2009-12-30_REVISION - C1982056 - Laserfiche WebLink

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• Erection of escape hoist /elevator buildings and equipment and installation of fence(s) i Installation and connection of escape hoist/elevator control equipment and systems • , Final clean -up and dressing of roads and site pads J Site-specific investigations required for design and permitting of the new ventilation shaft included a cultural resource survey of potential disturbance areas, 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 clearly marked before initiating surface disturbance activities. The shaft pad surface area 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 disturbance area of approximately 2.4 acres, approximately 1,940 cubic yards of topsoil were salvaged from the road corridor, and 8,954 cubic yards from the 11.1 acre pad area. 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 -121 will stabilize the stockpiled topsoil. Following topsoil removal from ditch and pond areas, 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 the 6MN Sedimentation Pond. A typical road drainage ditch design, prepared by Water & Earth Technologies, Inc. (August 2004), is provided as Figure 2 of Exhibit 8X. Two culverts are required to intercept and transfer flow from the runoff ditches to limit runoff velocities in the ditches to less than or equal to 3.75 feet per second (fps). This systems approach assures that the ditches remain stable, with no channel scour or degradation during peak flow events. The pond was originally designed to totally contain the runoff from the 100 -year, 24 -hour precipitation event, but was modified (along with CPDS Permit CO- 0027154) to provide for discharge through a single open - channel spillway. The pond, as modified, is designed to contain three years of sediment, a small (0.46 af) permanent pool, and to retain and treat the runoff from the 10 -year, 24 -hour precipitation event. The sediment pond was constructed by excavating the pond basin to depth. The interior pond embankment slope (incised) was constructed at 2.511:1 V or flatter. The excavated open - channel spillway is designed to safely pass the peak discharge from a 25 -year, 24- hour storm event. A short discharge channel segment was constructed within the stream buffer zone to tie -in with an existing natural swale, and minor earthwork was completed to assure that the swale would provide adequate freeboard for design discharge flows. Both the connecting segment and any minor disturbance within the existing natural swale were regraded and revegetated on completion of construction, with the assumption that reclamation of the swale would not be necessary or appropriate at the time of mine closure. During shaft excavation, excess groundwater accumulations in the shaft were pumped to the main collection ditch and pond. Since shaft excavation occurred over a relatively short timeframe (9- months), the design sediment storage capacity was utilized for temporary mine water storage. TCC utilized a portable floating pump, on a temporary basis, to transfer and discharge excess pumped water accumulations from the pond through a riprapped ditch segment to the adjacent ephemeral drainage channel under temporary CDPHE -WQCD Construction Dewatering Discharge Permit Permit No. COG - 0072272. During active operations, the pond will have sufficient capacity to contain 3 -years of sediment and the designed permanent pool volume, and to contain and treat the TR06 -57 2.05 — 45.6b 07/28/09