Agenda - Council - 06/25/2013 - Laserfiche WebLink

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Enterprise Park to Crooked Lake 115 kV Transmission Line March 2012 Environmental Assessment route selected, including existing subsurface features and available ROW. Manholes placed along the route would be used to pull conductors through the duct system. Open cut trenching is the most commonly used construction technique to install underground duct systems. Depending on the natural features in the right-of-way, it may be necessary to shore up the trench for worker safety, dewater the trench due to the presence of shallow groundwater, and backfill the trench with selective fill material to improve heat transfer. Installation generally includes direct burial in backfilled trenches and concrete trenches with covers or concrete ductbanks. Constructing the trench for the underground transmission line would likely result in greater temporary construction impacts than the proposed overhead line. In general, construction of underground transmission lines takes longer than construction of overhead transmission lines. A typical progression rate for underground construction would be 200-feet of transmission line per day. Background research and engineering and design considerations are required before construction of underground transmission lines. Prior to construction of underground transmission lines, the Applicant would conduct soil sampling and testing to determine the thermal conductivity of the earth and ability to trench and bore in the ROW. An underground transmission line is expected to cost five times more per mile compared to construction of an overhead transmission line, due to time, materials, process, and the use of specialized labor. An underground transmission line must also be routed to avoid other underground installations such as water, gas, and sewer lines. Unstable slopes, hazardous material sites, wetlands, and bedrock must be avoided. Going under a road, highway, or river requires construction techniques, such as directional boring, that are more expensive than overhead installations. Although failure of underground transmission lines is rare, a disadvantage of building underground transmission lines is the difficulty of finding and repairing failures. Overhead failures can usually be found through visual inspection. The time and cost to repair an underground transmission line would be greater than those anticipated for an overhead transmission line. While overhead transmission lines fail, on average, once every 17.8 years, underground transmission lines fail once every 50.5 years. In addition, the average time to resolve a failure on an overhead transmission line is nine hours. The average time to resolve a failure on an underground transmission line is three weeks. No routine maintenance or operation costs are anticipated for underground transmission lines. Visual inspections of underground transmission lines are not possible and will not be conducted. Unlike overhead transmission lines that are susceptible to a number of sources of outages (e.g., weather, birds, vehicle impacts), underground transmission lines are susceptible to only two outage causes: cable fault due to overloading of the system and failure of the cable or splices. If a fault is sensed on the transmission system, the underground transmission line will need to be accessed. 21