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Vantage to Pomona Heights Chapter 2 <br />230 kV Transmission Line Project FEIS Proposed Action and Alternatives <br /> PAGE 2-35 <br />Information in this section is partially derived from the Electric Power Research Institute (EPRI) Report <br />on the Assessment of Current Underground and Overhead Transmission Line Construction and <br />Maintenance in the United States (EPRI 2008). EPRI is an independent non-profit organization that <br />brings scientists, engineers, academia, and industry together to conduct research, for development and <br />demonstration relating to the generation, delivery and use of electricity. EPRI applies stringent standards <br />of objectivity through their advisory structure and by recruiting independent researchers and technical <br />authorities from around the world. <br />High voltage underground transmission lines have markedly different technological requirements than <br />lower voltage underground distribution lines. Some types of underground high voltage transmission lines <br />require extensive cooling systems to dissipate the heat generated by the transmission of bulk electricity. <br />The extremely high cost of large cooling systems and other special design requirements has limited the <br />application of underground transmission systems for long distance electric transmission. In contrast, <br />overhead conductors are cooled by the open air surrounding them - placing the conductors on towers puts <br />these conduits of energy above most human activity on the ground and deals effectively with the issue of <br />heat build-up and dissipation. <br />Open Cut Trenching <br />The most commonly used method of installation for underground transmission lines is open cut trenching. <br />Utilizing primarily mechanized digging equipment, this method of installation creates a trench with given <br />dimensions per design criteria. Trenching activity in the work area is governed by OSHA standards; state <br />and local laws are often applicable, as well. To mitigate safety concerns for personnel and equipment, <br />sheeting and shoring are often required. A SWPPP would be implemented to reduce hazards caused by <br />excess water within the work area. If groundwater is encountered, dewatering would be performed (EPRI <br />2008). During trenching, topsoil would be stripped and stockpiled to prevent comingling of soil and <br />subsoil materials (rock, etc.) that may reduce seeding/revegetation success. Examples of open cut <br />trenching are shown in Figures 2-7 and 2-8. <br />When excavation is complete, the trench bottom would be graded per design for construction of the cable <br />system. Pipe or conduit, depending on the type of cable system, are placed within the trench using spacers <br />or others means of stabilization. This insures that the cable or conduit maintains the correct position and <br />dimension during backfilling. To allow for routine maintenance and cable installation, manholes may be <br />placed within the cable system. Low thermal resistive backfill maybe selected to allow for heat <br />dissipation from the trench, as excess heat can be detrimental to underground conduit systems. Backfills <br />are often created and tested at local batch plants. Topsoil would be placed above the excavated subsoil or <br />engineered backfill material.