Dames & Moore, 1999 - USDA Forest Service

The underlying native soil was noted to be generally relatively dense gravel and sand (alluviurn/reworked
till and glacial till). The' underlying bedrock was detected at depths ranging from approximately 15 to 30
feet below the tailings and native soil contact.
Tailings Pile 3
Geologic cross-section and seismic refraction line D-D' (Figures 4.2-6b and 4.2-10) crosses tailings pile 3.
The entire surface of the tailings pile, other than the steepest slopes facing Railroad Creek, is covered with
approximately 4 to 8 inches of rounded gravel placed by the Forest Service between 1989 and 1991. The
seismic refraction line confirmed the thickness of the tailings materials beneath the gravel surface, and the
apparent depth to bedrock.
The available subsurfack information from the borings completed on the tailings pile suggest the presence of
a soil layer overlying possible colluvium near the southern limit of the tailings pile. A test pit completed
near the southern end of the seismic line (DMTP3S-1) encountered what appeared to be weathered glacial
soil consisting of sandy silt with some sand, cobbles, and boulders to a depth of approximately 19 feet. This
portion of the cross-section is coincident with the lowermost extent of an avalanche chute. Colluvium may
have originated upslope of the area due to erosionof bedrock and/or glacial soils. The colluvium was
interpreted to be on the order of 10 to 25 feet thick.
The cross-section indicates the presence of up to 70 feet of tailings. The density and consistency of the
tailings materials were relatively consistent with tailings piles 1 and 2. The tailings piles were underlain by
a thin layer of organic soils (less than 2 feet thick). The organics were then observed to be underlain by a
layer of reworked glacial materials or alluvium consisting mostly of sand and gravel. Several of the borings
interpreted that the silt content and density increased with depth. Therefore, the reworked glacial till appear
to be limited to less than 10 feet in thickness.
The glacial till was noted in the seismic section to be underlain by bedrock. The bedrock varied in depth
from approximately 25 feet to 75 feet below the contact between the tailings and native soil.
In contrast to tailings piles 1 and 2, several of the subsurface explorations and seismic refraction lines
detected the presence of possible "hard pans" within the tailings pile; some partially cemented material was
encountered at a depth which is underlain by less denselhard material. The layer was encountered
approximately 5 to 6 feet bgs in test pits DMTP3-2 and DMTP3-3 (Figure 4.2-6b). The test pits were
completed at the eastern toe of tailings pile 3. The more dense layer appears to coincide with the zone
where seepage from the pile would contact underlying groundwater. The layer was observed to be partially
cemented, hard, and less than approximately 4 feet in thickness. The seismic refraction lines did not appear
to detect the layer, likely due to the resolution .of the seismic survey being too large to detect a non-
continuous, relatively thin zone or layer of varying hardness and cementation.
Immediatelv East of Tailings Pile 3
Seismic refraction line E-E' (Figures 4.2-6b and 4.2-1 1) was placed immediately east of tailings pile 3, and
extended from approximately 200 feet north of Railroad Creek to approximately 200 feet upslope of the
existing wetland area. In addition, four test pits (DMTP3E-1 through 4) were excavated in the area of the
17693-005-019Wuly 19.1999;4:51 PM;DRAFT FINAL RI REPORT