State Route 520
pontoon casting basin

Location: Aberdeen

Owner/developer: Washington State Department of Transportation

Project team: Kiewit-General Joint Venture, general contractor; HNTB Corp., structural engineer; Farrow Construction Specialties, concrete contractor; Bayview Redi-Mix, ready-mix supplier

The 55-acre pontoon construction site for the state Route 520 bridge replacement project has a concrete batch plant, on-site water treatment and a 4-acre casting basin.

Crews are using the facility to construct 33 pontoons, 21 of which — at 360 feet long, 75 feet wide and nearly 30 feet tall — are the largest ever built in Washington state. Building the casting basin involved excavating 280,000 cubic yards of soil, driving over 900 piles, installing 2,000 tons of rebar and pouring 19,000 cubic yards of concrete. A crane rail system, launch channel and basin gate were also installed.

Up to six pontoons at a time can be built in the basin. The 18-inch-thick basin floor measures 200 by 900 feet and is supported by 643 steel piles, and required 11,000 cubic yards of concrete.

Because the slab is located 30 feet below ground level and surrounded by steep slopes, placing the concrete was a challenge. Crews used concrete pumps, placing booms and crane-supported trapezes to get the concrete down to position.

Typical slab pours were 20,000 square feet and required approximately 1,100 cubic yards of concrete. After pontoons are completed, crews flood the casting basin so the pontoons can be towed out to Grays Harbor for off-site moorage.

The slab-on-grade basin foundation was designed to reduce the risk of ground settlement during pontoon construction, ensuring a higher-quality pontoon with less risk of cracking.

The cast-in-place basin walls are 4 feet tall and rise to the base of the wall slopes. To prevent uplift on the casting slab, the basin has a permanent dewatering system embedded below the slab and around the basin walls that collects and pumps process water to treatment facilities before being discharged off site.

The casting basin was completed last December. Pontoon construction is under way, and is scheduled to finish in 2014.