Dames & Moore, 1999 - USDA Forest Service

vertical extent of the underground mine workings, and approximate thickness of bedrock overlying the
principal mine openings, or stopes. In addition, an article written at the time of mine operation (Huttl, 1938)
confirmed a "surface pillar" of 50 feet. Referring to Figure 4.2-2 1 a, the underground mine maps reviewed
for this evaluation confirmed that the areas highlighted between the 1100 level and 300 level portals
(Honeymoon Heights area), have bedrock approximately 50 feet thick overlying the stopes.
No historical information was identified which addressed the potential for subsidence at the Site.
Consequently, as part of the RI, a review of available underground maps, and a field reconnaissance of
the areas identified as having potential for mine subsidence was conducted. The reconnaissance of the
ground conditions above the 1 100-level to the 300-level of the mine was conducted in September 1997. No
indication of openings'to the subsurface andlor likely subsidence features were noted. In addition, no
indications of surface water flow into the subsurface were observed in the area of the intermittent drainage
during periods of flow. In addition, color aerial photographs (stereo coverage, 1 :12,000 scale, shot near
midday, June 6, 1997) were reviewed as well for possible indications of subsidence features; none were
observed.
However, at the request of the agencies,,the potential for subsidence at the Holden Mine Site was further
evaluated using the method of Golder (1990), as presented by Betourney (1996). The method is a
specialized adaptation of the underground opening stability analysis method developed by the Norwegian
Geotechnical Institute (Barton et al., 1974). The method is applied by graphically comparing key
geologic and geomechanical characteristics of the rock mass to a "normalized" mine opening dimension,
then using case history data to identify the ranges of data that are known to be associated with subsidence.
The geologic and geomechanical characteristics are combined into a Rock Mass Quality parameter Q,
which is defined in the Barton et al. publication. The Q value is a function of six individual features of
the rock mass and its environment:
1. RQD - Rock Quality Designation, which measures the continuity of core retrieved
during drilling
2. Jn - Number of Joint Sets, which is estimated using outcrop or underground geologic
mapping data
3. Jr - Joint Roughness Number, which is estimated from observations of the geometry of
exposed joints
4. Ja - Joint Alteration Number, which is estimated from observations of the aperture and
condition ofjoints and joint wall rock
5. Jw - Joint Water Reduction Factor, which is related to the water inflow rate or pore
pressure conditions
6. SRF - Stress Reduction Factor, which is related to the nature of the in-situ stress regime
High Q values indicate a good quality rock mass which would have a longer stand-up time and would
require less structural support to maintain stability.
Data used for estimating the Q valuewere obtained during an October 1998 field visit which included
geologic mapping along the ore body where it outcrops above the mine at Honeymoon Heights (Figures
4.2-21a and 4.2-21 b). The field data are presented in Appendix M. The joint strike and dip data noted on
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