GENERAL39437

• Permit No: M2002120
• IBM Index Class Name: General Documents
• Doc Date: 6/18/2004
• Doc Name: Insp Follow Up
• From: DMG
• To: Hall-Irwin Corporation
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: (303} 866-3567
FAX: [phone]
COLORADO
DIVISION OF
MI[V SRALS
GEOLOGY
REC LA MATZO N•MINING
S Af ETY•SOENCE
DATE: June 9, 2004 GolveOrnor s
Russell George
TO: Kate Pickford Executive Director

Ronald W. Cattany
FROM: Allen Sorenson ~ Division Director
/// Natural Resource Trustee
RE: Review of Operator's Slope Stability Analysis for Protection of Permanent Va/uable Manmade
Structures, Hal[ Irwin Corporation, Bernhardt Pit, Permit No. M-2002=120
The Division of Minerals and Geology {DMG) engineering staff have completed a review of stability analyses
provided by the Operator of the Bemhardt Pit. These analyses are dated November 19, 2002 and were prepared for
the Operator by Rocky Mountain Consultants, Inc„ a Tetra Tech Company. The analyses indicate that the slope with
the lowest safety factor, as it relates to a proximal manmade structure worthy of protection, is the pit slope where
mining pod 2 is within 55 feet of a Merit gas line. The Operator's analysis of this location yielded a safety factor
1.46 (rounded in the Operator's analysis to 1.5) where the hypothetical failure surface intersects the pipeline. The
Operator's analysis included the following two assumptions that the DMG determined required analysis of
parametric sensitivity:
The Operator's analysis assumed that the bedrock beneath the sand and gravel deposit is of consistently high
shear strength. DMG has produced an analysis where it is assumed that there is athree-foot thick layer of
weathered bedrock overlying intact bedrock, that the weathered bedrock unloaded by mining has degraded to
its residual strength, and that the weathered bedrock still buried under the sand and gravel is at its fully
softened strength.
The Operator's analysis assumed that the working face of the pit slope would be no steeper than 0.5:1
(horizontal:vertical). DMG has produced an analysis where the pit slope is mined to a near vertical slope.
Essentially, the DMG analysis anticipates the worst-case scenario for potential slope failure, as it is unlikely that the
pit could be mined to a 42-foot high vertical slope, and it is unlikely that weathered bedrock would have zero
cohesive strength. Hence, the DMG analyses are conservative and any safety factor in excess of unity for temporary
slopes would be considered acceptable.
The results of the DMG analyses are enclosed with this memorandum. The results show that consideration of lower
strength weathered bedrock has no effect on the stability of the modeled slope in this case, i.e., the DMG safety
factor for the slope is essentially equal to that of the Operator. This result indicates that the stability of the slope is
largely due to the thick layer of high shear strength sand and gravel within the slope profile. The consideration of a
near vertical pit wall does yield a lower safety factor of 1.26, However, as discussed previously a safety factor of
1.26 is acceptable in this case due to the conservatism built into the model.
Based on the DMG review, the mining setbacks to structures proposed by the Operator may be approved.
enclosures(s)
cc: Car] Mount, DMG (via email and w/o enclosures)
c:\acs\LOANER MACHME\Ivly Documents\bemhardt stability analysis review.doc
Office of Office of Colorado
Mined land Reclamation Active and Inactive Mines Geological Survey