Dames & Moore, 1999 - USDA Forest Service

The bedrock in the mine has been mapped as containing a number of fault and Fracture systems. Two
primary faults have been mapped within the Holden Mine with measured lateral and vertical movement: no
indications of recent movement were noted in literature andlor the field. The presence of the glacial till
overlying the bedrock in the lower portion of the valley would appear to decrease the likelihood of
groundwater movement From the faults to the near-surface groundwater in the valley alluvial deposits.
The potential for subsidence at the Holden Mine was evaluated as part of the RI. The scope of work
included conducting relatively detailed field mapping along exposed portions of the ore body between the
300- and 550-level portals, and the 700- and 1100-levels portals. The results indicate that the bedrock
spanning the underground openings is "stable" based on comparisons with historical data; refer to section
4.2.5 for additional discussion.
~ i l lMine i Suooort, and Waste Rock Piles
Referring to Figures 8.1-3 and 8.1-4 for the locations of the mill, mine support, and waste rock piles area, it
appears that this portion of the Site is generally underlain by limited man-made fill soils overlying the
glacial till soils which have been reworked near the existing ground surface by Railroad Creek.
The waste rock piles are composed of angular rock, including non-mineralized rock removed during initial
development of the 1500-level tunnels and other portions of the mine that did not contain sufficient
mineralization to warrant processing in the mill. The two piles located next to the abandoned mill are on the
order of 120 feet thick and are underlain by apparent glacial till soils; the contact between the waste and
glacial till slopes to the north. Water that permeates through the waste rock, therefore, travels along the
contact with glacial till materials, forming several intermittent springs or seeps observed near the base of the
piles.
The surface of the west waste rock pile is currently being used by Holden Village for the remediation of
petroleum hydrocarbon-contaminated soils encountered during the removal of underground storage tanks in
the village.
Eastern Portion of Site
Tailings Piles
The tailings materials have been determined to consist generally of silty fine sand to fine sandy silt. The
soils were noted by others to consist of higher percentages of sand near the outer boundaries of the piles.
The depths of the materials range from approximately 50 feet for tailings piles 1 and 3 to approximately 120
feet for tailings pile 2. The tailings appeared slightly cemented at the surface based on infilh-ation tests
completed by Dames & Moore and others. The permeability of the surfaces of the tailings piles is relatively
low, and is discussed further in the groundwater subsection.
Based on the results of borings completed by others, the tailings piles are directly underlain by reworked
glacial soils, which have a relatively high permeability. The reworked deposits appear underlain by
relatively low permeability glacial till. The northern portions of the tailings piles are also likely underlain
by the abandoned Railroad Creek drainage which was relocated at the time of the placement of the tailings
piles.
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8-5
17693-005-019Wuly 28. 1999;10:24 AM;DRAFT FINAL RI REPORT