Dames & Moore, 1999 - USDA Forest Service

Tailings pile 3 is situated near the base of an avalanche chute. An avalanche reportedly ter'minated near the
southern edge of the pile in 1996 (persdnal communication with Keith Anderson, USFS, 1997). The
potential exists for an avalanche to deliver avalanche debris to the southern margin of the tailings pile.
4.2.4.3 Erosion Potential
Summary of Historical Findings
Erosion is the breakdown of soils and bedrock by natural processes including water, wind, and glaciation.
Of these processes, water-related erosion during storm events has the most potential for adverse impact.
Fine-grained soil is also susceptible to wind erosion. The susceptibility of any material to erosion is
dependent upon: (1) chemical and physical characteristics, (2) topography, (3) the amount and intensky of
precipitation and surface water, and 4) the type and density of vegetative ground cover, if present.
The tailings material was found in the ORB 1975 report to be "highly susceptible to erosion by wind and
water" prior to the placement of the gravel surfacing and grass mats. The report noted that windblown
deposits were measured 500 feet downwind of the piles in amounts over 3,000 pounds per acre. Based on
the reported average dry unit density of 92 pcf, this would equal approximately 0.009 inches of tailings
material at that location. The report also concluded that a wind velocity of 30 cmlsec was needed to start
eroding the tailings material; the assumptions associated with this estimate were not provided.
RI Findings
Railroad Creek flows eastward for approximately 3,900 feet directly adjacent to the three tailings piles. The
tailings piles lie from 30 to 120 feet above the main channel on the south side of the creek. A field
reconnaissance along the creek and slopes of the tailings was conducted. During the reconnaissance, the
3,900 feet of creek reach along the tailings slopes was classified into 18 different slope reaches with
apparent similar erosion processes and erosion potential (e.g., slope angle, setback fiom creek, amount of
vegetation, relationship to high water, etc.). Schematic cross-sections for each slope reach are presented in
Appendix F. The criteria for grading and ranking the slopes in terms of erosion potential are presented in
Tables 4.2-2 and 4.2-2% respectively. Physical characteristics for each slope reach are summarized in
Table 4.2-3.
The logs of the test trench excavations are presented in Appendix C. In general, the trench excavations
indicate that the undisturbed tailings soils are predominantly laminated fine-grained sands that grade to silts
and have pockets or lenses of clay; however, due to an uneven decrease in oxidation fiom the slope inward
and from the top down, the soils appear laterally and vertically heterogeneous in color and the degree of
cementation. For example, there are abrupt and irregular contacts of well oxidized hard, dry, cemented,
medium-orange fine-grained sands in contact with loose, moist to dry, non-cemented, medium- to dark-gray
fine-grained sands. In some cases, the upper 2 to 4 ft of tailings soils are softer, looser and more uniform in
color and have no internal structure (i.e., the sands and silts are well mixed), and appear to have been
remolded. The remolding most likely took place when the slopes were reconfigured during the USFS
reclamation activities. 'soft, loose remolded soil also occurs on the slbpes and at the base forming a thick
talus in places. Wood pieces were also observed in most trenches, and in a few places, cut timber logs were
found buried in a vertical position.
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17693-005-019Uuly 19, 1999:4:Jl PM:DRAFT FINAL RI REPORT