Dames & Moore, 1999 - USDA Forest Service

stable slope while factors of safety at or below 1.0 indicate impending failure of the slope. Slopes with
factors of safety between 1.0 and 1.2 are considered marginally stable. This seismic event has an estimated
return period of approximately 40 years at the Site location. Graphical representations of the analyses for
seismic conditions, including site stratigraphy, groundwater elevation and topography, are presented on
Figures 4.2-19 and 4.2-20. Only results for the smaller 40-year return period seismic event are shown on
the figures.
As noted on Figures 4.2-17 through 4.2-20, the slope stability analysis included modeling a "cemented
layer" on the surface of the slopes. It was not possible to complete subsurface explorations on the slopes of
the tailings piles to characterize the thickness of the cemented layer due to the steepness of the slopes.
However, based on boring and test pit data collected from the tailings piles, including test pits excavated by
Dames & Moore in 1997 near the top and base of tailings piles 1 through 3 (DMTPI-2 through DMTP3-4,
shown on Figure 4.2-6b), it appears that the thickness of the "cemented layer" ranges from several inches to
approximately 10 feet. The slope stability analysis utilized the lower end of the range (approximately 3
feet). Since the slope stability failure circles noted on Figures 4.2-17 through 4.2-20 extend below the
maximum observed thickness of the cemented layer, the effect of cementation is thought to be minimal.
The analyses did not include scenarios of either no cemented zone or a 10-foot-thick cemented zone. In
addition, groundwater levels were assumed to be near the base of the tailings piles and not perched higher
within the tailings. Given the various observations from the RI reconnaissance, boring logs, groundwater
monitoring well data, test pits and slope performance over the years, the scenario utilized in the analyses
appears to be reasonable.
As noted for the liquefaction analyses, there is some level of uncertainty associated with the findings of the
slope stability analyses due to uncertainties related to the engineering properties of the tailings and native
materials, groundwater levels, the presencelabsence and degree of cementation, and the modeling utilized.
The observed long-term performance of the slopes suggests they are actually more stable than indicated by
the results of the static and seismic analyses. The tailings pile slopes are relatively steep, but have not
experienced a reported failure since a flood which occurred in 1948 and appeared to have been a 50-year
event (Crates, 1966); the 1948 storm event resulted in the eroding of the toe of the northwest portion of
tailings pile 2 and the delivery of tailings to railroad Creek. The slopes have held the relatively steep angles
since the 1948 event, but are steeper than the angle of repose as determined by the angle of internal friction
(37 degrees) resulting from laboratory testing completed by Hart Crowser. In addition, the Site apparently
experienced a relatively significant seismic event in 1990, which did not result in slope failure. Therefore, it
is our opinion that the soils have been appropriately and possibly conservatively modeled during the slope
stability analyses. Possible measures to mitigate the mass movement potential will be addressed in the FS
document.
Any erosion at the toe of the slope (such as Railroad Creek cutting into the slope) will reduce the factors of
safety described above. Erosion is discussed further in Section 4.2.4.3 below.
Based on the observed angular nature of the waste rock piles, it is estimated that potential for slope failure is
relatively low. However, isolated, shallow failures have the potential to occur on the steepest slopes.
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