2008-01-16_PERMIT FILE - C1994082 ‎(6)‎

• Permit No: C1994082
• IBM Index Class Name: Permit File
• Doc Date: 1/16/2008
• Doc Name: Slope Stability Evaluation Pond 012A Design
• Section_Exhibit Name: Tab 13 Attachment 13-21
• Media Type: D
• Archive: No

Geotechnical /Environmental Engineering • Materials Testing
August 20, 2007
Peabody Energy
P.O. Box 670
Hayden, CO 81639
Attn: Roy Karo
Job Number: 07-7600
Subject: Slope Stability Evaluation, Proposed Pond
12A, Yoast Mine, Routt County, Colorado.
Gentlemen:
As requested, NWCC, Inc. has completed a Slope Stability Evaluation of the proposed embankment slopes to be
constructed for the proposed Pond 12A to be constructed at the Yoast Mine in Routt County, Colorado. The purpose
of this stability evaluation was to calculate the factor of safety under static conditions for the proposed design
conditions of Pond 12A embankment in order to document design compliance with applicable regulatory standards
under Rule 4.05.9(8)(b).
Methodology: The stability of proposed embankment structure was assessed by computing theoretical factors of
safety (against slope failure) for the proposed slope configurations. A factor of safety in this case is calculated by
summing the forces resisting slope movement (i.e. soil strength generated along a theoretical failure plane) and dividing
that sum by the driving forces of the slope (i.e. the weight or mass of the soil and groundwater). We have chosen a
minimum factor of safety of 1.5 for this project.
By inputting slope geometry and soil strength values into the slope stability program SB-SLOPE, strength and
geometry values can be easily varied to help assess the factors of safety for the proposed slopes under varying
conditions. This program is a comprehensive slope stability program for microcomputers and uses the "Simplified
Bishop Method of Slices" for factor of safety calculations. The slope configurations used in our analysis were obtained
from the cross section drawing (Exhibit 13-12.7) provided by the client and dated July 2007. A copy of this cross
section that shows the dam configuration at its highest point is provided in Appendix A. The slope stability analyses
and data are presented in Appendix B and a discussion and summary of the results are discussed below.
Procedure: The analyses were completed on the maximum embankment section that will be constructed with an
upstream slope configuration of 3(H) to 1(V) and a downstream slope configuration of 3(H) to 1(V). The first case that
was analyzed was with the reservoir full and water at the principal spillway elevation (7344.5'). A phreatic surface has
also been added to this model to analyze the embankment under a full reservoir condition with steady state seepage.
The second case illustrated represents the same cross section with a rapid drawdown condition. In both cases, the
slopes were analyzed for what was considered to be the most likely failure modes: (1) A failure of the downstream
embankment slope during peak water storage and; (2) A failure of the upstream embankment face after a rapid
drawdown of impounded water. These analyses were conducted using conservative strength estimates based on the
laboratpry test results completed on remolded samples of the proposed fill materials, as well as estimated soil shear
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