_GENERAL DOCUMENTS - C1981017 (219)

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r °F ` 5TATE OF COLORADO �� q9 tee' * Roy Romer, Governor 1876 DEPARTMENT OF NATURAL RESOURCES MINED LAND RECLAMATION DIVISION FRED R. BANTA, Director ? DATE: November 9, 1988 TO: Candace Thompson J FROM: Jim Pendleton RE: Coal Basin P ' es - P it Renewal - Subsidence Considerations (Permit # C-81-017 ) In response to your request, I have reviewed the materials submi-:.ted by Mid-Continent Resources (MCR) regarding ground subsidence above their Coal Basin Mines. In completing my review I have revisited earlier revision submittals made by MCR, specifically their Permit Revision deleting subsidence monitoring approved in Spring of 1986. In completing their original permanent program permitting, MCR had assumed a subsidence factor of 0.55 (0.55 feet vertical subsidence for each foot of coal extracted) and had proposed a maximum angle of draw of 15.0 degrees from vertical . The Division concurred with the subsidence factor assumption but required the applicant to project potential disturbance boundaries using an angle of draw of 20.5 degrees from vertical . In December of 1985, MICR submitted a report by Bruce Collins, PhD. , acting as a consultant to the operator, addressing the subsidence monitoring results. Dr. Collins ' report summarized the previous three years subsidence monitoring data collected above the Coal Basin Mines. The report concluded that the subsidence data collected as of the Fall of 1985 documented an actual subsidence factor of 0.26 with a variation in observed angles of draw of between 13 and 21 degrees from vertical . Based upon the 1985 survey data, the Division agreed "that 0.30 was an acceptable subsidence factor for room & pillar and single-pass longwall operations at the Coal Basin Mines", and "that 18 degrees from vertical is an acceptable average angle of draw for purposes of future permitting subsidence projections (Excerpts from Dr. Collins December, 1985 report). [Note: The ability to discriminate actual angles of draw is dependent upon the separation of subsidence monuments. Above the Coal Basin Mines the separation of monuments was fairly large. Dr. Collins ' recent submittal objects to the Division's statement that the maximum angle of draw observed as of 1985 was 21 degrees. He contended in his December, 1985 report that "the actual angle of draw is probably in the vicinity of 18 degrees". This may have been the case. In fact, the Division agreed "that 18 degrees from vertical is an acceptable average angle of draw". However, failing data to specifically verify his supposition, the Division felt it appropriate to caution the operator: "A determination as to whether 18 degrees or 20.5 degrees from vertical will be acceptable for future permitting situations will depend upon whether the 2.5 degree difference would expose any critical f 215 Centennial Building, 1313 Sherman Street Denver, Colorado 80203-2273 Tel.(303) 866-3567