1987-12-01_PERMIT FILE - C1981017 ‎(195)‎

• Permit No: C1981017
• IBM Index Class Name: PERMIT FILE
• Doc Date: 12/1/1987
• Doc Name: Technical Revision Subsidence Monitoring Frequency
• Section_Exhibit Name: Chapter IV Appendix IV-F-5
• Media Type: D
• Archive: No

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p DEPARTMENT OF NATURAL RESOURCES
n David H.Getches,Executive Director
,8,6 * MINED LAND RECLAMATION DIVISION
DAVID C.SHELTON, Director
i
Richard D.Lamm
Governor
r
MEMORANDUM
1 January 1986
TO: Candace ThompsonFROM: Jim Pendleton
RE: Request to Suspend �bsidence Monitoring at the Coal Basin Mines
(Permit C-81 -017)
Pursuant to your request, I have reviewed Mid-Continent Resources' request to
suspend subsidence monitoring at the Coal Basin Mines. The technical summary,
prepared by Bruce Collins, consultant to the operator, presents a clear and
accurate rendition of the subsidence monitoring results to date.
Very briefly summarized, the operator assumed a subsidence factor (maximum
vertical ground surface subsidence / thickness of the extracted seam) of
0.55. The operator proposed an anticipated angle of draw of 15 degrees from
vertical , but was required by the Division to assume 20.5 degrees from
vertical . The past three years observations have demonstrated that the
maximum observed subsidence factor was 0.26. The operator concludes that a
maximum projected subsidence factor of 0.30 is more appropriate, based on the
observations to date. Further, the operator demonstrated that observed angles
of draw have varied from 13 degrees to 21 degrees from vertical . The lack of
precision is caused by the fairly wide interval spacing between subsidence
monuments. From these observations the operator concludes that it would be
sufficiently conservative to assume an average angle of draw of 18 degrees.
The evidence from three years of subsidence monitoring, both survey and visual
inspections, has detected no visible subsidence damage, cracking or
landsliding at the Coal Basin Mines. Based on the above conclusions,
Mid-Continent Resources, Inc, requests that the subsidence monitoring
requirements within its permit be suspended.
The Division does not dispute any of Mid-Continent Resources' observations.
The subsidence monitoring data has been painstakingly collected under very
adverse conditions. The analysis prepared by Dr. Collins well summarizes the
collected subsidence data. The Division agrees that 0.30 is an acceptable
subsidence factor for room & pillar and single-pass longwall operations at the
Coal Basin Mines, based on the observed subsidence data. The Division also
agrees that 18 degrees from vertical is an acceptable average angle of draw.
A determination as to whether 18 degrees or 20.5 degrees from vertical will be
acceptable for future permiting situations will depend upon whether the 3.5
degree difference would expose any critical structures or renewable resource
lands to possible material damage.
423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. [phone]