Dames & Moore, 1999 - USDA Forest Service

Infomation and data gathered about the potential source areas is summarized in Table 4.2-6. For each
source area, the following criteria were evaluated: (1) the approximate haul distance to the center of the
Site, (2) the composition and relative size of sources, and (3) the relative feasibility of developing the
source.
4.2.8.2 Riprap
Summary of Historical Findings
The riprap currently covering portions of the southern streambed of Railroad Creek along the toes of the
tailings piles was placed as part of the mine tailings rehabilitation project conducted by the USFS between
1989 and 1991. A review of the project files disclosed that several potential sources of riprap were
identified by the USFS in the Railroad Creek watershed downstream of Glacier Peak Wilderness boundary
(C. Blackbum, 1988). Referring to Figure 4.2-23, these areas included: (1) an outcrop of granitic bedrock
north of the Dan's Camp existing gravel pit, (2) outcrops of granitic bedrock exposed in roadcuts between
Dan's Camp and Holden Village, and (3) a talus slope approximately one mile east of Holden Village.
The talus materials were deposited on the slope as a result of fracturing and rockfall from upslope bedrock
outcrops. The pile of rock on the slope was considered by the USFS to be a potent'ial source of riprap.
However, the risk for injury andlor road closure due to possible rockfall during development resulted in the
source area being eliminated from further consideration (personal communication with AI Murphy, 1997).
The bedrock outcrop north of Dan's Camp was, therefore, developed as a rock quarry, -and portions of
isolated exposures of the bedrock within roadcuts were removed to provide the riprap utilized for the
project.
RI Findings
An assessment of potential riprap sources within appropriate portions of the Railroad Creek watershed were
evaluated. The scope of work included the initial review of available geologic maps and aerial photographs
for the Railroad Creek watershed from the Glacier Peak wilderness boundary down to Lucerne. The
objective of the assessment was to identify a source, or sources, of riprap as close to the Site as reasonably
as possible.
In October 1997, a field reconnaissance was conducted of potential source areas adjacent to the Holden
Road, from approximately milepost 1.6 to Holden Village. The source areas were evaluated with respect to
compositional lithologies, rock genesis (i.e., till, plutonic stock, etc.), potential riprap or soil cover quality,
potential haul distance, and accessibility. In addition, Schmidt-hammer tests were conducted at the potential
riprap source areas on a reconnaissance level to get a relative gauge of potential hardness of the various
sources that were encountered in the identified source areas.
The riprap source used by the USFS in 1989 is noted herein as the Existing Rock Quarry (Figure 4.2-23).
Based on the assessment of current riprap quality along Railroad Creek, this source area would not appear to
be a viable riprap source. This was confirmed upon visual inspection of the quany site and by the Schmidt-
hammer test survey. Although, some portions of the quarry appeared to have relatively competent
granodiorite (Type A in Section 4.2.4; unweathered, low fracture density and relatively high Schmidt-
hammer percentage recovery), most of the quarry was composed of Types B and C. Thus, use of this quarry
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17693-005-019Uuly 19. 1999;4:51 PM;DRAFT FINAL RI REPORT