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V Dorchester Coal Company United Bank of Arapahoe Bldg. Suite 300 9350 East Arapahoe Road Englewood, Colorado 80112 Writer's Direct Dial Number (303) 790-8444 November 15, 1984 iiiiiiiiiiiiiiiiiii RECEIVED NOV 19 1984 Mr. Bob Liddle Colorado Mined Land Reclamation Division 1313 Sherman Street - Room 423 Denver, Colorado 80203 MINED LAND RECUIMAiION DIVISION Calo. Dept. of Natural Resources Re: Semi-Annual Subsidence Survey Dear Bob: Enclosed is a summary of the data collected to date for the subsidence grid located in the southern portion of the permit area and the subsi- dence monuments along the Florence Water Line. The enclosed map shows the orientation of these monuments in relation to the underground work- ings. The subsidence grid was installed in early March, 1983, while development mining was underway in the underlying panel. Second mining was initiated in late March and completed by June, 1983. Maximum subsidence detected to date is 0.9 feet at Monument 4 (western- most point on the E-W arm of the grid). This is comparable to the maximum expected subsidence, as predicted by Rocky Mountain Geotechnical (Volume 6, Sec. 2.05.6(6) of the permit application) of 1.46 feet. For the other monuments within the grid which have been undermined, subsi- dence ranged from less than 0.2 feet to 0.7 feet. Data collected during the last three quarterly surveys show no significant change in elevation over that period of time. It is anticipated that additional subsidence in this area will be minimal. Data from the southern arm of the subsidence grid are useful for assessing the angle of draw. The southern limit of pillaring was in vicinity of 14onument 16. Surface subsidence decreases regularly from Monument 16 to between Monuments 19 and 20. These data indicate that the angle of draw is between 17 and 22 degrees. This is consistent with the projected angle of draw of (Rocky Mountain Geotechnical, 1983) 20 to 25 degrees. A Subsidiary of Dorchester Gas Corporation