2010-07-01_REVISION - C1982056 (2)

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continuously at full capacity, providing buffer storage for pump maintenance or failure, and limiting the distance over which the mine water must be transferred. As a separate, but related project, TCC will establish a new pipeline running approximately 6,200 feet from the 19- Right Dewatering borehole to the 6MN Storage Reservoir. The pipeline will be constructed of up to 12.5-inch HDPE pipe, buried to a depth of approximately five feet, with two trenched crossings of unnamed ephemeral drainages. The 19-Right Mine Dewatering Pipeline will discharge to the 6MN Reservoir near the eastern edge of the Reservoir. Pipeline construction will involve placement of sediment control materials (straw wattles or silt fence) downslope of the disturbance at any location where the pipeline corridor is within 50 feet of a drainage; removal and windrowing of available soil material (total soil material volume for the pipeline project is estimated at 5,750 cy) to one side of the 50 foot pipeline construction corridor; excavation of the pipeline trench using a track- hoe, with temporary placement of the excavated material on the opposite side of the trench; placement, connection, and bedding of the pipe; backfilling and compaction of the excavated material around the pipe; re-spreading of the soil material; and re-seeding using the standard rangeland seed mixture. Because the soil materials will be replaced immediately after pipeline placement, as construction advances, stabilization measures are not necessary. The 6MN Storage Reservoir is designed as a closed reservoir, with no discharge, and provision for adequate operating free-board to allow for storage of runoff from a small upslope drainage area. The 6MN Mine Water Storage Reservoir will be constructed south of the current 6MN Shaft pad and northwest of the NW Mains Ventilation installation.. The reservoir embankment will be an earthen dam constructed of suitable fill material excavated from the reservoir basin and placed and compacted consistent with engineering design specifications and construction recommendations. The engineering designs are based on site geotechnical investigations, which included drilling of six, 4-inch hollow-stem auger test holes; examination and testing of soil samples; placement of 1.5-inch slotted PVC pipe in two of the auger holes to measure water levels (piezometers); and excavation of seven shallow (6-11 feet in depth) test pits. The location of the 6MN Mine Water Storage Reservoir geotechnical investigations is shown on Map 24 (sheet 2 of 3), Surface Facilities and Freshwater Systems, and by the detail drawings in Exhibit 490, 6MN Mine Water Storage Reservoir - Design Drawings. Prior to reservoir construction, sediment control materials (straw wattles or silt fence) will be placed downgradient of the construction area, and an estimated 12,800 cy of topsoil (average depth 12-inches) will be recovered and placed in a stockpile approximately 90 feet wide and 600 feet long adjacent to the existing topsoil stockpile for the 6MN Ventilation Shaft Pad. The stockpiled topsoil will be stabilized by seeding with the standard temporary stabilization seed mixture within 6 months following completion of placement, and drainage from the pile will be controlled through placement and maintenance of erosion control material (straw wattles or silt fence) around the stockpile perimeter until effective vegetative cover is established. The reservoir embankment will be an earthen dam constructed of suitable fill material excavated from the reservoir basin (approximately 15,800 cy) and placed and compacted consistent with the engineering design specifications and construction recommendations provided in Exhibit 490, 6MN Mine Water Storage Reservoir - Design Drawings The resulting embankment structure will be approximately 14 feet high at its highest point, with the dam crest at an elevation of 6,717 feet. Actual constructed height will take into account anticipated embankment settlement, which is estimated at a maximum of 5 percent (constructed top elevation 6,717.7 feet). The surface area of the maximum pool (operational mine water storage + pump buffer storage + design stormwater storage)at an elevation of 6,714.4 feet will be approximately 5.1 acres, resulting in a maximum design storage volume of approximately 34.3 acre-feet of water. Even though the 6MN Storage Reservoir has been designed and will be operated as a closed reservoir, a single open-channel emergency spillway, with an invert of 6,714.9 feet, has been incorporated into the design in compliance with applicable regulatory requirements for impoundments. The design of the reservoir also incorporates a compacted soil liner (approximately 4,800 cy with an estimated permeability of 2 x 10.7 cm/in) to minimize infiltration and water loss. Due to the capacity and embankment height for the reservoir, reservoir design and construction must also meet the requirements of the Mine Safety and Health Administration (MSHA, 30CFR 77.216). Reservoir design and construction specifications meet all applicable design and regulatory requirements. Separate applications for approval were submitted to this agency, with a copy of final approval provided to the CDRMS. TRIO-73 2.05-45.13 06/30/10