Dames & Moore, 1999 - USDA Forest Service

from the roof, then evaluating the forces causing and resisting the roof fall process. This approach was
not used due to the obvious lack of roof fall activity observed during and after mining of the stope, and
the relatively weak indications ofjoint sets shown in the stereonets.
The method of evaluating mine subsidence used for this project contains no direct means of incorporating
the presence of a single fault, unless the characteristics of the fault (e.g., strikeldip for "number of joint
sets," "joint" roughness and alteration) are averaged along with those of the other joints affecting the
opening.
There is some level of uncertainty associated with the findings of the subsidence assessment due to
uncertainties related to the engineering properties of the bedrock, the accuracy of the underground mine
maps, and the methods utilized to analyze the subsidence potential. However, the finding that the
"surface pillar" is marginally stable is consistent with the field observations and the case studies utilized
as comparisons.
4.2.6 Windblown Tailings
An assessment of the lateral extent of windblown tailings was completed as part of the EU. The assessment
include the review of aerial photographs with limited site reconnaissance and sampling. The results of the
assessment are presented on Figure 4.2-22 and indicate that the amount of wind-blown deposits detectable
utilizing the methods employed, are generally limited to the area to the northeast of three tailings piles. The
depth of the deposits were noted to generally decrease with distance from the piles. The thickness of the
deposits near the mapped limits were estimated to generally be less than one inch. Those deposits in the
immediate vicinity of the Railroad Creek north and east of the tailings piles were noted in isolated areas to
be on the order of several inches thick.
4.2.7 Existing Railroad Creek Riprap
4.2.7.1 Summary of Historical Findings
The existing riprap lining the south bank of Railroad Creek was placed during the Site reclamation efforts
completed between 1989 and 1991. The rock reportedly originated from an existing rock quarry developed
approximately nine miles to the east of the Site, north of the "Dan's Camp" existing gravel pit, as well as
from road cuts in bedrock situated west of the existing rock quarry and Dan's Camp (Figure 4.2-23). A
review of historical files dating back to the mine tailings rehabilitation project in 1989 to 1991 indicated that
the riprap source was qualitatively assessed.
4.2.7.2 RI Findings
An assessment of the condition of the riprap was conducted as part of the RI. The riprap found at the base
of the tailings was classified into six different types. Types A, B, and C were apparently derived from the
granodioritelmonzonite quarry along the Holden Road, types D and E are volcanic in nature (source
unknown) and type F (fenicrete blocks) was derived locally from the creek bottom (Figure 4.2-24). The
criteria for ranking the riprap are presented in Table 4.2-4. Type A is comprised of large coherent blocks
of competent granodiorite, but is found in significant portions only along reach 2-B (Table 4.2-5). Types B
(quartz diorite to quartz monzonite) and C (diorite to monzonite) are the most prevalent riprap type, but by
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17693-00S-Ol9Uu\y 19.1999;4:51 PMDRAFT FINAL RI REPORT