Dames & Moore, 1999 - USDA Forest Service

were transferred to AutoCAD maps of the Site. Polygons of erosion potential were mapped in the field
based on the observations. The field findings and results of office analyses are presented in Section
4.2.4.3.
3.1.5.4 Mine Subsidence Potential Assessment
Based on a review of underground mine maps for the Holden Mine, the ore body was noted to have been
present within metamorphic bedrock, was nearly vertical in orientation, had a strike in a northwest-
southeast direction, and a width of approximately 80 feet. The ore body was mined by developing a
number of underground openings, or stopes, which are noted on mine maps. The upper limit of the
underground openings are indicated on the mine maps to approach within approximately 50 feet of the
ground surface. The potential for subsidence exists in these areas. The zone of mapped shallow mine
workings is noted to exist in the area noted as Honeymoon Heights (Figure 3.1-7).
The mine subsidence potential assessment evaluat,ion included mapping of the field conditions along the .
mapped strike of the exposed ore body, including the documentation of the following:
Frequency and orientation of bedding, fractures, and joints
Rock type and consistency
Slope angles
Vegetative cover
Presence or absence of surface water drainages
Indications of potential subsidence features
The data were analyzed as described in Section 3.1 1.2. The field findings and results of office analyses
are presented in Section 4.2.5.
3.1.5.5 Existing Riprap Investigation
A visual assessment of the existing riprap was conducted for the sections of Railroad Creek and Copper
Creeks adjacent to the three tailings piles (Figure 3.1-7). During the reconnaissance, the riprap condition
was characterized for each section (as defined in the Erosion Potential subsection above). The
characterization included a reach by reach assessment of the predominant and maximum boulder size,
boulder lithologic color and composition, and weathering characteristics. Boulder types were classified
into six different types on the basis of lithologic composition and weathering condition. In addition,
Schmidt-hammer tests were performed on representative boulder types to assess the relative hardness and
competency of each riprap type. This information was used by Dames & Moore in estimating the quality
of the riprap for each slope reach. The data collected in the field were utilized to complete an office
analysis of whether the riprap is sufficient to protect the toes of the tailings pile slopes from erosion by
Railroad Creek and Copper Creek. The field findings and results of the office analyses are presented in
Section 4.2.7. .
3.2 TASK 2 - HYDROLOGIC IIWESTIGATION
Referring to Figures 3.2-1 and 3.2-2, the RI hydrologic investigation was designed to provide a
comprehensive database in support of the feasibility study and ecological risk assessment, as well as
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17693-005-019Uuly 19. 1999:2:07 PM;DRAFT FINAL RI REPORT