Crews on Monday started excavating more than a quarter-million cubic yards of dirt for a casting basin in Aberdeen – a crucial step for building concrete pontoons to replace the aging and vulnerable State Route 520 floating bridge.

Crews will spend the next few months digging down 30 feet to allow construction of the 33 concrete pontoons that will eventually be towed to Lake Washington. The Washington State Department of Transportation (WSDOT) hired Kiewit-General to build the $367 million project, which includes building the basin and pontoons.

Crews began pile driving and site draining in February. They will use an excavator with an 8-yard bucket – large enough to fill a dump truck in one scoop. A smaller excavator and a grader will also be used to dig out and smooth the 4-acre basin and ensure a consistent depth. The completed casting basin will measure 900 feet long and 200 feet wide.

The entire casting facility is scheduled for completion by the end of the year; however, crews will begin producing interior concrete walls for the first of six pontoon cycles in areas near the casting basin as early as July. The first cycle is scheduled to be complete in 2012. The final pontoons are scheduled to be completed by mid-2014.