Dames & Moore, 1999 - USDA Forest Service

gravels are variable in thickness and density. The seismic refraction lines disclosed the center of the pile to
consist of lower density materials than the underlying and surrounding tailings. This likely reflects the
location of the pre-existing municipal dump area which was reportedly covered with tailings and/or soil fill
during the 1989 to 1991 mine tailings rehabilitation project (PNL, 1992). Based on the results of the .
seismic refraction survey, it appears that the soils placed as backfill were not compacted to a density
standard (i.e., 90 to 95 percent of the laboratory maximum density).
The thickness of the tailings ranged up to approximately 70 feet. The tailings materials encountered in both
borings and test pits were found to be relatively consistent in grain size; ranging from a silty fine sand to
fine-sandy silt; the more sandy soil was suspected to be present nearer the edges of the piles. Some soils
with clay-size particles were encountered nearer the center'of the pile and likely reflect the fine-grained
"slimes" generated during the mill processing of the ore and settling within the ponds atop the piles during
placement; these materials are likely cohesive in nature (Hart Crowser, 1975).
A relatively thin layer of organic-rich soil, less than 2 feet thick, was encountered at the contact with the
native soil. The borings did not disclose the presence of a cemented layer at the contact between the tailings
and native materials. However, a test pit completed immediately northeast of tailings pile 1 (DMTPIE-I)
(Appendix C) disclosed the presence of a partially to cemented soil layer (ferricrete), likely associated with
the seepage of mineral-rich water from the tailings pile.
The boring logs for boring B- l and groundwater wells TPI-3 and TPI -4, completed by others, and both the
seismic refraction lines and downhole geophysical surveys, disclosed the underlying native soil to be
.moderately dense sand and gravel (alluvium/reworked till). The density of the soil increased with depth,
grading into a glacial till. The underlying bedrock was found to be 30 to 60 feet below the tailingslnative
soil contact. The bedrock was more shallow near the center of the pile, and slightly deeper to the north and
A seismic refraction line was also completed along the eastern margin of tailings pile 1, adjacent to Copper
Creek, as well as across Railroad Creek; the results are presented in the following subsection titled "Copper
Creek and Railroad Creek Confluence."
None of the subsurface explorations completed within the tailings by Dames & Moore and others, and the
seismic refraction lines appeared to detect the presence of "hard pans" (layers of hard/dense, cemented
materials) within the tailings pile, other than near the surface of the piles where a partially cemented layer
was occasionally encountered.
Tailings Pile 2
Geologic cross-section and seismic refraction line C-C' (Figure 4.2-6b and 4.2-9) crosses near the center of
tailings pile 2. The results of the borings by others and both the seismic refraction line and downhole
geophysics completed for the RI, indicated that the tailings piles range in thickness from approximately 50
feet in the area of the PZ-1 series wells to approximately 120 feet near the north limit of the pile. The
tailings encountered in the borings were found to be relatively consistent with 'the materials observed in
tailings pile 1.
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17693-005-019Uuly 19,1999:4:51 PM:DRAFT FINAL RI REPORT