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Page Twelve Preliminary Adequacy Review Grand Mesa Coal Co. - Red Canyon Mine , April 28, 1981 2.05.6(6) Subsidence The applicant's inventory of structures and renewable resource lands determine that 12 structures, the alluvial aquifer of Ward Creek and assorted agricultural lands exist within the potentially affected area. The application projects that no subsidence will occur because mining will be limited to SSa extraction with no retreat mining. In addition, safe areas limited to SOo overall extraction are proposed beneath the 12 structures and a 200 to 400' outcrop barrier is projected to be left adjoining the subcrop of the coal seams. The existing Red Canyon No. 1 mine extracts approximately 5 feet of coal from the "E" seam. The proposed Red Canyon No. 2 mine will extract approximately 8 feet of coal from the "D" seam located approximately 130 feet stratigraphicaIly beneath the "E" seam. No projection is included within the application concerning the combined affects of extraction in the "E" and "D" seams. Such a projection must be included in the application by amendment. The application observes that no roof fall is believed to have occurred in post mining from 1967 through 1978 and, therefore, none is expected to occur during future extraction. However, Grand Mesa Coal Company recently, subsequent to submission of the pernut application, encountered severe roof stability problems while driving the No. 1 west submains toward Ward Creek in the southwest 1/4 of the northeast 1/4 of Section 11. The company informed the Division by telephone and letter of this difficulty and re- quested permission to drill two additional exploratory core holes. (Letter to Dan Mathews from Larry M. Reschke, dated March 31, 1981). The brief description of the problem encountered included within those comvtunications theorized that greater alluvium thickness and thinner bedrock overburden exists beneath Ward Creek than earlier projections had anticipated. The results of the coring project will be utilized to re-design the mining plan. These amendments will have to be reflected in the application. Furthermore, the implications of possible roof fall and subsidence for the alluvial aquifer beneath Ward Creek and any overlying structures and wells will have to be discussed within the amended application. The application includes a proposal for safe areas of 50~ extraction beneath the 12 structures identified above potentially affected areas. These safe areas are delineated assuming a 15 degree angle-of-draw. The angle-of-draw is less than those commonly observed by investigators such as Dunrud (U.S.G.S. Professional Paper No. 969). Empirical data and/or literature references should be provided to support the assumed 15 degree angle-of-draw, or the angle-of-draw should be increased and the calculations revised. Further- more, the applicant's analysis assumes that limiting extraction to 50~ will inherently guarantee stability. Appropriate pillar strength calculations should be included within the application to justify the assumption of stability for SOa extraction configuration. This analysis should include analyses of both the superimposed "E" and "D" seams. The applicant should produce a map which clearly shows safe areas superimposed on the mine plan for seam "E". An analogously detailed mine plan map including safe areas of extraction should be prepared and submitted for the "D" seam workings proposed during this permit period.