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December 2003 Senora Caa! Compvny * Senora TT W Mine Sedrmentatian Pond 011A Dengn StabrGly Analynr ~ 1 <br />1.0 INTRODUCTION <br />• 1.1 PURPOSE <br />The purpose of this stability analysis is to calculate the factor of safety (FOS) under static conditions for the <br />design conditions of Sedimentation Pond O11A embankment in order to document design compliance with <br />applicable regulatory standards under Rule 4.05.9(8)(b). <br />1.2 METHODOLOGY <br />The static FOS for Sedimentation Pond O11A was calculated using a basic, two-dimensional equilibrium <br />analysis program called SLOPElW® (Version 5.1). The calculated FOS is the ratio of those resisting <br />forces that would prevent embankment movement or failure to the corresponding driving forces. Resisting <br />forces include fhe internal srength parameters for both the soil materials used in the construction of the <br />embankment and the underlying base materials on which the facility was constructed. Potential driving <br />forces include gravitational forces on the embankment and foundation materials and the gravitational and <br />lateral forces associated with the water volume that could be impounded behind the embankment. For this <br />analysis, a circular failure mode was assumed, and [he FOS was calculated using the Bishop's method. <br />1.3 PROCEDURE <br />The design embankment cross-section shown on Drawing 13-SB of the Yoast Permit (Leidich, 2003) was <br />used to evaluate embankment stability. A water surface elevation behind the embankment of 7,421.5 feet <br />above mean sea level (amsl) was used for the analysis. This water surface elevation is equivalent to the <br />elevation of the invert to the principal spillway. <br />The embankment geometry analy2ed is based on the design drawings. The embankment is designed with <br />• 3H:1V slope for both the upstream and downsveam slopes. The upstream slope will be protected from . <br />erosion by vegetation. <br />The phreatic surface for the stability analysis is based on a water surface elevation at the invert elevation of <br />the principal spillway. The phreatic surface through the embankment was estimated based on Dazcy's law <br />and observation of similar homogenous pond embankments at the mine site. At these locations seepage <br />does not occur above the embankment toe. The resulting surface is shown on the SLOPFIW computer <br />printouts contained in Appendix A. <br />The soil that comprises both the Pond I lA embankment and the foundation of the embankment is spoil <br />material. Soil shear strength properties for the pond foundation and embankment were estimated from the <br />angle of repose for loosely deposited spoils piles measured visually in the field. The senior engineer <br />estimated [he effective stress internal angle of friction to be 39° with no cohesion. The loose deposits of <br />spoils piles were observed to have angles of repose steeper than this estimated internal angle of friction. <br />Therefore, it was considered conservative to use [his value for the compacted spoil material that makes up <br />the foundation and embankment at Pond 11A. The unit weigh[ of the spoil material was estimate a[ 120 pcf. <br />The parameters described above were input into the SLOPE/W progam to evaluate static stability of the <br />Pond 011A structure and are shown in the attached program printouts with associated results. <br />MIY/H, Tnc. 1801 Ca!+jomra Street, Suite 2900 * Denver, Colorado 80202 * (303) 291-2222 <br />J:\2470108 Senora Coa!\TTt~14\Pond 011 A Duign St°brliy.kr <br />11/04/03 ryin <br />