PERMFILE114059

• Permit No: M2000144
• IBM Index Class Name: Permit File
• Section_Exhibit Name: ATTACHMENT A
• Media Type: D
• Archive: No

STATE OF COLOl~DO
DIVISION OF,NINERALS AND GEOLOGY
Depanment or Natural Resources ~/!
1717 Sherman SL, Room 21i `
Denver, Colorado 80?03 D I v 15 l O N O F
Phone: [phone]; MINERAL S
FAX. [phone] &
GEOLOGY
REC LAM ATIpN
MIN ING•SAFE 7Y
DATE: December 26, ?000 8,11 Owens
Crnernor
Grcg E. watcher
TO: Tom Schreiner Executive Duecror
Michael 8. Long
D,vis,on D,rector
FROM: Allen Sorenson
RE: Stability Analysis, Hall-Irwin Corporation,
North La Poudre Resource, File No. M-200-144
I have reviewed the stability analysis provided with the above referenced application. The closest
structure to the proposed neaz vertical pit walls is a Sinclair gas pipeline. The application specifies a
mining setback of 50 feet from the pipeline. The application further states that the pit depth will
generally not exceed 20 feet. Typically, any mining setback in an alluvial gravel pit that is equal to or
greater than two-times the maximum pit depth may be considered a safe, conservative, and prudent
protection measure. Also, the Applicant provided limit equilibrium stability analyses yielding a slope
stability safety factor of 1.5 for the gas pipeline. Although I do not necessarily agree that the sheaz
strength for the claystone bedrock input to the analyses is sufficiently conservative for a critical cross
section of the proposed slope, the Applicant's demonstration is additional evidence that the proposed SO
foot setback for the pipeline is satisfactory. (It is [he Division's experience that residual strength
q weathered shale or claystone at the toe of a pit wall may be the weak layer that causes slumping or slope
~/~ failure; residual siren h weathered shale ma be modeled usin c=0 and 14 de ees
r 1 gt Y g ~= gr )~
~~~~~ ~
In summary, the~Division can approve the mining setbacks proposed in the application. The Applicant,
shotiTd'commit td mazking the underground utilities and establishing the setback lines on the ground
prior to mining. Of greater concern than the setback distances, is the potential for the lateral berm that
will support the Sinclair pipeline following mining to be washed out during a flood. The Applicant
must provide a design for the Division's review and approval that will demonstrate and ensure
equalization of hydrostatic pressure between the two mining pods during a flood. This design must be
approved prior to consideration of the reclamation permit application for approval.
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