PERMFILE135270

• Permit No: M2002120
• IBM Index Class Name: Permit File
• Doc Date: 3/6/2003
• Doc Name: DMG staff review of pitwall stability analysis
• From: ACS
• To: KAP
• Media Type: D
• Archive: No

STATE OF COLOlZADO
DIVISION OE MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman SL, Room 215
Denver, Colorado 80203
Permit #: ^^ +~ ~°~ Confidental?: ~ 1 v t s t o N o r
Phone: [phone] CIeSS: ~~/m ~ Type-~~ 11 N BRA L S
FAX: [phone]
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DATE: March 6, 2003 ,_
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~O(,. ~atl: ~It n0 date Stamp): 3 ECLA MATION
I N I N G• S A F E T Y
TO: Kate Pickford
~~ Bill Owens
FROM: Allen Sorenson ~
~ covemor
~ Greg E. Walther
Executive Director
~': Pit WaU Stabi/Ily flnalySlS, Hall-Irwin Corporation, Ronald W. Cottony
Bernhardt Resource Gravel Pit, Permit No. M-2002-I20 Acting Division Director
The Division of Minerals and Geology (DMG) has completed a review. of the Tetra Tech RMC slope stability
analysis for the Berhardt Resource site. The purpose of the stability analysis was to demonstrate that the
excavation setbacks proposed in the mine plan aze sufficient to prevent damage to structures that aze not owned
by the Applicant. While the DMG has determined that the proposed mining setbacks are adequate, we do not
agree with the mechanisms and assumptions employed in the stability analyses the Applicant has provided. The
DMG has identified the following issues with the Applicant's stability analyses:
1. The analyses model the mined highwall at a slope of O.SH:1 V. In practice, it is virtually impossible to
control the slope of a gravel pit highwall, and pits that aze mined from the toe of the scope are typically
neaz vertical to vertical. The DMG requires that flit the purpose of analysis, gravel pi[ walls be modeled
as vertical unless the Operator can demonstrate that the slope angle of the highwall will be controlled
during mining.
2. It is expected that the upper several feet of the bedrock underlying the sand and gravel deposit will be
weathered. The DMG requires that the residual strength of the weathered rock layer be considered in
the stability analyses. The mine area is expected to be on a foundation of Fox Hills Sandstone or the
Upper Transition Member of the Pierre Shale. The weathered sandstone bedrock can be expected to
have higher residual sheaz strength than the claystone that may be present in the pit foundation as a
component of the Pierre Shale. Since claystone could be exposed in the pit floor, the anticipated
residual strength of claystone must be considered in stability analyses. Since it is unlikely that the
weathered bedrock will deteriorate to the point where the residual strength would be mobilized along a
potential failure surface, the analyses using the residual strength as an input factor will be conservative
and essentially any safety factor greater than unity will be demonstrative of adequate temporary pit
slope stability.
The DMG has re-analyzed the Applicant's Pod 2 cross section in consideration of the forgoing pair of
reservations, and determined that given the conservative nature of the analysis, e.g. vertical pit wall and residual
strength weathered bedrock, the safety factors are adequate to protect the gas lines. A copy of the DMG
analysis is enclosed. It is significant that both the Applicant's and DMG's analyses of the gas line stability are
predicated upon a setback distance of 55 feet measured from the crest of the pit slope to the location of the
pipeline. Resultingly, no mining may occur closer than 55 feet to the gas lines, or any of the other structures,
without first securing a written agreement with the structure owners; any such agreement must be reviewed and
accepted by the DMG prior to mining within the minimum 55 foot setback.
enclosure(s)
cc: Carl Mount, DMG (w/enclosures)
C:~acsVvly Documents~Bemhardt stabi]ity.doc