2008-10-31_PERMIT FILE - M2008017 ‎(3)‎

• Permit No: M2008017
• IBM Index Class Name: PERMIT FILE
• Doc Date: 10/31/2008
• Doc Name: Slope Stability Report Review
• From: DRMS-acs
• To: DRMS-mac
• Email Name: MAC
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF RECLAMATION, MINING AND SAFETY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
COLORADO
D IV IS I ON OF
RECLAMATION
MINING
SAFETY
DATE:
TO:
FROM:
RE.
October 31, 2008
Michael Cunningham
Allen Sorenson

Slope Stability Report Review, Broken Arrow Investments, LLC,
Derr Pit, File No. M-2008-017
Bill Ritter, Jr.
Governor
Harris D. Sherman
Executive Director
Ronald W. Cattany
Division Director
Natural Resource Trustee
The Division of Reclamation, Mining, and Safety engineering staff (DRMS) have reviewed the Slope Stability
Report provided with the above reference permit application. DRMS does not necessarily concur with all of
the assumptions and determinations included in the report, as listed here:
1. The internal friction angle input to the analyses for the slurry wall is too high. However, the slurry
wall is not a factor in any of the lowest safety factor potential failure surfaces, so the results and
conclusions contained in the report are unaffected.
2. The minimum safety factor selected for the report is said to have been taken from the Colorado State
Engineer's Dam Safety Regulations. In fact, the selected safety factors are from the State Engineer's
regulations for dam stability at end of dam construction. The State Engineer's criteria for long term
dam stability are much more restrictive (1.5 safety factor as opposed to 1.3). Also, the safety factors
for dam safety where shale or claystone foundation is present, as it is at the toe of the proposed Derr
Pit mined slopes, may require consideration of the residual strength of the shale or claystone, and the
residual strength would be much less than the shear strength input for weathered bedrock for the Derr
Pit analyses.
Regardless of the foregoing, the DRMS has determined that the setbacks and mine slopes proposed for the
Derr Pit are protective of nearby infrastructure and can be approved. Critical to this determination is the mine
plan requirement to maintain excavated slopes at an angle no steeper than 311:1 V at all times during operation
of the pit. Therefore, the Operator must be notified that the DRMS will consider mining at an excavation
slope of steeper than 3H:1 V to be a serious violation of the permit that will initiate an immediate enforcement
action.
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Office of Office of
Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines