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Subsidence Projections <br /> The operator retained Geo-Hydro to complete a technical projection of subsidence <br /> expected within the revised permit area. A typical subsidence profile is graphically <br /> depicted on Plate I I of the subsidence study in Volume 3. Vertical subsidence was <br /> projected to reach a maximum of approximately six feet. In consultation with the Division, <br /> and through the employment of Dunrud's research (1976), the operator estimates the <br /> conservative angle of draw to be 20.5 degrees from vertical. The technical projections <br /> included in the subsidence study conformed to the prudent state-of-the-art of subsidence <br /> projection. <br /> Subsidence monitoring has been conducted within the Bowie No. 1 Mine permit area, in <br /> compliance with the originally approved permit. The data collected to date corresponds <br /> with the technical projections for vertical subsidence and draw angle(See Table 8,Volume <br /> 3 of the PAP and annual subsidence reports). <br /> The operator has completed strain projections for the structures inventoried on Map 9-7. <br /> Strain projections have also been completed for the Pitkin Mesa and the Bowie No. I Mine <br /> water supply water pipelines,which cross the potential subsidence zone of the life-of-mine <br /> area. The Pitkin Mesa pipeline supplies potable water to roughly 160 domestic taps. <br /> Subsidence Control Plan <br /> The operator proposed a subsidence control plan to prevent material damage from <br /> occurring to the renewable resource lands within Stevens Gulch and East and West Roatcap <br /> Creeks. The U.S. Bureau of Land Management Lease C-37210 also bears stipulations <br /> addressing these same concerns. <br /> The operator proposed to control subsidence within these renewable resource lands by <br /> limiting the extraction of coal beneath potentially impacted surficial aquifers. In areas of <br /> the renewable resource lands separated from the D Seam by less than 600 feet, extraction <br /> was to be limited to seven 20-foot wide mains developed on 120-by-80-foot centers, <br /> resulting in permanently retained 100-by-60-foot pillars. Pillar stability calculations <br /> project Ashwin/Wilson safety factoring in excess of 2.0 for pillars of that size at 600 feet <br /> of overburden depth. Further, in areas of the renewable resource lands separated from the <br /> D seam by between 600 and 800 feet of overburden,permanent 60-by-60-foot pillars,with <br /> Ashwin/Wilson safety factoring in excess of 1.5, were to be retained. In both of these <br /> areas, mining was limited to development without retreat extraction. Technical Revision <br /> No. 17 and Technical Revision No. 19 address mine plan modifications to Panel B and <br /> Panel C respectively. These revisions modify the pillar size and spacing scheme described <br /> above,while maintaining the required safety factors. Both revisions were approved by the <br /> Division. <br /> 38 <br />