Structural Design Challenges to Upgrade from a Single-Circuit 115-kV Line to a Double-Circuit 230-kV Line in a Salt Marsh Area

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In late 1997, Gulf Power Company and Mesa Associates, Inc. entered into a contractual agreement for engineering services to rebuild and upgrade 40 miles of 115-kV transmission line to a 230-kV transmission line in the vicinity of Pensacola, Florida. Approximately 30.7 miles of the new line was designed for concrete and steel pole structures on reasonably good terrain with a sandy soil. The remaining 3.3 miles of transmission line was situated across the marshland of the Escambia River Swamp, presenting access problems, environmental issues and potentially insurmountable construction problems. This paper describes the structural design challenges encountered in the transmission line through the marsh area. The existing transmission line through the marshland area was constructed with laced steel towers, using barges to drive pilings and form pile caps upon which the towers were erected. In the 33 years since the towers were installed, the access cut-ins in the marsh had been filled in with silt and eventually vegetation growth. The current environmental regulations ruled out any water access. Marsh Master equipment allowed access for inspection and analysis of both structures and foundations, with a minimum of disturbance to the marshland environment. The Gulf Power Company planning department had indicated a need for future double-circuit 230-kV transmission capacity along the route occupied by the existing single-circuit 115-kV line. A decision was made to limit any future environmental impact by an attempt to design and construct the section of line through the marshlands to meet both present and future needs.