1991-01-22_REVISION - C1981035

• Permit No: C1981035
• IBM Index Class Name: REVISION
• Doc Date: 1/22/1991
• Doc Name: Review Letter
• From: Jim Pendleton
• To: Harry Ranney
• Type & Sequence: PR2
• Media Type: D
• Archive: No

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STATE OF COLORADO
MINED LAND RECLAMATION DIVISION oF.0
Department of Natural Resources
1313 Sherman St., Room 215
Denver,CO80203 * a�
303 866-3567 ** *
FAX: [phone]
Roy Romer,
Governor
Fred R Banta,
DATE: January 22, 1991 Division Director
TO: Harry Ranney
FROM: Jim Pendleton
RE: National g Co Mine - Refuse Pile Stability Analysis, PR-02
(Permit No: C-81-035)
I have reviewed the stability analysis which you forwarded to my attention.
This analysis, prepared by Fred Johnson, consultant geologist to NKC, was
submitted in response to our earlier aaequacy comments.
The methodology applied by Mr. Johnson consists of a manual evaluation of the
original Terzaghi and Rankine pile stability equations. This technique is
valid, but has several serious constraints which might render it inconclusive
in this situation. The analytical technique graphically divides the body of
the pile into two zones, that portion applying the load acting to displace the
pile, and that zone supplying the resistance to displacement. In its simplest
form, as applied by Mr. Johnson, the technique assumes a level and planar
foundation beneath the pile. In addition, the evaluation of the equations
requires the determination of values for the refuse density, the coefficient
of friction of the waste material , and the angle of repose of the refuse. Mr.
Johnson has uniformly applied a refuse material density of 86
pounds/cubic-foot, a coefficient of friction of 0.93, and an angle of repose
of 42 degrees. No explanation is provided to document the derivation of these
important mechanical properties for the waste. The applicant should provide a
thorough discussion of the derivation of each of these mechanical properties ,
including a discussion of the expected uniformity of these properties
throughout the refuse.
Nay recollection of the Nat King Coal mine's surface facility area suggests
that the foundation of the proposed refuse pile is neither level nor planar.
In fact I believe the area dips away from the high wall of the face up. If my
recollection is correct, this configuration is not conducive to stable
placement of refuse. Further, the simplistic analysis performed by Mr.
Johnson would not be adequate to determine the projected mass stability of the
proposed refuse pile. Since Terzaghi and Rankine developed the calculation
techniques in the early 1900's, considerably more comprehensive techniques
have been developed. Slope stability analytical techniques, such as the
Bishop Method and the Simpson Method, have been developed which can evaluate
the stability of more complex pile configurations.