Dames & Moore, 1999 - USDA Forest Service

known. The sand appears to be more dense than the tailings pile material. Based on seismic refraction data
gathered by Northwest Geophysical for this report, the sand is underlain by dense glacial till.
The following engineering characteristics presented in the 1975 Hart-Crowser report were used for
evaluation of slope stability:
Tailings
Cemented Tailings
Sand
Glacial Till
Soil Angle of Internal Friction Total Unit Weight Cohesion
(phi) (pc9 (ps9
The values for the tailings materials utilized in the slope stability analyses were based on laboratory testing
performed by Hart-Crowser for their 1975 report and typical published values. The values for the sand were
increased slightly directly beneath the tailings pile due to overburden pressures (as opposed to at the toe of
the slope where the materials were less thick). For the purposes of the slope'stability analyses, it is assumed
that the wood cribbing that was used to suspend the tailings discharge line around the pond perimeter had
rotted away.
In addition, the review of borings completed by others, as well as field observations during the RI, discussed
the presence of soil units in addition to those utilized by Hart Crowser in the slope stability analyses
completed in 1975. The parameters for these soil units were determined based on experience gained on
other sites. The following engineering characteristics were used in the slope stability analyses completed by
Dames& Moore for the units observed at the Site, but not utilized in the 1975 Hart Crowser study:
Topsoil
Densified Topsoil
Talus
Densified Sand
Soil Angle of Internal Friction Total Unit Weight Cohesion
(phi) (pcf) (psf)
An organic layer was mentioned in several logs of borings completed by PNL in 1990. Only one of the logs
of borings completed by Hart Crowser in 1975 (B-6; see Figure 4.2-6b) indicated the presence of an organic
layer at the base of the tailings. The layer was noted to be roughly at the level of the toe of the slopes for
tailings piles 2 and 3 for those borings completed closest to the slopes facing Railroad Creek. The PNL logs
appeared to indicate a relatively continuous layer of organic material near the tailingslnative soil contact.
A soil friction angle of 34 degrees was used in the slope stability analyses for this layer, identified as
"densified topsoil" in the above table and on Figures 4.2-17 through 4.2-20. The 34-degree soil friction
angle is consistent with fibrous peat and silt or sandy silt with wood fragments, as described in the PNL
logs.
17693-005-0 I 9 9 ~ 19. 1 1999:4:5 ~ 1 PM;DRAFT FINAL RI REPORT