Dames & Moore, 1999 - USDA Forest Service

Tailings pile 1 was utilized as a municipal dump andlor sewage lagoon by both Howe Sound and Holden
Village; however, no information was discovered regarding the disposal of petroleum products in the dump.
The refuse was reportedly placed in a depression in the tailings materials. The placement of municipal @
waste was discontinued in 1989. The depression was filled with soil andfor tailings materials during the
tailings rehabilitation project conducted between 1989 and 1991.
Holden Village, Winston Home Sites, and Baseball FieldlCampground Area i
The near-surface geology for the area to the north of Railroad Creek, beneath Holden Village, Winston
Home Sites, and the Baseball Field/Campground area appears to consist of glacial soil overlain by
colluvium originating from the slopes to the north.
8.2.2.3 Tailings Pile Slope Stability
Seismic Potential I
The Site is located in an area that is moderately active seismically. The Site has a low to moderate potential
to experience an earthquake, which could liquefy the relatively loose alluvium underlying the toes of the
.tailings piles, based on the analysis of historic seismic data and geologic data. The soils directly beneath the
tailings piles have been compacted by the weight of the tailing materials, and are, therefore, less susceptible
to liquefaction.
Release Potential I
The slopes of the tailings piles facing Railroad Creek were found generally to range in angle between
approximately 22 degrees and 58 degrees. The majority of the lower- to mid-slopes for tailings pile 1
range between 22 degrees and 33 degrees, with isolated portions of the upper slopes observed in excess of
60 degrees. The lower slopes of tailings pile 2 were observed to be less than 34 degrees, with the majority
of the mid- to upper slopes in excess of 44 degrees. The majority of the lower- to mid-slopes for tailings
pile 3 are less than 34 degrees, with the upper slopes ranging from 33 degrees to more than 42 degrees.
Based on engineering analyses of the tailings piles, the slopes of the tailings piles are stable at the present .
time under static conditions, assuming that the base of the slopes are not eroded by Railroad Creek.
However, the "factor of safety" under static conditions was found to be 1.04 (a factor of safety below 1.0
indicates instability, and slopes with factors of safety between 1.0 and 1.25 are considered marginally
stable). It was also found that a realistic hypothetical earthquake event with a horizontal peak acceleration of
0.05 gravity (g) and a return period of 40 years would result in a factor of safety of 0.98 for the steepest
portions of tailings pile 2 and 1.04 for tailings pile 3 slopes. Tailings pile 1 slopes were not modeled but
were generally considered similar to tailings pile 3 slopes, except for isolated steep sections that are similar
to tailings pile 2. The factor safety increased with the depth of modeled failures; a slope of approximately 33
degrees was determined to have a factor of safety of 1.25 or greater under seismic conditions and is,
therefore, considered stable.
The results of the analyses suggest movement would involve displacement of 1 to 15-foot deep sections of
the steepest tailings. The horizontal length of a particular failure zone along the face of the slope could
reasonably be expected to range from less than 100 feet to more than several hundred feet. The likelihood
of a release is highest .for the portion of the tailings piles immediately downstream of the confluence of
\U)M-SW\I\VOLI\COMMOMWP\WDATAMOS\REPORTSWOLDEN-2W\8-O-Od~ 8-6 DAMES & MOORE
17693-005419Uuly 28, 1999;10:24 AM;DM FINAL RI REPORT