Dames & Moore, 1999 - USDA Forest Service

efore the tailings piles had reached any significant height. There were no documented reports andor
accounts that the latter earthquake caused any landslides at the Site.
An analysis of potential liquefaction at the Site was conducted using empirical methods (described in Seed
et al. 1983) as part of the RI. The analysis included comparing the Standard Penetration Test (SPT) values
(soil density derived from a standardized field test) collected during the drilling of the borings on the tailings
piles in 1975, with cyclic stress ratios (ratio of estimated earthquake-induced shear stress divided by the
effective overburden stress at a given point) based on a magnitude 6.75 seismic event with a horizontal
acceleration of 0.18 g, and a return period of 475 years. The soils analyzed were a clean sand, a sand with
less than 15 percent passing the No. 200 sieve, and a silty sand with more than 35 passing the No.
200 sieve.
In addition to SPT values from the Hart Crowser borings, the SPT values of soils encountered in one test pit
excavated at the toe of the slope were estimated. The estimate was based on judgment using the resistance
offered by the soil during excavation. The distribution of SPT values was not considered explicitly, but the
lowest recorded values were used in the analysis. As required by the empirical method, the recorded SPT
values are converted to "normalized" values, which are shown on Figure 4.2-16 in the Danies & Moore
report.
Referring to Figure 4.2-16, the results appeared to support the findings by Hart Crowser in 1975 that the
typical potential for liquefaction of the tailings material andlor native underlying soil is relatively low. A
moderate potential for liquefaction may exist in zones of clean, loose sand near the toe of the slopes.
There is some level of uncertainty associated with the findings of the liquefaction assessment due to
uncertainties related to the earthquake magnitude, the grain size of the soil underlying the tailings piles, the
tailings SPT (density)'data, and the liquefaction boundary curves. However, the analyses utilized the lower
of the density values. Consequently, the findings are relatively conservative and consistent with the
methods employed by the geotechnical engineering profession.
Due to the variability of both tailings density and moisture content values, the potential for conducting the
liquefaction analyses utilizing a probabilistic approval was evaluated as part of the RI. However, existing
literature provides insufficient evidence of sustained liquefaction above the free water level, which was
utilized for the liquefaction potential analyses conducted for the RI.
A discussion of potential seismically-induced landslides are presented in the subsequent Section 4.2.4.2 of
this report.
4.2.4.2 Tailings Pile Slope Stability
Summary of Historical Findings I
Mass movement is generally considered the downslope movement of a mass of soil, rock, andor snowlice.
The stability of.soil is directly related to the physical characteristics of the soils and underlying bedrock.
The mechanics and rates of slope movement are controlled by a variety of factors, including: slope gradient,
water content and soil pore water pressure, and engineering properties of the materials, such as cohesion and
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17693-00S-019Uuly 19. 1999:4:5 1 PM:DRAFT FINAL RI REPORT