PERMFILE50246

• Permit No: C1994082
• IBM Index Class Name: Permit File
• Doc Date: 12/13/2004
• Doc Name: As-Built Report Sediment Structure Pond 011A
• Section_Exhibit Name: Tab 13 Attachment 13-19
• Media Type: D
• Archive: No

September 2004 Senera Coal Company * Yoart Mine Sedimenlalian Pond O11A Ar-Built Stobility Anofyrir O1
• 1.1 PURPOSE
1.0 INTRODUCTION
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 Ol]A embankment in order to document
compliance with applicable regulatory standards under Rule 4.05.9(8)(6).
1.2 METHODOLOGY
The static FOS for Sedimentation Pond O]IA was calculated using a basic, two-dimensional
equilibrium analysis program called SLOPE/W® (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 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
was constmcted. 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 the FOS was calculated using the Bishop's method.
1.3 PROCEDURE
The embankment cross-section shown on Exhibit SC was used to evaluate embankment stability. A
water surface elevation behind the embankment of 7,428.84 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 shown in Exhibit SC. 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 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 SLOPE/W
computer printouts contained in Appendix A.
The soil that comprises both the Pond ]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 the effective stress internal angle of friction to be 38° 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 this value for the
compacted spoil material that makes up the foundation and embankment at Pond 11A. The unit
weight of the spoil material was estimate at 120 pcf.
The parameters described above were input into the SLOPE/W program to evaluate static stability of
the Pond O11A structure and are shown in the attached program printouts with associated results.
•
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