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• SOUTHFIELD MINE SEMI-ANNUAL SUBSIDENCE REPORT FOR PERIOD: NOVEMBER, 1985 - APRIL, 1986 (STIPULATION NO. 11 OF PERMIT C=81-014) r 1 U • Introduction This report was prepared to summarize subsidence monitoring re- sults and to state subsidence predictions based upon the present subsidence monitoring program. Survey monitoring for the above time period was performed on the South Grid, City of Florence Water Line, Thompson Ranch Structures, and the Newlin Creek Streambed as required. Survey data has been presented on four summary tables, each of which compares results of the most recent survey to the pro- ceeding survey and to the original survey to indicate the magni- tude and direction of movement for each monument and structure. The survey data for both monument lines of the South Grid is portrayed in cross sectional form to depict the total subsidence as of the most recent survey. The report presents observations and predictions in sections for each item monitored in the subsidence monitoring program. South Subsidence Grid The subsidence grid was installed by Dorchester Coal Co. in March, 1983, and subsequently undermined by the 2 South "A" Panel through June, 1983. Additional mine development has since occurred under the north leg of the grid. Energy fuels Coal completed the devel- opment of the 2 South "B" Panel and retreat mined the pillars through August, 1985. A third mining section, "C' Pnnel, wes developed and retreat mined from August, 1985 through March, 1986 adjacent to "B" Panel and some 150 Eeet from the most northerly subsidence monument. The barrier pillar between these panels was partially extracted during retreat. Survey data to date indicates the maximum subsidence detected is 1.72 feet at Monument 15. This monument also exhibits the great- est amount of change since the previous survey (9/85) of 1.43 feet. It is evident from the data presented on the summary table that Monuments 9 through 15 are subsiding due to the combined influence of the "B" and "C" Panels. The maximum predicted sub- sidence, assuming the combined width of `B" and 'C" Panels of 700 feet, is 5.0 feet based on the Rocky Mountain Geotechnical (RMG) report in Volume fi of the Mining permit. Total subsidence et Monument 15 is only 34B of the predicted maximum value, although this monument may not be located at the point of maximum subsi- dence. Monuments undermined by "A" Panel continue to show relatively little subsidence, ranging from 0 to 0.7 feet as compared to the