Dames & Moore, 1999 - USDA Forest Service

The groundwater levels used for the stability analyses were conservatively chosen as 20 feet above the
highest values measured by Dames & Moore during the spring and summer of 1997 at groundwater
monitoring wells located closest to the slopes. This increase in water level over measured values was
adopted to account for possible highly unfavorable rainfall and snowrnelt conditions that might cause
future water levels to exceed those observed. The water was assumed to emerge at the toe of the slope,
since no reports of water emerging higher on the slope could be found. The water surface was depicted as
a straight line between the nearest piezometer and the toe of the slope. Since the closest piezometers were
approximately 300 feet from the slope face, the 20-foot "highly unfavorable" increase for purposes of
analysis resulted in less than a 5-foot increase in water levels below the slope itself.
Slope stability analyses for static conditions (i.e., not under seismic conditions) were performed on cross-
sections developed through tailings piles 2 and 3. For the assumed cross-section of tailings pile 2, the
minimum static factor of safety for sliding was found to be 1.034. For the assumed cross-section of tailings
pile 3, the minimum static factor of safety was found to be 1.10. The lowest factors of dety are for
relatively shallow failures extending 10 to 15 feet below the face of the slope at the deepest point. Deeper
failures were found to have higher factors of safety. Failure of the slope is defined as any down slope
movement of the failed soil mass. The horizontal length of a particular failure zone along the face of the
slope is expected to range fiom less than 100 feet to more than several hundred feet. Graphical
representations of the analyses for static conditions are presented on Figures 4.2-17 and 4.2-1 8.
The graphical representatives of the analyses include cross sections of tailings piles with the hypothetical
slope failures resulting from the stratigraphy, soil parameters, and groundwater conditions. The factor of
safety is presented as contours in the upper left-hand comer of the figure. The contours represent slip circle
centers having equal factors of safety. Each contour line is the collection of the centers of rotation for slip
circles (i.e., circles representing the plane of sliding of potential slope failure masses within the slope) which
have the same factor of safety. The contours typically converge at a central minimum value, as displayed on
the figure. The contours can be used to assess how much of the slope may be near the minimum factor of
safety.
An analysis of the potential for seismic induced landslides was performed using ground acceleration values
recommended by the U.S. Geologic Survey in Frankel et al., 1996. The peak ground acceleration for a 475-
. year return period earthquake was accordingly estimated at 0.18 g for specific soil conditions at the tailings
location. The pseudo-static method was used for the landslide analysis. The seismic coefficient typically
used for pseudo-static analyses is two-thirds of the peak ground acceleration. Slope stability analyses
performed on the tailing pile slopes above Railroad Creek indicate that the tailings pile 2 slopes would fail
during a seismic event of this magnitude (safety factor of less than 0.98). The results of the analyses
indicated a safety factor of 1.04 for tailings pile 3 slopes.
Knowing that slopes for two of the three tailings piles will clearly fail during the relatively large 475-year
seismic event triggered a search for the smallest seismic event that could bring the slopes to the point of
failure. The analyses indicated that a seismic event generating a peak ground acceleration of 0.05 g is large
enough to cause slope failure, i.e., to reduce the "factor of safety" to a value of 1.0 or less. The "factor of
safety" is,the ratio of the magnitude of forces tending to resist slope failure (e.g., shear strength of the soil)
to the magnitude of the forces tending to cause failure (e.g., weight of soil mass on the upper slope). In the
opinion of the geotechnical engineering community, factors of safety higher than 1.2 indicate a suitably
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17693-005-019Uuly 19.1999:4:5 1 PM;DRAFK FINAL RI REPORT