PERMFILE47584

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 11/22/2004
• Doc Name: As-Built Report Sediment Structure Pond 013
• Section_Exhibit Name: Tab 13 Attachment 13-20
• Media Type: D
• Archive: Yes

September 2004 Seneca Coal Company * Yowt Mine Sedimemlatiox Pond 013 At-Bull! Stability Analysis ~ 1
• 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 013 embankment in order to document compliance
with applicable regulatory standards under Rule 4.05.9(5}(b}.
1.2 METHODOLOGY
The static FOS for Sedimentation Pond 013 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 [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 Bishop's method.
1.3 PROCEDURE
The design embankment cross-section shown on Exhibi[ 13-7 was used to evaluate embankment
stability. A water surface elevation behind the embankment of 7,367.3 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 (see Exhibit 13-7). 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 pond embankments at the mine site. The resulting surface is shown on the
SLOPE/W computer printouts contained in Appendix A.
Based on soil surveys performed by Jim Nyenhuis (1993 - 1996) and as indicated in Table 1, the soil
and subsoil materials in the Sedimentation Pond 013 area are predominantly slightly sandy to sandy
days. These materials are of a similaz type to the soils previously collected and analyzed for
Sedimentation Pond 010 based on soils mapping information and NRCS soils chazacterizations.
Given the reasonable comparability based on soil type, the soil physical parameters previously
determined for Sedimentation Pond 010 have been used in the stability analysis for Sedimentation
Pond 013.
TABLE 1
SEDIMENTATION POND 013 SOIL DATA
Soil No. Soil Name Atterberg Limits Particle Size Data USCS Classification
ILL - PO 1% assin No. 200)
11 C Binco 35 to 60 - 15 to 72 - 92% CL-CH
35
35E Aaber No data 60 - 95% CL
In order to estimate the engineering properties of the available soil, field data collected for
• Sedimentation Pond 010, and haul roads Y-A and Y-B were reviewed (Northwest Colorado
Consultants, 1994 and 1996). The data were compazed to the NCRS index data (NCRS, 1999)
available for the area of Pond 013 according to [he Yoast Mine soil survey (Nyenhuis, 1993-a). The
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