2010-04-26_PERMIT FILE - C1980007A ‎(3)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 4/26/2010
• Doc Name: Refuse Pile Expansion East
• Section_Exhibit Name: Exhibit 82 Refuse Pile Expansion East
• Media Type: D
• Archive: Yes

2.2.3 Model Results
A slope of 2.5H: IV was selected for the north face of the RPE East refuse pile to optimize refuse pile
capacity. A buttress with a key trench is included in the design, as is a drainage blanket under the
entire RPE East refuse pile and a rock drain along the thalweg of No Name Gulch and a rock drain to
convey seepage water past the base of the buttress. In -place colluvium in the refuse pile footprint
will be removed to expose bedrock prior to placement of refuse, and so the colluvium is not included
in the geotechnical model as a continuous layer. However, it will be mixed with the refuse typically
in windrows perpendicular to the slope contours and included in the refuse pile, but not placed as a
layer. The modeled section is placed in the thalweg of the draw because this will be the deepest zone
of refuse and is, therefore, the critical design section.
2.2.3.1 Refuse Pile Configuration
The proposed RPE East refuse pile stability was modeled with the characteristics of the LRP refuse.
Two refuse pile design cases were further modeled in response to seepage and stability model results.
Design Case A has the north face of the RPE East extending from the crest of the buttress at
elevation 6550 at a 2.5H: IV slope to elevation 7440, just lower than the elevation of the ridge
between No Name Gulch and the box canyon to the south, as shown on Figures 2 -5 and 2 -6. Design
Case B added 300 - foot -wide benches at elevation 6835 and elevation 7185, with a top elevation of
7440, as shown on Figures 2 -7 and 2-8.
2.2.3.2 Model Results
Rule 4.10.1(1)(a) (per 4.09.1(7)) requires a long -term static factor of safety of 1.5 or higher and a
seismic factor of safety of 1.0 or higher. Design Case A meets this requirement. Design Case B
applies if it is assumed the RPE East refuse pile can develop undrained conditions but the static
factor of safety for this case is 1.2 instead of 1.5. End -of- construction (undrained) factors of safety
are typically lower than long -term (drained) factors of safety. The lower factor of safety is justified
because it is considered very unlikely that fully undrained conditions will be present in the refuse
pile and the actual refuse material is expected to be better drained than the surrogate material
assumed from the LRP. It should be noted that the required seismic factor of safety for Case B
remains 1.0.
Revised March 19, 2010 P65
Revised April 2, 2010 P6-5 `` ,6
23 A p �.
P:\Mpls \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit \WorkFiles\Permit Application \Text\2010 -04 -19 RPE East Permit Application.doc 6