GENERAL30529

• Permit No: M1980244
• IBM Index Class Name: General Documents
• Doc Date: 11/26/1996
• Doc Name: REVIEW OF HIGHWALL STABILITY REPORT CRESSON PROJECT PN M-80-244
• From: DMG
• To: BERHAN KEFFELEW
• Media Type: D
• Archive: No

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• • III IIIIIIIIIIIII III
STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Deparlmenl of N. rural Resource,
131 3 Sherman St., Room !15 J+y`

Denver, Colorado AO'_Ol ~
`
Phone: [phone]h7 ~
IIII
FAX: [phone]-A1CI6
DEPARTMENT OF
NATURAL
DATE: November 26, 1996 RESOURCES
Roy Romer
TO: Berhan Keffelew c°ver^or
///~~3 lames 5. Lochhead
~ Esecuuve Director
~
FROM: James Dillie
Michael B. Long i
Division Direcor
RE: Review of Highwall Stability Report, Cresson Project, Permit No. M-80-244
Per your request, I reviewed CC&VG's report regarding the results of mine highwall
mapping and additional sheaz box tests to assess the internal friction angle of crushed
Cresson ore as required by Amendment 6.
As I interpret the data you provided, the operator submitted amine wall design that would
provide adequate operational stability which was also assessed to be stable under longer-
term (post-mining) conditions. Based on the input data, the factors of safety for the mined
walls ranged from 2.6 to 10. The operator was required to conduct geologic mapping of the
pit slopes after initiating mining operations and periodically evaluate the maps to determine
the effect of mapped structures and rock types on slope stability. The mapping will
specifically address the attitude of major structures to confirm or alter the submitted
interpretation and the lengths of joints that dip into the pit. If the mapping indicates that
planned bench and mine wall angles should be reduced because the assumptions of the
planned slopes was faulty, CC&VG shall notify the Division and shall submit an alternative
plan for modified angles for Division approval.
As stated, the original slope design was based on surface openings and core hole data. The
data indicated several joint sets (which will not be repeated here). The mapping of actual
highwall joint information has refined the joint set data (see page 2 of John Hardaway's
November 6, 1996 letter). Briefly, the new data indicates that the Cresson mine rock mass
has one dominant joint set which is oriented approximately north-south with a near vertical
dip. The subordinate and minor fractures are noted on page 2.
The operator evaluated 77 structures in the field to determine a value of the effective stress
friction angle. In-situ shear planes were measured at 77 wedges and blocks resulting in a
40 degree effective stress friction angle. In addition to the in-situ evaluation, CC&VG
recently performed direct shear tests of the Cresson ore after crushing .The friction angle
for the Cresson ore was 38 degrees and the cohesion was 1275 psf.
As stated on page 6 of Mr. Hardaway's letter, the operator proposes to revise the highwall