Since October 2014, we have been engaged in an EPC project that will repair and replace an existing overhead 115-kV transmission line for Eglin Air Force Base in Florida. The project is approximately 3.4 miles long and is comprised of both overhead and underground transmission line designs. The objective is to replace the current overhead transmission line with improved overhead transmission line which includes an underground transmission line section in the “clear zone” of the Base’s runway. Some of the challenges faced include limited right of way, wetlands, and outage constraints.
The underground portion of the project is approximately 3400 feet long and will consist of two parallel circuits that will be placed in a concrete ductbank except for the portion under the wetlands which will directionally bored. We have worked with Power Secure (the constructor) and permitting agencies to develop a construction plan to protect the wetlands. This includes having completed a “biological study” to identify and protect wildlife in the area. The final design was completed with continuous runs of cable to avoid splice vaults for increased reliability. The underground to overhead transition was designed to have a structure with a two-way switch at each end for maintenance and to allow load to be transferred from one circuit to the other.
Much of the overhead line was designed to allow construction to be performed while the existing line remains energized which will minimize outages. This required continuous communications with the construction team to incorporate construction methodology and provide adequate electrical clearance for live line construction. The overhead portion of the transmission line was designed with spun-concrete poles. Due to sandy soil conditions, we designed the foundations with steel caissons. The steel caissons will be vibrated into the ground in a matter of minutes rather than conventional foundations that may take hours, which will shorten construction installation time. Concrete poles will then be socketed into the steel caissons.
The final design also includes replacing a motorized gang operated switch in one of the substations along with a new supporting structure. The design of this project will be issued for construction as soon as all the permits have been obtained and construction can be released. Construction is scheduled to be complete by summer 2016.