2010-06-04_PERMIT FILE - C1982056A (9)

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collection pit would have significant excess capacity. The combination of excess capacity and flow controls should be adequate to prevent any off -site discharge. The substation area will be constructed at the same time as the shaft pad, as previously discussed. The substation area • will be a 50 x 100 foot gravel- surfaced pad. Pole- or pad - mounted transformers and switch -gear will be installed on the pad, and a grounding field will be established under the near edge of the shaft pad during construction. If transformer pads are utilized, a maximum of three 20 x 20 foot pads would be constructed. The substation area will be enclosed by a chain -link security fence with locked gate, to limit access. A 69KV powerline will be extended up to 3,000 feet to the new substation from the existing Yampa Valley Electric Association line to the northeast. Installation of this line will require placement of approximately 12 -16 new powerpoles. Primary Method of Construction — Blind Drilling _Prior to the beginning of the blind drilling process, a pressurized pre -grout operation utilizing up to ten small diameter (6.25 ") holes, approximately 600 -800 feet deep around the perimeter each shaft will be considered and undertaken, as needed, to limit groundwater flows between the shaft and the formations. Individual grout holes will be drilled, and a cement grout pumped under pressure into each borehole before completing the next hole, to seal any wet zones encountered. Each borehole will be filled with grout and allowed to set -up, effectively sealing the borehole and any fractures or voids in the grouted formation(s). Any surface casing extending above ground surface will be cut -off flush with, or below, natural ground level. If needed, and once the pre -grout operation is completed, approximately 10 -foot diameter shaft collars will be excavated for each shaft to competent rock, at a depth of approximately 40 feet. Material from the collar excavation will be placed within and on either end of the cuttings pit. Surface casing approximately 8 feet in diameter (inside) and large enough to accept the shaft casing and drilling tools will be installed in each shaft collar. This surface casing will be either steel casing grouted in -place or cast -in -place concrete. Maximum 18 -inch diameter auger holes will be drilled into competent rock and reinforced concrete piers will be poured to anchor the shaft pads and any other significant structures. It is anticipated that up to 40 anchor piers may be required. Two concrete pads, either "T" shaped or rectangular, will be poured to accommodate the drilling and casing equipment; emergency escape hoist, and shaft heater, for the ventilation intake shaft; and fan, fan shroud, mechanical building, • and silencer, for the ventilation exhaust shaft. Average thickness for the reinforced concrete pads will be 2 feet, and the pads will be a maximum of 4,050 square feet. Specialized blind drilling equipment consisting of A -leg derricks, drive motors, and drill -string handling components will be installed on the shaft pads and used to drill the pilot holes and, complete shaft drilling. Nominal 17.5 -inch pilot holes will be drilled for each shaft to an approximate depth of 1,360 feet and surveyed to determine vertical shaft deviation and location underground for mine entry tie -ins. The pilot holes will then be reamed to a nominal diameter of 7.5 -8.0 feet to accommodate the final steel liner. The reaming technique utilizes an inverted disc cutter equipped hemispherical raise -bore head, to cut. the rock. During the drilling of the pilot holes and the 7.5 -8.0 -foot diameter excavation, all cuttings will be removed from the excavation using an airlift system, with water (reverse circulation) being used as the circulation media. Supplemental water, for drilling and to adjust the density of the drilling medium, may be supplied, as needed, by hauling water from a nearby location where TCC's holds valid existing water rights. The cuttings will be placed in 115 x 425 x 10 foot, four cell, lined temporary cuttings pit excavated within the shaft pad adjacent to the shaft excavation. The cuttings pit will be lined with a nominal 30 -mil PVC flexible membrane liner having a permeability of _10 - ' Z cm/sec. The cutting head will be rotated from the surface by the drill pipe, which also serves as a conduit to transfer cuttings from the shaft bottom to the surface collection pit. The drill pipe also serves to suspend the drilling assembly in the hole and is tensioned to adjust the cutting weight on the head and to keep the drill pipe straight. The cutting head moves the water from near the center of the head and discharges it at the periphery of the head. This water then flows down around the cutters and washes the cuttings towards the pick -up pipe in the center of the head. The drill pipe's upward flow is activated by compressed air injected down the center of the drill pipe. Rock cuttings are moved by the circulation of the water down the hole, across the shaft bottom and up the drill pipe to the surface collection pit. Within the collection pit, the rock cuttings are separated or settled out and the water is allowed to drain back or is • pumped back into the hole for re -use. The cuttings volume will be approximately 12,500 cy. Once the cutter head reaches the coal seam, the cutter head will be disassembled and removed from the A -leg derrick. Using the same A -leg derrick, the excavated shaft will then be lined to a nominal 6 -foot inside diameter TR09 -70 2.05-45.17 11/16/09