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Vantage to Pomona Heights Chapter 2 230 kV Transmission Line Project FEIS Proposed Action and Alternatives PAGE 2-41 lubricating the passage of the conduit pipe. Bentonite is non-toxic and commonly used in farming practices. The drilling fluid is sent into a machine called a “reclaimer” which removes the drill cuttings and maintains the proper viscosity of the fluid. Drilling fluids are designed to match the soil and cutter. They are monitored throughout the process to ensure that the bore hole stays open, pumps remain operational, and drilling fluid circulation throughout the borehole is maintained. Drilling muds are “thixotropic” and thus thicken when left undisturbed after bore removal. However, unless cementitious materials are added, the thickened mud is approximately as stiff as very soft clay. There is a potential for drilling fluid release or “frac-out” during installation, which can occur when pressure in the drill hole is not maintained and a loss of circulation of drilling fluid occurs. Frac-out is typically caused by pressurization of the drill hole beyond the containment capability of the overburden soil material, which allows the drilling fluid to flow to the ground surface. Releases can be caused by fractures in bedrock or other voids in the geologic strata that allow the fluid to surface even if down hole pressures are low. Providing adequate depth of cover for the installation can substantially reduce the potential for inadvertent releases or frac-out. 2.2.5.2 Typical Underground Construction Activities Duct Bank Requirements for Off-Road Construction For off-road construction or construction not occurring within an existing roadway, installation of an underground cable would require a dedicated area for construction consisting of a permanent access road for future maintenance and repair activities and an additional temporary access road during the initial construction for equipment and temporary storage of materials. The entire length of the ROW would be cleared of all vegetation to accommodate installation of the underground cable. The total construction surface impact area for underground cable construction and installation would be approximately 55 to 60 feet or greater in width along the entire underground route to accommodate trenching machines or excavators, truck mounted rock drills, dump trucks to haul out excavated material unsuitable for backfill and to haul in backfill material. A permanent access road of approximately 14 feet in width would be required to perform operation and maintenance (see Section 2.2.3.2). A permanent cleared surface area would be required for the width of the cable structure duct bank: 5 to 10 feet wide plus the permanent 14 foot access road for a total permanent cleared area of approximately 24 feet wide (Figure 2-10). Disturbance assumptions along the proposed centerlines for access roads are considered identical for the Overhead Design Option and Underground Design Option detailed in this FEIS. Splice Vault Requirements The outside dimensions of splice vaults for 230 kV underground cables are approximately 9 feet wide by 10 feet tall and up to 28 feet long. The installation of each splice vault therefore typically requires an excavation area approximately 10 feet wide, 11 feet deep, and 30 feet long. The actual burial depth of each vault would vary, based on the cable manufacturer’s splice and cable racking requirements, site- specific topographic conditions, and on the depth of the adjacent cable sections that must interconnect within the vault. Splice vaults would require a permanent cleared area for future access for maintenance and transmission cable repairs, and an additional temporary cleared area for construction activities. Within the easements for the splice vaults, certain uses such as the development of structures and growth of shrubs and trees would be prohibited to avoid duct bank damage and impacts to the operation of the cables.