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ENVIRONMENTAL ASSESSMENT <br /> Topsoil would first be stripped over the new low-cover crossing area and stored in a topsoil stockpile for <br /> reuse in reclamation (Diagram 2-4). Sequentially, a trench for each entry about 40 ft deep and about 40 <br /> ft wide would then be excavated across the valley floor and would also expose competent bedrock on <br /> both ends. Excavated material from the first trench would be stored in a pile separate from the topsoil <br /> and would be used in backfilling the last trench. An estimated 65,000 cubic yards (CY) of dirt would be <br /> excavated in total to construct the tunnels.The excavation would be phased to keep excavated stockpiles <br /> at a reasonable size. An estimated area of 43,560 square feet is available to stockpile material (Diagram <br /> 2-4), which assuming a 10-ft average depth represents a temporary storage for 16,000 CY of material. <br /> Therefore,the Project would require at least 4 phases of excavation. <br /> P OPOSEO IN INES LI Cr EXCAVATION -- <br /> I- <br /> i <br /> TEMPORARY SOIL <br /> LTI P EAR <br /> STOCKPILE-1 ACRE <br /> SP 9 RISE I <br /> OPO O 0 CLI S <br /> r - L_— <br /> STAGING AREA _ <br /> J 170 200 <br /> L <br /> Diagram 2-4. Map View of the Staging and Stockpile Areas <br /> At each phase, the area would be excavated to subgrade, arched culverts installed, and the culverts <br /> backfilled before beginning the next phase.Once each trench is excavated,concrete would be transported <br /> to the site and foundation footers for the multi-plate arches would be installed. After foundation <br /> construction,a multi-plate corrugated steel arch, 18 ft wide and 9 ft tall,would be constructed.The arches <br /> would be extended into the bedrock at each end approximately two to three ft and then sealed using <br /> shotcrete. Once each arch is constructed, the trench would be backfilled using material excavated from <br /> the next sequential trench,the arches would be covered with excavated material, and compacted. <br /> When excavating in the channel bottom, the contractor would install a reinforced HDPE rigid pipe sized <br /> for the 5-year storm water runoff event with a compacted berm constructed on the upstream side to <br /> funnel the channel towards the pipe inlet above the excavation.The pipe would be moved to one side of <br /> the channel during initial excavation, and then relocated to other side once this work is complete and <br /> Dunn Ranch Area Coal Lease by Application COC-78825 and Mine Plan Modification EA 2-5 <br />