2009-04-01_REVISION - C1981010

• Permit No: C1981010
• IBM Index Class Name: REVISION
• Doc Date: 4/1/2009
• Doc Name: Preliminary Adequacy Review Comments ‎(Memo)‎
• From: Kent Gorham
• To: Jason Musick
• Type & Sequence: PR6
• Email Name: JDM
• 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]
To: Jason Musick
Interoffice Memo
COLORADO
DIVISION OF
RECLAMATION
MINING
SAFETY
Bill Ritter, Jr.
From: Kent Gorham W6 Governor
Harris D. Sherman
CC: Dan Hernandez; Marcia Talvitie Executive Director
Ronald W. Cattany
Division Director
Date: April 1, 2009 Natural Resource Trustee
RE: Preliminary adequacy review comments, Horse Gulch Fill, PR-6, Trapper, Mine
Based on Appendix A of the Agapito Report titled Horse Gulch Fill Stability Analysis,
December 2008 (The Report), there is no question that Horse Gulch Fill is a valley fill by
definition. A valley fill "means a fill structure consisting of any material other than coal waste
or organic material that is placed in a valley where the side slopes of the existing valley
measured at the steepest point are greater than 20 degrees or the average slope of the
profile of the valley from the toe of the fill to the top of the fill is greater than 10 degrees."
(Rule 1.04(148)). Appendix A, Horse Gulch General Arrangement, shows three areas
highlighted in yellow that exceed 35% slope (19.17 degrees). Preliminary calculations
indicate an average slope for the drainage profile of 8.6% (4.5 degrees). Given that some
side slope areas are represented as exceeding a 19 degree slope, I think the Horse Gulch Fill
meets the requirements of a valley fill because the side slopes likely exceed 20 degrees in
places. Therefore, the requirements of 4.09.1 and 4.09.2 apply. The fill may also meet the
definition of a head-of-hollow fill, in which case the additional requirements of 4.09.3 may also
apply. I will leave the final determination of fill category to the DRMS engineer who reviews
the submittal.
The Report includes a demonstration that the constructed Horse Gulch Fill will have a
minimum safety factor of 1.5 as required by Rule 4.09.1(7). However, I could not find any
commitment in the report to construct the fill with any specific thickness for horizontal lifts.
Based on the fact that the underdrain is proposed for placement by gravity separation during
spoil placement, it appears that the applicant is not proposing to place excess spoil in
horizontal lifts. Again, whether this is a concern regarding stability, I will defer to the DRMS
engineer.
The Report (p. 23) states that there will be no need to channel storm runoff away from the fill
once the fill is constructed. The Division does not agree with this statement. Small areas on
the south side of the fill will flow toward the fill and require diversion into channels designed
for the 100 year, 24-hour storm event. (Rule 4.09.2(7))
Additionally, Rule 4.09.2(7) also requires that drainage from the fill be diverted into channels
designed for the 100-year, 24-hour storm event. No diversion of flow from the fill is proposed
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