, 2001) was used to evaluate embankment stability. A water surface elevation behind the embankment of 7,385 feel above mean sea level (amsl) was used for the analysis. This water surface elevation is equivalent to the elevation of [he invert to the principal spillway. • The embankment geometry analyzed is based on the as-built survey. The phreatic surface for the stability analysis is based on a water surface elevation of 7,385 f[-amsl. The phreatic surface 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 phreatic surface is shown on the SLOPE/W computer printouts contained in Appendix B. In order to obtain shear suength properties of the soils, two samples were collected for laboratory shear strength testing. The strength properties for the foundation soils were obtained from a Shelby tube sample taken from a test pit at the location of the embankment. The strength testing was performed at the in-place density for the foundation soils. The Pond ]0 embankment fill strength results were selected to represent the strength properties of the Pond 14 embankment fill due to the similarity in index properties between the foundation soil sample, which is representative of the soils that will be used for the embankment fill, and the Pond 10 sample. The Pond 10 embankment shear strength test was performed on a remolded sample at 95% of the maximum dry density from the standard Proctor test (ASTM D698). The strength parameters used in the stability analysis are shown in Table 1. The results of the biaxial consolidated undrained tests and classification tests are included in Appendix A. TABLE 1 INDEX AND STRENGTH PROPERTIES OF SOILS FOR PONDS SAMPLE ATTERBERG PARTICLE SIZE EFFECTIVE EFFECTIVE LIMITS DATA FRICTION ANGLE, COHESION, (LL - PI) (% passing No. ~' a (psi 200 Pond 14 Foundation 26 - 11 75 24.5 0 Pond 10 Borrow Source 38 - 22 86 27.5 338 tl1°mlgomer)~ U%Ir°n Harla. 1G5 Soul6 Union B~uleuard, Suite 410 ~ I~kemood, Co/orrd° 80778 ~ (303J 7G3-5140 J:\7J70109Srna,uCaiAPondfAtb/t\U%rcAPONDla Atb~tbSlABILIIYda !!// 17/OI rat