Dames & Moore, 1999 - USDA Forest Service

the coefficient of Friction. Geomorphic. hydrologic, and vegetative factors determine the occurrence,
frequency, and significance of the processes in an area.
In terms of the Site, the tailings pile slopes facing Railroad Creek and Copper Creek are relatively steep. A
perimeter dike failure was documented to have occurred at the east end of tailings pile 3 in 1946 resulting
from the plugging of a decant tower with ice and overtopping of the containment dike which resulted in
erosion of the tailings (miscellaneous written communications, 1946). It should be noted that the decant
towers were reportedly backfilled and sealed during work efforts performed by the Forest Service between
1989 and 199 1 ; however, the risk of overtopping of tailings ponds has been mitigated with the construction
of surface water swales, and the closure of the mill which provided the water to the tailings ponds during the
I946 event.
Several other failures occurred later within the relatively steep slope of tailings pile 2 east and near the
confluence of Railroad Creek and Copper Creek, during storm events (Crane, 1966). No other significant
mass movement events were documented on the slopes of the tailings piles facing Railroad Creek and
Copper Creek.
A report was completed in 1970 by the Washington Department of Natural Resources which discussed the
mine tailings at Holden Mine (Thorsen). Conclusions were presented that cementation through oxidation of
iron sulfides and settlement appeared to have made the tailings piles at least as stable or possibly more stable
than when originally irnplaced. However, no actual slope stability analyses were apparently conducted.
A qualitative assessment of the stability of the relatively steep slopes of the tailings piles adjacent to
Railroad Creek and Copper Creek was conducted as part of the ORB study. Geotechnical engineering-
related laboratory analyses were conducted on soil samples collected from borings drilled mostly into the
perimeter of the tailings piles. The internal angle of friction of the material ranged from 34 degrees for the
coarser material to 38 degrees for the finer material. This angle is less than the steepest slope faces of the
tailings piles. However, only limited samples were collected for direct shear testing. The slopes appeared to
be stable in the present configuration. The report concluded that the tailings piles were relatively stable in
their present configuration, but that flattening the slopes of the piles would likely improve the long-term
stability of the piles.
RI Findings
Based on data collected by Dames and Moore and others, the groundwater elevations in the northern
portions of tailings piles are relatively close to the bottom of the piles, as discussed in Section 4.4. This is
confirmed by the observation that seeps occur only near the toe of the slopes of the three tailings piles.
Based on exploration logs of Dames and Moore and others, the tailings consist of silty sands and sandy silts.
Up to 2 to 3 feet of tailings at the surface are lightly cemented. Geotechnical laboratory tests performed by
Hart-Crowser for the 1975 ORB report indicate silt contents of the tailings ranging from 28 to 100 percent.
Grain size analysis by Dames & Moore indicated silt contents ranging from less than 20 to more than 50
percent (Appendix D). Based on the borings drilled by Hart-Crowser in 1975, the soils underlying the
tailings are dense sands and gravels. A test pii excavated at the toe of tailings pile 3 as part of the RI
(DMTP3-4) (Figure 4.2-6b) revealed loose sand under the tailings; the aerial extent of this loose sand is not
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17693-005-019VuIy 19.1999;4:5 1 PM;DRAFT FINAL RI REPORT