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:. ~~ • Grand Mesa Coal Company -. June 17, 1986 ~ ~~ P. O. Box 1351 Palisade, CO 81526 (303) 464-7951 ,,. ~uN 2 o rsas John T. Doerfer Reclamation Specialist Colorado Mined Land Reclamation 423 Centennial Building 1313 Sherman Street Denver, Colorado 80203-2273 Subject: Subsidence Monitoring - File No. C-81-034 Dear John: The following table tabulates the past two quarters of subsidence monitoring on part of the monitoring points. The points being monitored are houses 2, 6, 7 and 8 and the control to these houses. These houses are being monitored because mining has been done within their projected zone of influence as shown on Map 2.05.6(6)(f)(ii)(c)-1 in our permit application. The initial elevations were measured as baseline information in early 1982. The "Elevation Change" is the current elevation minus the original elevation. The "Relative Elevation Change" is the "Elevation Change" of the point minus the "Elevation Change" of the backsite. Elevations for the baseline and subsequent monitoring on the sub-points have been calculated using vertical angles. Sub 0, Q and S show the greatest relative elevation changes. The surveys of these points were done according to the attached Second Order, Class II guidelines. These points show movement according to the guidelines. All three points are outside the projected zone of influence of mining, therefore, the movement is not believed to be associated with mining. Elevations for the baseline and subsequent monitoring on the house have been calculated using a level loop from the closest sub-point. None of the houses show major re]ative elevation changes that would indicate subsidence. We propose to monitor these points one additional time and then suspend monitoring '' unless there are visual indications of subsidence. The original survey and four quarterly surveys will be used to calculate new baseline elevations.