PERMFILE64512

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 7/24/2007
• Section_Exhibit Name: Exhibit 77 Deer Creek Shaft Plan
• Media Type: D
• Archive: No

Barr Engineering Company
4700 West 77th Street • Minneapolis, MN 55435-4803
ARD Phone: [phone] • Fax: [phone] • WWW.barr.COm An EEO Employer
B~= Minneapolis, MN • Hibbing, MN • Duluth, MN • Ann Arbor, MI • Jefferson City, MO
July 10, 2007
Mr. Philip Sclunidt
Engineering Manager
Mountain Coal Company
West Elk Mine
P.O. Box 591
Somerset, CO 81434
Dear Mr. Schmidt:
Pursuant to our telephone conversation, Bart has performed a stability evaluation of the impact of blasting
at Deer Creek Shaft on Monument Dam. This letter summarizes the results of that evaluation.
Gordon Revey made some estimates of the predicted accelerations at the Dam site induced by the
proposed blasting at the Deer Creek shaft. The estimated peak ground acceleration was 0.007g according
to Revey. This value is much lower than the design acceleration value used in the analysis for the
modified dam. The earthquake acceleration typically cortesponds to low frequencies whereas the
blasting-induced acceleration corresponds to high frequency. However, since there is a chance that
blasting can take place before completion of the proposed dam enhancement, an analysis was conducted
of the existing dam subjected to the estimated peak ground acceleration of 0.007g.
The results of the analysis indicate that the computed factor of safety decreases from 1.28 to 1.26 for the
drained case (ESSA) and from 1.07 to 1.05 for the midrained case (USSA). This corresponds to an
approximately ].56 to 1.89 percent decrease in the factor of safety from existing conditions which is
considered insignificant.
Sincerely,
Phl ip B. So se g' P.E. . .
Vice President
Z:\CLIENTS\MUQountain Coal Company\Deer Creek Shag Blasting.doc