The cruise ship terminal in Juneau, Alaska, is essential for the city’s economy. However, the recent evolution of Panamax and post-Panamax cruise ships, which can be more than 1000 ft long, was overwhelming the port. The capacity of the old pile-supported timber dock was limited to simultaneous berthing of one 800-ft-long and one 1000-ft-long cruise ship, preventing larger ships from docking simultaneously.
As a solution, the harbor owners wanted a replacement dock with floating berths that created enough space to accommodate simultaneous berthing of one 1000-ft-long and one 1100-ft-long cruise ship, explains Yeliz Firat, senior engineer at BergerABAM Engineers Inc. in Federal Way, Wash. “The floating nature of the pontoons provided the added value of being able to load and unload passengers even during significant tidal fluctuations, without the need for complex operations.”
The owners needed a low-maintenance solution that would remain durable for 50 years, which first led them to precast concrete. Unlike steel berths, which are highly dependent on periodic dry-dock maintenance, precast concrete requires minimal effort, Firat notes. “The maintenance of concrete pontoons is minimized by using appropriate concrete cover, detailing, materials, and corrosion-prevention measures during design and construction.”
The precast concrete pontoons could also meet strict criteria for the design freeboard, which is the vertical distance at the ship’s side between the waterline and the deck. Freeboard requirements were set at 8 ft, with a tolerance of just plus or minus 1 in. “The freeboard is governed by the weight and the height of the pontoons, so the geometry and thickness of the hull plating had to be monitored and controlled diligently,” Firat says. “Precasting the walls and the deck panels in a controlled environment was instrumental in controlling the weight.”
Precast concrete also lent itself to constraints of the jobsite. The project had to be completed between October 2015 and May 2017, with no disruptions to terminal operations from May through September. The two all–precast concrete pontoons—measuring 300 and 400 ft in length—were fabricated simultaneously in the dry-dock of Concrete Technology Corporation’s precast plant in Tacoma, Wash., then towed 1000 nautical miles to Juneau to meet the designated timeline for on-site installation. “Simultaneous fabrication of both pontoons in the dry-dock not only enabled meeting the critical towing schedule and favorable towing conditions, it also made the construction of two pontoons of significant size cost-effective by optimizing mobilization,” says Firat. Both pontoons were completed and launched from the dry dock in February 2016.
In May of 2016, the Juneau Cruise Ship Terminal completed installation of the all-precast, prestressed concrete floating pontoons that now serve as loading berths for the one-million-plus tourists who pass through the Alaskan harbor every year. Both the designers and owners are pleased with the results of this novel use of precast concrete. “We love the concrete pontoons because they are aesthetically pleasing and not obtrusive along our waterfront,” says Gary H. Gillette, port engineer for the city and borough of Juneau. Along with providing the harbor with two sturdy, durable, and non-slip structures that are stable in all sea conditions, the wide, open docks have become a gathering spot for Juneau residents and home to events throughout the year.