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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 />The access road follows the natural topography, generally consisting of rolling terrain, and will be gravel -surfaced,
<br />so access road drainage control requirements are minimal. Structures along the access road include two road
<br />drainage ditches, compacted road -base surfacing of the access road, revegetation of cut and fill slopes and the
<br />topsoil stockpile, and two road crossing culverts. A typical road drainage ditch design, prepared by Water & Earth
<br />Technologies, Inc. (August 2004), is provided as Figure 2 of Exhibit 8X. Two culverts are required to intercept and
<br />transfer flow from the runoff ditches to limit runoff velocities in the ditches to less than or equal to 3.75 feet per
<br />second (fps). This systems approach assures that the ditches remain stable, with no channel scour or degradation
<br />during peak flow events. All drainage calculations and documentation for the shaft pad, sedimentation pond, and
<br />access road are provided in Exhibit 8X, and drainage structures are shown on Map 24 (Sheet 3 of 3).
<br />Construction of the ventilation shaft access road involved topsoil recovery and stockpiling, installation of required
<br />drainage structures, scarification and re -compaction of surface materials, and placement and compaction of
<br />approximately 8 inches of pit -run gravel and 3 inches of suitable road -base material. Following topsoil removal,
<br />the exposed sub -grade was scarified to a depth of approximately 12 -inches and re -compacted to 95 percent of
<br />maximum standard Proctor Density (ASTM D698), with random density testing to verify compaction. Road
<br />construction required minimal cut and limited fill, so the actual road disturbance area averaged approximately35
<br />feet wide. Cut slopes were established at a maximum of 1.5H:1 V and fill slopes were graded to 2H:1 V or less, with
<br />all disturbed slopes stabilized by seeding with the topsoil stockpile stabilization seed mixture. The access road top
<br />surface is approximately 24 feet wide and is graded and crowned to promote effective drainage. For permitting
<br />purposes, a road disturbance corridor 100 feet wide has been defined as encompassing all project -related activities,
<br />including road construction and topsoil stockpiling.
<br />Preparation of the ventilation shaft pad involved topsoil recovery and stockpiling, installation of the required
<br />drainage structures and sediment pond, grading and compaction to establish a level pad working area, excavation of
<br />a temporary underground mine development waste disposal pit for surface collar material and conventional
<br />excavation of the shaft, haulage or stockpiling of the excavated borrow material, and placement of gravel surfacing
<br />material to control dust and sediment from the pad area. Excavated borrow material from the collection pit was
<br />used to construct the embankment of the waste disposal pit, which was constructed in 8 to 12 inch lifts, compacted
<br />MR11-259 2.05-45.9 11/17/11
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