2011-01-13_REVISION - C1981038 (8)

Home Browse Search

applicant completed a subsidence survey of hydrologic features which is described in Section 2.05.b(b)(e) Additional shallow alluvial monitoring wells were installed within the alluvial/colluvial deposits for the purposes of monitoring potential effects of subsidence upon the hydrology of the surficial deposits. Subsidence Projections - The operator retained Geo-Hydro Consultants, Inc. to complete a technical projection of subsidence expected within the revised permit area. A typical subsidence profile is graphically depicted on Plate 11 of the subsidence study in Volume 3. Vertical subsidence was projected to reach a maximum of approximately six feet. In consultation with the Division, and through the employment of Dunrud's (1976) research, the operator estimates the conservative angle-of-draw to be 20.5 degrees from vertical. The technical projections included in the subsidence study conformed with the prudent state-of-the-art of subsidence projection. Subsidence monitoring has been conducted within the Bowie No. 1 Mine permit area, in compliance with the originally approved permit. The data collected to date corresponds with the technical projections for vertical subsidence and draw angle (See Table 8, Volume 3 and annual subsidence reports). The operator has completed strain projections for the structures inventoried on Map 9-7. Strain projections have also been completed for the Pitkin Mesa and the Bowie No. 1 Mine water supply water pipelines which cross the potential subsidence zone of the life-of-mine area. The Pitkin Mesa pipeline supplies potable water to roughly 160 domestic taps. Subsidence Control Plan - The operator has proposed a subsidence control plan to prevent material damage from occurring to the renewable resource lands within Steven's Gulch and East and West Roatcap Creeks. The U.S. Bureau of Land Management Lease C-37210 also bears stipulations addressing these same concerns. The operator proposed to control subsidence within these renewable resource lands by limiting the extraction of coal beneath these potentially impacted surficial aquifers. In areas of the renewable resource lands separated from the D seam by less than 600 feet, extraction will be limited to seven 20-foot wide mains developed on 120-by-80-foot centers, resulting in permanently retained 100-by-60-foot pillars. Pillar stability calculations project Ashwin/Wilson safety factoring in excess of 2.0 for pillars of that size at 600 feet of overburden depth. Further, in areas of the renewable resource lands separated from the D seam by between 600 and 800 feet of overburden, permanent 60-by-60-foot pillars, with Ashwin/Wilson safety factoring in excess of 1.5, will be retained. In both of these areas, mining will be limited to development without retreat extraction. Technical Revision No. 17 (TR-17) and Technical Revision No. 19 (TR-19) address mine plan modifications to Panel B and Panel C respectively. These TR's modify the pillar size and spacing scheme described above, while maintaining the required safety factors. Both revisions were approved by the Division. With the exception of the ruins depicted as Structure No. 27, the inventoried buildings have been protected by the retention of solid coal pillars of 300-foot radius beneath each of the potentially affected buildings. The operator's strain projections anticipate potential, repairable damage (non-material damage) to the two existing water pipelines, the Steven's Gulch Road, assorted trails, ponds, and springs 39