2008-01-11_PERMIT FILE - C1982056A (2)

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Site-specific investigations required for design and permitting of the new ventilation shaft included a cultural resource survey of potential disturbance azeas, soil and foundation characterization as the basis for shaft pad and access road design, and completion of a wetland/AVF survey for the shaft pad and access road. The cultural resource survey was conducted by Metcalf Archaeological Consultants, the • engineering soils characterization by Northwest Colorado Consultants, and the soils/wetland/AVF survey by Habitat Management, as described in Section 2.04. The perimeter of all areas affected by surface facilities was cleazly marked before initiating surface disturbance activities. The shaft pad surface azea is approximately 11.1 acres in size with minimal cut and fills. The associated access road corridor is approximately 2,930 feet long, with a finished top width of 24 feet (road corridor 35 feet wide, 2.4 acres). See Exhibit 25U, 6-North Mains Ventilation Shaft - Geotechnical Pavement Design, for details on the road, and Map EX49A-M1, a new road profile and cross-section. Given an average topsoil depth of approximately 6 inches and a total road distwbance area of approximately 2.4 acres, approximately 1,940 cubic yazds of topsoil were salvaged from the road corridor, and 8,954 cubic yards from the 11.1 acre pad azea. Prior to initiating topsoil removal activities, stream and wetland buffer zones were marked with identifying signs to limit surface disturbance within these areas to the planned and approved activities. Topsoil from the road corridor was recovered and placed in a stockpile at the north end of the road corridor beyond the edge of the 35-foot road disturbance area. Topsoil from the pad area was placed in a stockpile within the shaft pad on the southwest edge of the pad and along the west edge of the access road, just south of the pad, for future reclamation use. Topsoil salvage volumes are indicated on Table 49A. Natural vegetative materials (mulch) incorporated into the topsoil, and seeding with the topsoil stockpile stabilization seed mixture identified on page 2.05- 12] will stabilize the stockpiled topsoil. Following topsoil removal from ditch and pond azeas, required drainage and sediment control structures • were constructed or installed. Surface drainage for the shaft pad disturbance is controlled by two downgradient ditches on the north side of the pad, compacted road-base surfacing of the shaft pad, revegetation of cut and fill slopes and the topsoil stockpile, and a small, non-dischazging sediment pond that will completely contain three years of sediment plus the runoff from the 100-yeaz, 24-hour precipitation event. During shaft excavation, excess groundwater accumulations in the shaft aze also pumped to the main collection ditch and pond. The sediment pond was constructed by excavating the pond basin to depth. The interior pond embankment slopes (incised) are constructed at 2.SH:1 V or flatter. Design calculations have been completed to determine the elevation corresponding to the 25-yeaz, 24- how storm runoff volume. A marker has been placed in the pond at this elevation and any accumulations of runoff or pumped groundwater above this level aze pumped from the pond within 24-hows to maintain adequate stormwater storage capacity. TCC utilizes a portable floating pump to transfer excess water accumulations from the pond to a water tanker or apumper-truck, which hauls the water to the existing Pond D for final treatment and discharge; to discharge the water from the pond to one of the utility boreholes, with routing of flows to the abandoned underground mine workings; or to dischazge through a riprapped ditch segment to the adjacent ephemera] drainage channel under temporary CDPHE-WQCD Construction Dewatering Discharge Permit Permit No. COG-0072272. The limited road drainage is controlled by two road drainage ditches, compacted road-base swfacing of the access road, revegetation of cut and fill slopes and the topsoil stockpile, and two road crossing culverts. Active use areas are graveled to control dust and drainage, other disturbed azeas (including topsoil stockpiles) are stabilized with temporary vegetation, and any structures are painted in neutral earth-tone colors to blend with the natural surroundings. The access road follows the natural topography, generally consisting of rolling terrain, and will be grave]- • swfaced, so access road drainage control requirements are minima]. Structwes along the access road include two road drainage ditches, compacted road-base surfacing of the access road, revegetation of cut and fill slopes and the topsoil stockpile, and two road crossing culverts. MR06-218 2.05-45.6b 07/26/07