Float-in cofferdams have been used on several major bridge projects in the U. S. since the concept was first introduced in 1998 on the Bath–Woolwich Bridge in Maine, and then on the new Carquinez Bridge at San Francisco Bay in 2001. For this construction method, all foundation piles are pre-installed and cut- off, and concurrently, a concrete shell is fabricated offsite that acts as both a form and cofferdam for casting of the underwater pile cap. The float-in cofferdam installed in 2009 for the new Port Mann Bridge, southeast of Vancouver, British Columbia, Canada, is distinctive for several reasons: • Size of Installation – It is the largest float-in cofferdam ever installed, 109 ft (33.2 m) by 138 ft (42 m) by 32 ft (9.8 m) high and weighing in excess of 6,000 tons (5,440 tonnes). • Number of Piles –The cofferdam was installed over the top of 63 piles, each 6 ft (1.8 m) diameter pipe pile cut-off 10 ft (3 m) underwater. • Pre-Installed Pre-Cast Support Collars for Initial Landing of Cofferdam – Prior to underwater cut-off, each pile was outfitted with a precast collar hung from the top of the pile driving template and grouted to the outside of the pile. • Load Distribution System During Cofferdam Landing on 63 Piles – To ensure that any single collar was not overloaded by the 6,000 ton (5,440 tonne) cofferdam during landing, a 3 in (7.6 cm) pipe ring with an outside diameter of 9.43 ft (2.9 m) was pre- positioned underwater on top of each precast support collar. AUTHORS: 40 • DEEP FOUNDATIONS • SEPT/OCT 2012 Float-In Cofferdam Over 63 Large Pipe Diameter Piles Design-Build Joint Venture A Peter Kiewit Sons Co. and Flatiron Corporation Joint Venture (JV) won the design-build contract for the new Port Mann Bridge in 2008. The bridge crosses the Fraser River at Coquitlam, British Columbia, approximately 10 mi (16 km) southeast of Vancouver. The main in-water tower of the cable-stayed bridge is mid channel, 45 ft (13.7 m), with tidal variation of about 14 ft (4.3 m). The final foundation design for N-1 Pier consisted of 63 piles, the tops of the piles were embedded in a 24.6 ft (7.5 m) deep, by 138.13 ft (33.2) by 108.6 ft (42 m) pile cap positioned just below the High Water Elevation (HWE) of +9.25 ft (+2.8 m). The original pre-bid cofferdam design was a conventional cofferdam using AZ-36 sheet piles up to 107 ft (32.6 m) long. The gap between the bottom of the pile cap and the sloping river bottom was to be filled with clean, granular fill and topped with 6.56 ft (2 m) of gravel. After the contract award, the JV evaluated the float-in cofferdam concept and concluded that it offered significant cost savings over the pre-bid conventional cofferdam. Float-in cofferdams are typically used for underwater Robert Bittner, P.E., Bittner-Shen Consulting Engineers, Portland, OR Bruce Patterson, P.E., and Dave Russ Kiewit Bridge & Marine Vancouver, WA foundations that use large-diameter driven piles or large-diameter drilled shafts. Such foundation elements are extremely stiff, which allows the pile caps to be located up off the bottom of the waterway. The tops of the pile caps for these elements are typically located at or just above the HWE for aesthetic reasons. Float-in cofferdams are characterized by other common features:
