2009-10-01_PERMIT FILE - C1994082A

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 10/1/2009
• Doc Name: Slope Stability Evaluation, Runoff Control Structures
• Section_Exhibit Name: Tab 20 Attachment 20-2
• Media Type: D
• Archive: Yes

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August 10, 2009
Peabody Energy
P.O. Box 670
Hayden, CO 81639
Attn: Mr. Jay James
Job Number: 07-7600
Subject: Slope Stability Evaluation, Proposed Yoast
Mine - Runoff Control Structures, Seneca Yoast
Mine, Routt County, Colorado.
Gentlemen:
As requested, NWCC, Inc. has completed a Slope Stability Evaluation for the proposed Yoast Mine - Runoff Control
Structures to be constructed within the Seneca Yoast Mine in Routt County, Colorado. Our firm previously completed
a Slope Stability Evaluation for Pond 12A at the Yoast Mine, which was prepared under this job number and dated
August 20, 2007.
The stability of proposed runoff structure was assessed by computing theoretical factors of safety (against slope failure)
for the proposed slope configuration. 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 4.4 for
this project.
By inputting slope geometry and soil strength values into the slope stability program GEO-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 a variety of
methods for factor of safety calculations.
The slope configurations used in our analyses were obtained from the Typical Runoff Control Structure Drawing
provided to us by the client on August 4, 2009. Copies of the plan and cross section provided by the client are shown in
Appendix A.
A discussion and summary of the slope stability evaluations for the proposed structure are discussed below.
Proposed Runoff Control Structures: A slope stability analysis was completed for the proposed runoff control
structures to be constructed across the mine site, in hopes to reduce erosion and flow rate of the existing drainage
. channels in the area. It appears that the proposed runoff control structures will be constructed to a maximum height of 8
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