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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 />Is 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.
<br />A typical road drainage ditch design is provided in Figure 2 of Exhibit 8X, prepared by Water & Earth
<br />Technologies, Inc, August 2004. Two culverts are required to intercept and transfer flow from the runoff ditches to
<br />limit runoff velocities in the ditches to less than or equal to 3.75 feet per second (fps). This systems approach
<br />assures that the ditches remain stable with no channel scour or degradation during peak flow events. All drainage
<br />calculations and documentation for the shaft pad, sedimentation pond, and access road are provided in Exhibit 8X,
<br />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 />to 95 percent of maximum standard Proctor Density. The collection pit is approximately 14 feet deep, is sized to
<br />contain all material excavated from the shaft, and is lined with a PVC membrane liner to minimize infiltration of
<br />any moisture contained in the excavated material. In the event unanticipated groundwater flows are encountered
<br />during shaft sinking operations, portable pumps will be used to continuously dewater the shaft, with discharge to
<br />one of the utility boreholes, which would route the water back into the abandoned underground mine workings, or
<br />to the adjacent ephemeral drainage channel. Facility construction details and configuration are illustrated by Maps
<br />EX16E -M1 and M2; Map EX49A -M2, Shaft Pad Profiles; and Map EX49A -M3, Waste Disposal Pit Profiles, in
<br />Exhibits 16E and 49A. The cuttings pit and associated foundation area were inspected during topsoil stripping;
<br />placement of the liner; and temporary closure, on completion of shaft sinking operations. As -built drawings will be
<br />provided to the CDMG prior to cuttings placement and following temporary closure.
<br />Primary Method of Construction — Conventional Blind Sink Excavation - The 6 -North Mains Ventilation Shaft was
<br />developed by conventional blind sinking excavation. Pre - grouting was not required for conventional blind sinking
<br />operations. A probe -hole was typically carried down the hole and ahead of the shaft as shaft excavation progresses.
<br />Water was grouted off from within the excavated shaft as it was encountered. A shaft collar was excavated to an
<br />approximate 20- to 22 -foot diameter to competent rock at a depth of approximately 20 -25 feet. Material from the
<br />• collar excavation was placed within waste disposal pit. A surface casing approximately 12 -18 feet in diameter
<br />(inside) and large enough to accept the shaft casing and conventional drilling and mucking tools was installed in the
<br />shaft collar. This surface casing is cast -in -place concrete. A nominal 25 by 25 -foot by 4- to 6 -foot thick concrete
<br />pad was poured to accommodate the winches, drilling, mucking and casing equipment, and headframe. Specialized
<br />drilling and mucking equipment in the form of a hoist, headframe, backleg foundation, and muck dumping system
<br />TR06 -57 2.05 -45.6c 07/28/09
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