Laserfiche WebLink
Mountain Coal Company Exhibit 69 <br /> West Elk Mine Sylvester Gulch Facilities <br /> • The existing Pond 017 will be used as the primary aeration cell in the revised treatment system. <br /> To enhance aeration, air diffusers will be installed on or near the bottom of the existing pond <br /> with appropriately sized compressors on the banks to provide air flow. The system will be <br /> designed so that aeration can be continuous, except during maintenance activities. For mine <br /> water inflows containing higher iron concentrations, a float-mounted surface aerator system <br /> designed for high-volume water movement may be used to fi.0 ther enhance oxygenation. <br /> The Pond 017 volume at the existing operating water surface elevation (El.) of 6822.8 feet above <br /> mean sea level (ft MSL) is approximately 200,000 gallons. This provides a normal detention <br /> time of approximately 8.3 hours at the design flow rate of 400 gpm. To provide a larger pond <br /> operating volume and prevent unintended discharges from the existing Pond 017, the exi ¢ing <br /> outlet structure will have a sealed steel plate attached with a top El. of 6826.5 ft MSL. The total <br /> pond volume is anticipated to be approximately 325,000 gallons at El. 6825.0 ft MSL,the normal <br /> maximum operating water surface, as shown on the area-capacity curve (Drawing.Sheet C2). <br /> The minimum freeboard in the existing Pond 017 will be 1.5 feet with the maximum emergency <br /> spillway water surface at El. 6826.5 ft MSL and the pond crest at El. 6828.0 ft MSL, which <br /> exceeds what is required by Rule 4.05.9(7)(d). - <br /> Design of Second Pond <br /> • To provide for settling after aeration, a Second Pond is planned for construction. The pond is <br /> designed to be in compliance with applicable portions of Rules 4.05.6 and 4.05.9. The Second <br /> Pond will cover a total area of approximately 1.1 acres from the edge of the excavation east of <br /> Pond 017 to the toe of the embankment fill east of the Second Pond. <br /> The depth to bedrock in the pond vicinity is approximately 25 to 36 feet below existing ground <br /> surface (ft bgs). Prior to excavation,topsoil, which is present at a thickness of approximately 6 <br /> to 12 inches in the Second Pond area,will be stripped and hauled to a nearby topsoil stockpile for <br /> later use on external embankment slopes and adjacent disturbed areas. During excavation,pond <br /> embankments will be constructed using select compacted material from the excavation. These <br /> embankments will be located on the north, east and south sides of the pond. The embankments <br /> will have a crest width of 12 feet and 2.5h:Iv external side slopes. The maximum height of these <br /> embankments will be approximately 9.5 feet above existing ground surface on the embankment <br /> centerline. Based on the existing pond slopes and geotechnical data reviewed for the project,the <br /> interior pond excavation slopes are designed to be 2h:ly and the maximum excavation depth will <br /> be approximately 17 feet. <br /> The combined upstream and downstream side slopes of the settled embankment are designed to <br /> be 4.5h:ly, as opposed to the minimum 5h:ly stipulated per Rule 4.05.9(7)(e). However, as <br /> allowed by the same Rule, slope stability analyses have been performed(Attachment 1)to <br /> support the design as being stable, with a worst-case factor of safety of 1.36 assuming very <br /> conservative soil strength parameters and a design seismic load (more than the minimum safety <br /> • factor of 1.3 for static conditions per Rule 4.05.9(8)(b) and more than the minimum seismic <br /> 4 <br /> Rev. 10/97-TR83,Rev.04109-TR116 <br />