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• Installation of a modular nitrogen injection system
<br />• Final clean -up and dressing of roads and site pads
<br />• These activities are briefly described in the following sections:
<br />Site - specific investigations required for design and permitting of the new ventilation shaft included a cultural
<br />resource survey of potential disturbance areas, soil and foundation characterization as the basis for shaft pad and
<br />access road design, and completion of a wetland/AVF survey for the shaft pad and access road. The cultural
<br />resource survey was conducted by Metcalf Archaeological Consultants, the engineering soils characterization by
<br />Northwest Colorado Consultants, and the soils /wetland/AVF survey by Habitat Management, as described in
<br />Section 2.04. The perimeter of all areas affected by surface facilities was clearly marked before initiating surface
<br />disturbance activities. The shaft pad surface area is approximately 11.1 acres in size with minimal cut and fills. The
<br />associated access road corridor is approximately 2,930 feet long, with a finished top width of 24 feet (road corridor
<br />35 feet wide, 2.4 acres). See Exhibit 25U, 6 -North Mains Ventilation Shaft - Geotechnical Pavement Design, for
<br />details on the road, and Map EX49A -M1, a new road profile and cross - section.
<br />Given an average topsoil depth of approximately 6 inches and a total road disturbance area of approximately 2.4
<br />acres, approximately 1,940 cubic yards of topsoil were salvaged from the road corridor, and 8,954 cubic yards from
<br />the 11.1 acre pad area. Prior to initiating topsoil removal activities, stream and wetland buffer zones were marked
<br />with identifying signs to limit surface disturbance within these areas to the planned and approved activities.
<br />Topsoil from the road corridor was recovered and placed in a stockpile at the north end of the road corridor beyond
<br />the edge of the 35 -foot road disturbance area. Topsoil from the pad area was placed in a stockpile within the shaft
<br />pad on the southwest edge of the pad and along the west edge of the access road, just south of the pad, for future
<br />reclamation use. Topsoil salvage volumes are indicated on Table 49A. Natural vegetative materials (mulch)
<br />incorporated into the topsoil, and seeding with the topsoil stockpile stabilization seed mixture identified on page
<br />2.05 -121 will stabilize the stockpiled topsoil.
<br />• Following topsoil removal from ditch and pond areas, required drainage and sediment control structures were
<br />constructed or installed. Surface drainage for the shaft pad disturbance is controlled by two downgradient ditches
<br />on the north side of the pad, compacted road -base surfacing of the shaft pad, revegetation of cut and fill slopes and
<br />the topsoil stockpile, and the 6MN Sedimentation Pond. The pond was originally designed to totally contain the
<br />runoff from the 100 -year, 24 -hour precipitation event, but was modified (along with CPDS Permit CO- 0027154) to
<br />provide for discharge through a single open - channel spillway. The pond, as modified, is designed to contain three
<br />years of sediment, a small (0.46 af) permanent pool, and to retain and treat the runoff from the 10 -year, 24 -hour
<br />precipitation event. The sediment pond was constructed by excavating the pond basin to depth. The interior pond
<br />embankment slope (incised) was constructed at 2.5H:1 V or flatter. The excavated open - channel spillway is
<br />designed to safely pass the peak discharge from a 25 -year, 24 -hour storm event. A short discharge channel segment
<br />was constructed within the stream buffer zone to tie -in with an existing natural swale, and minor earthwork was
<br />completed to assure that the swale would provide adequate freeboard for design discharge flows. Both the
<br />connecting segment and any minor disturbance within the existing natural swale were regraded and revegetated on
<br />completion of construction, with the assumption that reclamation of the swale would not be necessary or
<br />appropriate at the time of mine closure.
<br />During shaft excavation, excess groundwater accumulations in the shaft were pumped to the main collection ditch
<br />and pond. Since shaft excavation occurred over a relatively short timeframe (9- months), the design sediment
<br />storage capacity was utilized for temporary mine water storage. TCC utilized a portable floating pump, on a
<br />temporary basis, to transfer and discharge excess pumped water accumulations from the pond through a riprapped
<br />ditch segment to the adjacent ephemeral drainage channel under temporary CDPHE -WQCD Construction
<br />Dewatering Discharge Permit Permit No. COG - 0072272. During active operations, the pond will have sufficient
<br />capacity to contain 3 -years of sediment and the designed permanent pool volume, and to contain and treat the
<br />runoff from the 10 -year, 24 -hour storm event with 1.0 foot of freeboard between the 10 -year, 24 -hour water surface
<br />• and the invert of the open channel spillway. Even if the pond was full to the spillway invert at the start of the 10-
<br />year, 24 -hour storm event, the pond would adequately control sediment and retain, treat, and discharge the runoff
<br />from a 10 -year, 24 -hour event.
<br />TR09 -70 2.05 -45.8 12/31/09
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