2009-05-12_PERMIT FILE - C1980007

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 5/12/2009
• Section_Exhibit Name: Exhibit 69 Section A II. Mine Dewatering and Treatment Facilities
• Media Type: D
• Archive: No

Client Memorandum
Geotechnical and Slope Stability Analyses
Sylvester Gulch Facilities Area
. March 20, 2009
Page 5
STABILITY ANALYSES
This section summarizes the geotechnical stability analyses and consolidation analyses of the
Second Pond excavation slopes and maximum embankment section. These analyses utilize data
developed from the 2008 test pit and laboratory testing program and from the 1997 boring logs
and associated laboratory testing program.
Inclinometer data were reviewed for a device installed near the edge of the steep hillside (1-13)
just east of the Second Pond. These data indicate relatively minor soil movements of less than
1.5 inches over the past 12 years; therefore, the global hillside stability is judged to be stable
enough for construction of a Second Pond.
Analyses performed for stability include two-dimensional slope stability modeling and
consolidation analyses under the maximum embankment loads. The maximum excavation slope
was modeled in the southwest portion of the Second Pond and the maximum embankment
section was modeled in the northeast portion of the pond.
Material Strength Parameters
The various soil material parameters used in slope stability modeling include unit weight (y),
internal friction angle (o) and cohesion (C). These were assigned based on the laboratory testing
program results, published data for similar materials and engineering judgment. The following
table summarizes the various conservative material parameters utilized in the analyses:
Table 2. Stability Modeling Parameters
Material Zone Unit Weight Friction Angle Cohesion
O (o) (C)
118 pcf 20° 250 psf;
Compacted Embankment (Silty to Gravelly Clay, CL) (moist);
130 pcf 150 psf
(saturated) (minimum)
125 pcf 28° (normal
Native Silty-Sandy Gravelly Clay with some rocks (CL and (moist); effective stress) 190 psf
SC-GC) 135 pcf
(saturated) 23° (worst case)
45° (normal)
Rock at Embankment Toe 125 pcf 0
401 (worst case)
Bedrock 140 cf 45° 0
pcf - pounds per cubic foot
psf- pounds per square foot
Review of published data for similar materials indicates the material strength parameters to be
very conservative. Based upon standard penetration blow counts in the 1997 borings and typical
Rev. 04109-TR116