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Cow Camp. The applicant completed a subsidence survey of hydrologic features which is <br />described in Section 2.05.6(b) of the permit application. Additional shallow alluvial <br />monitoring vvells were installed within the alluvial/colluvial deposits for the purposes of <br />monitoring potential effects of subsidence upon the hydrology of the surficial deposits. <br />Subsidence Projections <br />The operator retained Geo -Hydro Consultants, Inc. to complete a technical projection of <br />subsidence expected within the revised permit area. A typical subsidence profile is <br />graphically depicted on Plate 11 of the subsidence study in Volume 3. Vertical subsidence <br />was projected to reach a maximum of approximately six feet. In consultation with the <br />Division, and through the employment of Dunrud's (1976) research, the operator estimates <br />the conservative angle of drawto 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 anddrawangle (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. 1 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 Steven's Gulch and East and West <br />Roatcap Creeks. The U.S. Bureau of Land Management Lease C-37210 also bears <br />stipulations addressing these same concerns. <br />The operator proposed to control subsidence within these renewable resource lands by <br />limiting the extraction of coal beneath these potentially impacted surficial aquifers. In <br />areas of the renewable resource lands separated from the D seam by less than 600 feet, <br />extraction was to be limited to seven 20 -foot wide mains developed on 120 -by -80 -foot <br />centers, resulting in permanently retained 100 -by -60 -foot pillars. Pillar stability <br />calculations project Ashwin/Wilson safety factoring in excess of 2.0 for pillars of that size <br />at 600 feet of overburden depth. Further, in areas ofthe renewable resource lands separated <br />from the D seam by between 600 and 800 feet of overburden, permanent 60 -by-60-foot <br />pillars, with Ashwin/Wilson safety factoring in excess o f 1.5, mere to be retained. In both <br />of these areas, mining was limited to development without retreat extraction. Technical <br />Revision No. 17 (TR -17) and Technical Revision No. 19 (TR -19) address mine plan <br />modifications to Panel Band Panel C respectively. These TR's modify the pillar size and <br />spacing scheme described above, while maintaining the required safety factors. Both <br />revisions were approved by the Division. <br />37 <br />