REP10582

• Permit No: M1977378
• IBM Index Class Name: Report
• Doc Date: 4/19/1991
• Doc Name: GEOTECHNICAL SERVICES
• Media Type: D
• Archive: No

' Tailings Pond No • •
Sunnyside Mine
Silverton, Colorado
Project No. 3120K114
r Geologic Historv; The Sunnyside Mine Tailings Pond No. 2 is
located in the western mountain province, 2 miles north of
Silverton, Colorado. The San Juan Mountains in the western
mountain province are believed to be tectonically stable. No
' known neogene faults are noted in "Earthquake Potential in
Colorado, Bulletin 43." Neogene faults are faults which were
active in the last 28 million years. In areas which are remote
from known faults, a horizontal acceleration equal to 10% of
' gravity (0.1g) is generally used in stability analysis to
simulate potential seismic conditions.
' The closest potentially active faults are the Ridgeway Fault,
and the zone of faulting associated with the Lake City Caldera
' collapse, as found in, "Earthquake Potential in Colorado,
Bulletin 43." The Ridgeway Fault is approximately 25 miles
,' north of the site. The Ridgeway Fault is Quaternary in age.
Quaternary faults are faults active in the last 2 million
' years. The faults associated with the Lake City Caldera
collapse are considered non-tectonically active.
' The closest recorded earthquakes to the site were located at
Silverton, Colorado. Recorded earthquake intensities are
' approximately IV on the modified Mercalli intensity scale.
Based on the literature reviewed, the highest probable rock
' acceleration value assigned in Colorado was 0.07g, in the
southwest portion of the state.
' Material Properties: The on-site clayey silts, or sliR:es, on
the interior of the pond exhibit properties of none to low
' cohesion and low to moderate angles of internal friction. The
outer materials in the pond which consist of silty sands, or
' the sand fraction of the tailings, had none to low cohesion and
moderate to moderately high angles of internal friction. The
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