Dames & Moore, 1999 - USDA Forest Service

3.1 1 TASK 11 - DATA EVALUATION
3.1 1.1 GeologicaVGeotechnical Data
Following 'is a discussion of the geological/geotechnical engineering analysis of data collected in the
field. The results of the analysis are described in Section 4.2.4 of this report.
3.1 1.1.1 Seismic Analysis
Potential seismic hazards at the Site are soil liquefaction and seismic induced slope instability. The
seismic aspects of slope stability are addressed in the slope stability sections of the text. According to the
Uniform Building Code (1997) the Site is in a Seismic Zone 2B. .The UBC states that Seismic Zone 2B
has a seismic zone factor, Z, (or peak ground acceleration) of 0.2 g for an earthquake with a return period
of 475 years. An earthquake with this return period is commonly used as the design basis for structures
and earth slopes. A separate site-specific evaluation was performed to revise the estimated peak ground
acceleration for the Holden tailings pile locations to 0.18 g. An evaluation of the liquefaction potential of
the tailings material was conducted by the empirical method described by Seed, ldriss and Arango (1983).
The objective of this analysis was to determine the probability of site soils liquefying during a seismic
event based on the UBC criteria.
Historical seismic data from the National Oceanic and Atmospheric Administration were reviewed to
determine what seismic events had occurred in the area around the Site since its inception. This review
was performed to evaluate the ground acceleration levels to which the tailings piles have been subjected
since their completion. The largest seismic events were evaluated by the method described in Dames and
Moore report to the State of California (Dames & Moore, 1995).
3.11.1.2 Slope Stability Analysis
The objective of the analysis was to evaluate the probability of the slopes failing under static conditions
and during a seismic event. Slope stability analyses were conducted on the tailings pile slopes by the
pseudo-static method using the software Slope/%' Version 3 (commercially available) that is produced by
Geo-Slope International, Ltd. Slope/%' uses the limit equilibrium theory to solve for the factor of safety
of earth and rock slopes. The Spencer Method of Analysis was chosen as most appropriate for analyses
of the Site. Data from Dames and Moore's current investigation and other previous investigations at the
Site (Hart Crowser, 1975) were reviewed to determine appropriate engineering characteristics for the Site
soils, as well as groundwater levels and topography. The objective of the analysis was to determine the
factor of safety against sliding for the tailings slope. Typically, two-thirds of the peak ground
acceleration is used as the seismic coefficient in pseudo-static analysis.
3.1 1.2 Mine Subsidence Potential Analysis
The results of the geologic mapping efforts along the strike of the surface expression of the ore body m
the Honeymoon Heights area were analyzed to better define the mine subsidence potential. The analysis
utilized the underground mine maps in conjunction with the method+of Golder (1990), as presented by
Betourney (1 996), which is a specialized adaptation of the underground opening stability analysis method
developed by the Norwegian Geotechnical Institute (Barten et al., 1974).
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17693405-019Uuly 19. 1999;2:07 PM;DRAFT FINAL RI REPORT