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t ;,~,~ <br />MEMORANDUM <br />' To: Christine Johnston RECEIVECI <br />Mountain Coal Company, LLC OCT 13 1999 <br />' From: Wright Water Engineers, Inc. Division of Minerals & lieobQy <br />C. Richazd Dunrud, P.E. <br />Date: October 7, 1999 <br />Re: Subsidence Field Observations-September 15-17, 1999 <br />Summary of Major Findings <br />' 1. No new cracks attributed to subsidence were observed above longwall Panel 10 in azeas <br />where cracks, at least partly attributable to subsidence, were observed last yeaz. The <br />1 cracks observed last yeaz were healed and not discernable, except for some cracks due to <br />mass-gravity movements in the crown of the topsoil pile and along the drainage ditch west <br />of the topsoil pile. <br />t 2. In the Apache Rocks Permit Area, smaller cracks were observed at revisited sites this yeaz <br />than were observed last year. Last yeaz there was no longwall extraction beneath these <br />areas. This yeaz many of these revisited sites had been undermined. <br />3. Subsidence cracks were observed in bedrock and also in colluvium and soil less than 2 ft <br />thick on a ridge above Panel 13 in the Apache Rocks Permit Area. Cracks were not <br />~~ observed in this area where the colluvium and soil was greater than 2 ft thick. <br />' 4. No new cracks were observed in the landslide terrain on the north side of Jumbo <br />Mountain. Dry conditions during last winter and early spring reduced water lubrication <br />that can renew slide activity. The wet periods during the late spring and summer <br />apparently did not provide saturation deep enough to cause renewed sliding. <br />5. Large, open pre-mining fractures as much as 2 feet wide were observed in sandstones of <br />the Ohio Creek Member of the Mesaverde Formation on West Flatiron near cliff edges. <br />Cracks were also locally observed on the narrow portions of West Flatiron. These cracks, <br />which aze present prior to mining, are the result of lateral spreading due to natural mass- <br />gravity forces acting in cliff areas of reduced lateral constraint. <br />Wright Water Engineers, Inc., 2490 W. 26'" A~nue, Ste. 100A, Demer, CO 80211 <br />Tel. 303/460.1700; Faz. 303/480.1020, e-mail:krHright@wrightwater.com <br />