One of Maryland’s largest electric utility providers, Baltimore Gas and Electric (BGE) owns and operates a high-voltage transmission grid. Most of BGE’s 230-kV lines circling Baltimore are above ground; however, at the Key Bridge, the utility has underwater lines, which were put into the riverbed in the 1970s. The 2.5-mile-long portion of the line that crosses the Patapsco River through the main shipping channel is located approximately 10 to 15 ft below the riverbed. Having been in service for over 50 years, this portion displayed signs of deterioration. Given that this section was critical for the resiliency of the grid system, BGE planned to replace it with a new transmission line crossing.
Various alternatives were analyzed, with due consideration given to cost, design complexity, environmental impact, stakeholder preferences, permitting complications, and interruption of shipping. In 2015, the project team selected overhead lines incorporating tall towers in the river as the preferred solution, and final design was completed by the end of 2019.
The crossing includes a total of eight towers, with heights that vary between 160 and 400 ft. The tallest towers are in the water adjacent to the shipping channel to provide a minimum of 230 ft of clearance for ship traffic. The towers in the water required independent vessel collision protection structures to prevent ships from striking the towers or their foundations. A detailed, probabilistic vessel-collision risk analysis was performed per requirements set forth in the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications. The protection structures have a continuous concrete ring around each foundation, the largest being 14 ft wide, 7 ft deep, and 633 ft long in perimeter. Both the foundation and protection structures are composed of layers of precast and cast-in-place (CIP) concrete, supported by steel pipe piles.
Precast concrete was incorporated into the earliest design concepts and was a dominant technology in all over-water construction, reducing the construction time of the in-water structures, improving the design life of reinforcement, reducing the amount of CIP concrete formwork, and improving the accuracy of perimeter fender bolt placement.
For the work over water, the use of CIP concrete would have been complicated and time-consuming. All concrete elements directly above open water were designed and detailed as precast concrete. Only narrow CIP closure pours between precast concrete planks were required over water. Most precast concrete elements were rectangular, but trapezoidal and bent-angle shapes were also used for unique structural boundaries. “Considering that all precast concrete pile caps and panels over water were 2-ft thick, the avoidance of horizontal formwork avoided significant effort,” says Mehedi Rashid, structural engineer, Moffatt & Nichol. “With so much work to be performed in an accelerated construction schedule, precast concrete was the most effective method to achieve a successful on-time completion of the project,” he adds.
Of the eight towers required to cross the Patapsco River, tower 1 is located at BGE’s Hawkins Point substation, towers 2 through 6 are located in the water, and towers 7 and 8 are located at Sollers Point. The largest span (2200 ft in length) crosses the Fort McHenry channel, which is the primary shipping channel entering the Baltimore Harbor.
The precast concrete configuration consists of precast concrete caps installed on top of the piles, with precast concrete planks spanning between the caps to form a continuous precast concrete working surface over the water. A total of 67 pile caps and 62 panels were used for the project. The precast concrete pile caps range in weight from 8 to 47 tons.
The contractor’s substitution of a single, monolithic precast concrete foundation piece at three of the towers produced a massive square precast concrete pile cap weighing 164 tons. The project used marine concrete to provide a 75-year design service life and conducted strict quality control measures along with field and production testing to achieve the design objectives.
Stakeholders for this significant infrastructure project were cognizant of environmental sensitivity issues, budget concerns, and the project’s economic impact on the community. Compared with the underwater option, which would have required jet-plowing submarine cable through the river bottom, the overhead option was more environmentally sound. Also, building underground cables would have cost approximately twice as much as the overhead project. Furthermore, installing overhead lines had less impact on operations in the Port of Baltimore than the underground option would have had.