Dames & Moore, 1999 - USDA Forest Service

Several faults have been mapped in the mine. The prominent faults strike nearly east-west and appear to
have been intersected by both the 1500-level tunnels. Referring to Figures 4.2-5, 4.2-6a, 4.2-14, and 4.2-
15, two of the faults have been mapped with lateral andlor vertical displacement. However, as noted above,
the faults in the Railroad Creek drainage. which generally parallel the direction of the faults intersected in
the mine, appear to be limited in lateral extent and continuity. No indications of recent fault movement were
noted.
4.2.4 Geologic Hazards
4.2.4.1 Seismic Liquefaction Potential
Summary of Historical Findings
Groundshaking occurs in conjunction with earthquakes. Foundation materials which are relatively dense
transfer energy associated with an earthquake more efficiently than less dense materials. Less dense soils
may be susceptible to seismically-induced settlement. Liquefaction is a phenomenon in which loose to
medium dense, saturated, granular soils lose their shear strength during dynamic loading (usually during an
earthquake) and behave as a fluid. Liquefaction causes soil settlements and sometimes lateral spreading or
slope failure of a soil mass. Loose granular, saturated soils which are commonly susceptible to liquefaction
are normally found in valley bottoms throughout the region.
The mine tailings piles were investigated in terms of geotechnical engineering characteristics as part of the
ORB 1975 study. In summary, the results of the field and laboratory program indicated that the mine
tailings consist of interlying sequences of clay-, silt- and fine sand-size material. The soil was noted to be
non-plastic with an average in situ wet and dry density of 116 pounds per cubic foot (pcf) and 92 pcf,
respectively. The optimum moisture content was noted to be 13 percent based on ASTM D-1557-70. The
relative density of the material based on the Standard Penetration Test (a field test where the sampler is
driven into the soil with a 140-pound hammer with a 30-inch stroke) was medium dense.
The 1975 Hart Crowser report indicated that the phreatic surface (groundwater level) was observed to be
consistently about two feet above the interface of the northern extent of the tailings pile with the original
ground surface, which indicated "nominal seepage pressure within the piles." The susceptibility of the
tailings piles to liquefaction, or "quicking" of the material during a seismic event, was determined to be low,
based on the assumption that the phreatic groundwater surface was 2 feet above the contact between the
tailings and the underlying native alluvial soil.
RI Findings
Historical seismicity data were reviewed for the Site. Based on data collected by the National Oceanic and
Atmospheric Administration (NOAA), the Site has been subject to two earthquakes of magnitude greater
than 4 with epicenters close enough to cause potentially significant peak ground accelerations (Figure 4.2-
2). The first earthquake occurred in 1942 at a distance of approximately 10 miles with a magnitude of 4.3'
on the Richter scale which resulted in an estimated peak ground acceleration at the Holden site of 0.04
gravity (g). The second earthquake occurred in 1990 at a distance of approximately 44 miles with a
magnitude of 6.0 on the Richter scale which resulted in an estimated ground acceleration at the Holden
site of 0.03 g (NOAA, 1998). The former earthquake occurred during the operating life of the mine but
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17693-005-019Uuly 19.1999;4:51 PM:DRAFT FINAL RI REPORT