PERMFILE55043

• Permit No: C1994082
• IBM Index Class Name: Permit File
• Doc Date: 3/2/2004
• Doc Name: Sedimentation Pond 011A Stability Analysis
• Section_Exhibit Name: Tab 13 Attachment 13-18
• Media Type: D
• Archive: No

December 2003 Senora Caa! Compvny * Senora TT W Mine Sedrmentatian Pond 011A Dengn StabrGly Analynr ~ 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
design conditions of Sedimentation Pond O11A embankment in order to document design compliance with
applicable regulatory standards under Rule 4.05.9(8)(b).
1.2 METHODOLOGY
The static FOS for Sedimentation Pond O11A was calculated using a basic, two-dimensional equilibrium
analysis program called SLOPElW® (Version 5.1). 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 fhe internal srength parameters 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 the 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 circular failure mode was assumed, and [he FOS was calculated using the Bishop's method.
1.3 PROCEDURE
The design embankment cross-section shown on Drawing 13-SB of the Yoast Permit (Leidich, 2003) was
used to evaluate embankment stability. A water surface elevation behind the embankment of 7,421.5 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 analy2ed is based on the design drawings. The embankment is designed with
• 3H:1V slope for both the upstream and downsveam slopes. The upstream slope will be protected from .
erosion by vegetation.
The phreatic 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 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 SLOPFIW computer
printouts contained in Appendix A.
The soil that comprises both the Pond I lA embankment and the foundation of the embankment is spoil
material. Soil shear strength properties for the pond foundation and embankment were estimated from the
angle of repose for loosely deposited spoils piles measured visually in the field. The senior engineer
estimated [he effective stress internal angle of friction to be 39° with no cohesion. The loose deposits of
spoils piles were observed to have angles of repose steeper than this estimated internal angle of friction.
Therefore, it was considered conservative to use [his value for the compacted spoil material that makes up
the foundation and embankment at Pond 11A. The unit weigh[ of the spoil material was estimate a[ 120 pcf.
The parameters described above were input into the SLOPE/W progam to evaluate static stability of the
Pond 011A structure and are shown in the attached program printouts with associated results.
MIY/H, Tnc. 1801 Ca!+jomra Street, Suite 2900 * Denver, Colorado 80202 * [phone]
J:\2470108 Senora Coa!\TTt~14\Pond 011 A Duign St°brliy.kr
11/04/03 ryin