2012-11-30_PERMIT FILE - C1980007A

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 11/30/2012
• Doc Name: RPEE Phase 1B Buttress
• Section_Exhibit Name: Exhibit 82 Appendix K
• Media Type: D
• Archive: Yes

To: Mike Peacock, Kathy Welt
From: Paul Swenson, Jed Greenwood
Subject: Summary of Additional 2D Stability Modeling for RPE East; Modified Phase 1 B Buttress
Date: October 12, 2012
Page: 5
Project: 06261003
c: File
The USSA results are not shown here because the potential failure surface exits above the top of the
buttress such that the buttress and the shear strengths of the materials in the buttress do not impact the
reported factor of safety compared to what was submitted previously. The buttress scale was not
examined because the buttress has already been constructed such that the end -of- construction conditions
should not apply. Furthermore, piezometer readings indicate generally unsaturated conditions in the
refuse and colluvium in the area of the buttress.
Modified Phase 1 B Buttress Design
The plan and typical sections for the modified Phase 1B buttress are shown on Figures 6, 7 and 8.
Conditions shown on these figures reflect construction record information where available. The Phase 1 A
buttress has been completed, as has the modified Phase 113 colluvium buttress. Although the modeling
indicates that placement of the Phase 1 B rock materials could be delayed until 2013, placement of rock in
the Phase 1B rock buttress zone has begun and should continue as rock becomes available from refuse
pile development work.
Conclusions
The analysis results indicate that the factors of safety are adequate according to Rule 4.10.1(1)(a) (per
4.09.1(7)), which requires a long -term static factor of safety of 1.5 or higher and a seismic factor of safety
of 1.0 or higher. This indicates that refuse placement can continue to a top of slope elevation of 6,930
with the modified Phase 1 B buttress configuration until the end of 2013. While the stability analyses
indicate that overall stability factors of safety conform to rule requirements and engineering standard of
practice, an observational approach to confirming the results should be adopted. Localized erosion or
movement can occur. Should such movement occur, actions to repair such areas should be implemented
to avoid impacts to the overall slope.
The Phase 113 rock buttress should be placed at the earliest convenient time, and no later than the end of
2013, to protect the colluvium from erosion and surface sloughing.
P:\Mpls \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit \WorkFiles\Slope Modeling\9-27-12\RPE_East_Modeling_Summary_10-
12-2012.docx