Dames & Moore, 1999 - USDA Forest Service

wetlands and adjacent to Railroad Creek. Three test pits (DMTP3E-4 through 6) were also completed in the
area southeast of tailings pile 3 as part of the borrow source evaluation.
The test pits completed south of the wetlands encountered medium dense to very dense, sand and gravel
with subangular cobbles and boulders, indicating that the materials may be relatively close to the source
bedrock, and are possibly reworked glacial till. However, two of the three test pits were terminated due to
practical refusal conditions at 6 feet and 13 feet bgs due to increasing boulders with depth. Consequently,
the soils encountered are likely weathered glacial till. The third test pit was completed to a depth of
approximately '17 feet bgs. The seismic refraction indicated that the relatively dense glacial till materials
underlying the looser materials ranged in thickness from approximately 40 to 50 feet. Bedrock was
apparently encountered below the glacial materials.
Referring to Figure 4.2-1 1, the seismic refraction, boring, and test pit data collected in the area of the
wetlands south of Railroad Creek and east of the tailings pile 3 suggest the presence of a trough-like feature,
in the area of monitoring well DS-1, filled with relatively loose soil; the feature is assumed to represent an
abandoned stream channel of Railroad Creek. The maximum depth of the feature appears to be on the order
of 25 feet. The test pits encountered alluvium consisting of interlying silt, sand, and gravel in the wetland
area. The relatively loose/sofl alluvium was noted to be underlain by approximately 70 feet of relatively
dense glacial till and bedrock. The bedrock surface also appears to form a trough-like feature as a result of
glaciation.
The seismic refraction line suggests that Railroad Creek and the area to the north of the creek area are ,
underlain by alluvial soil ranging from approximately 15 to 25 feet thick. The alluvial deposits are then
underlain by the relatively dense glacial till materials and bedrock at depth.
As noted for tailings piles 1 and 2, none of the subsurface explorations and seismic refraction lines appeared
to detect the presence of hardpans within the tailings pile, other than near the surface of the pile.
Tailings Piles 2 and 3 Adiacent to Railroad Creek
Section F-F' (Figures 4.2-6b and 4.2-12) is based solely on a seismic refraction line completed in an east-
west direction immediately north of tailings piles 2 and 3 and adjacent to Railroad Creek; the line extended
approximately 500 feet. The seismic section disclosed the presence of a relatively thin layer (less than 10
feet) of relatively loose to medium dense alluvium overlying approximately 5 to 25 feet of relatively dense
glacial till soil. The glacial till soil was noted to be underlain by bedrock. The bedrock in this area was
noted to be as shallow as approximately 15 to 20 feet bgs.
Copper Creek and Railroad Creek Confluence
Section G-G' (Figures 4.2-6b and 4.2-13) is based on a seismic refraction line trending generally north-south
between tailings piles 1 and 2, and across Railroad Creek to the north by approximately 250 feet. Referring
to Figure 4.2-6b and Appendix C, test pit DMTPI E- l was completed on the south bank of the creek as part
of the fenicrete assessment; due to encountering water, the test pit was terminated at a depth of
approximately 3-112 feet. The combined test pit and seismic refraction data indicate the presence of a native
soil layer (alluvium and reworked fill), approximately 10 to 20 feet thick overlying the entire section. The
seismic refraction data indicate the soil to be underlain by relatively dense glacial till ranging in thickness
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17693-005-019Uuly 19. 1999;4:5I PM:DRAFT FINAL RI REPORT