Dames & Moore, 1999 - USDA Forest Service

the tables were plotted on "stereonets" to assist in identifying the number of joint sets. Referring to
Figures 4.2-21c through 4.2-21g, the stereonets provide a 3-dimensional interpretation of the joint
features in order to evaluate whether the joint sets are coincident. The earlier data suggest that the
occurrence of the fractures andtor joints are relatively random in nature.
The field mapping effort confirmed the absence of mine subsidence-related features along the strike of the
ore body. Referring to Figure 4.2-21b, a depression was discovered southwest of the 700-level portal.
However, the depression appears to have been excavated, and drill rod found adjacent to the feature
suggests that the feature was associated with rock drilling to characterize the ore body.
Based on field data presented in Appendix M, Q values between 14 and 33 were obtained, indicating a
rock mass of "Good" quality. The method requires that these values be plotted against a "Scaled Critical
Crown Pillar Span" Cs, which is directly proportional to the span of the mine opening (Figure 4.2-2111).
The data points representing the Q versus Cs relationship for the Holden site fall into a zone described as
"Stable" based on similar data points obtained from a large number of mine opening case histories.
However, the Holden data is near the border between the "Caving" and "Stable" zones on the plot.
Accordingly, the stability of the mine "surface pillar" cannot be considered assured given the long time
period over which the performance must be addressed and the potential for seismic events during that
period.
The regional geologic structure, particularly faults in the vicinity of the mine, were also evaluated with
respect to the subsidence potential. Although published descriptions of the geology of the Holden Mine
area describe minor faults of varying size and orientation, only one fault appears to be significant enough
to possibly affect the potential for subsidence. This fault directly transects the mine axis and is oriented
in a roughly east-west direction (strike N90E, dip 70 to 80S), i.e., perpendicular to the direction of the
measured maximum principal tectonic stresses for this region (Zoback, M.L. and Zoback, M., 1980,
"State of Stress in the Conterminous United States," Journal of Geophysical Research, V.85, N. B11).
The fault transecting the mine was not included with the mapped joints and discontinuities in the stereonet
used to identify joint sets because this fault serves as the boundary between the two portions of the stope
for which the geologic data were collected (compare Figures 4.2-6a with 4.2-21a and 4.2-21 b). The data
for Scanline 1 were collected south of the fault, and data for Scanline 3 were collected north of the fault.
Mine maps show the stope being discontinued at the fault location (compare Figures 4.1-5b through 4.1-
13 with 4.2-6a). The Norwegian ~eotechnical Institute (NGI) method of evaluation Rock Mass Quality
(Q), which governs the subsidence potential assessment, does incorporate an option to consider
"weakness zones intersecting" or simply influencing the excavation in the selection of the 'stress
Reduction Factor (SRF) component of Q. Our opinion is that since the fault does not directly intersect the
stope itself, the SRF should be based on rock stress considerations rather than weakness zone
considerations. However, even if an unfavorable SRF category is selected based on weakness zone
considerations, the Q value increases by less than 1 percent. Overall, the value of SRF can exert only a
very small influence on the Q value.
The method used by Dames & Moore in this analysis is an empirical one based on case history
information and key features of the mine and geologic conditions. An alternative approach, one used
more frequently for civil works projects, is to identify specific blocks kinematically capable of falling
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11693-005-019Uuly 19.1999;4:51 PM;DRAR FWAL RI REPORT