APPCOR10239

• Permit No: C1980003
• IBM Index Class Name: Application Correspondence
• Doc Date: 8/17/1981
• Doc Name: MEMO REVIEW OF THE STABILITY ANALYSIS FOR THE HAYDEN GULCH MINES OVERBURDEN EXCESS SPIL PILE
• From: MLR
• To: BRIAN MUNSON
• Media Type: D
• Archive: No

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ST4TE OF COLORADO RiC MARfI ~MM. Gnvem nr '111111111111111 III
DEPARTMENT OF NATURAL RESOURCES 999
D. Monte Pascoe, Executive Director
MINED LAND RECLAMATION
423 Centennial Building, 1313 Sherman Slreet
Denver, Colorado 80203 Tel. [phone]
David C. Shelton
Director
August 17, 1981
TO: Brian Munson
FROM: Jim Pendleton
SUBJECT: Review of the St lity alysis for the Hayden Gulch Mine's
Overburden Exc Pile
Pursuant to your request I have reviewed the report entitled; "Geotechnical
Investigation of the Overburden Dump Area, Hayden Gulch mine, Routt County,
Colorado", prepared for H-G Coal Company by Morrison-Knudsen Company, Inc.
I forward the following comments for your consideration.
The geotechnical and stability anlaysi•s performed by M-K is relatively simplistic
in nature. No materials testing was performed to determine shear strength of
the spoil materials. In addition, no observation of actual hydrologic conditions
within the pile was made. The consultant assumed that the pile is "well drained
and no phreatic surface is to be found within". The computer technique
utilized is 'a'cceptable. Based upon the assumed material and hydrologic
properties, the static slope factor of safety was calculated to be in excess
of 2.
In the absence of discrete spoil materials strength and hydrologic data
concerning the phreatic surface within the pile, I believe it prudent to repeat
the computer stability analysis in order to determine the inter-related
sensitivity of the two assumed input parameters. The consultant did complete
two separate computer analyses assuming a variation of spoil material shear
strength. The first analysis assumed an angle of the internal friction of
22 degrees and a cohesion of 100 pounds per square foot. The second analysis
assumed an angle of internal friction of 28 degrees and a cohesion of 250
pounds per square foot. In my opinion this variation of shear strength is
adequately conservative to be acceptable. However, the consultant should
repeat the analysis projecting possible phreatic surface configurations other
than the assumed dry pile.
The consultant should project a maximum probable elevation of the phreatic
surface within the pile. The computer analysis should be repeated utilizing
that maximum probable phreatic surface assumption, as well as several other
intermediate configurations. If the resulting factors of safety exceed the
required 1.5, I would deem it acceptable to satisfy the requirements of Rule
4.09. If the resulting factors of safety are sub-standard, the operator should
then be required to install the appropriate monitoring well within the pile to
determine the maximum elevation of the existing phreatic surface. If this is
proven to be necessary, it may be possible to utilize an existing monitoring
well within the pile, which was originally designed to monitor an aquifer