Construction of 77 concrete pontoons for the replacement of the world's longest floating bridge has reached the halfway point. Redesigned cycle 3 pontoons are floating toward the Seattle jobsite.
The new $954-million, 7,710-ft-long State Route 520 bridge connecting Seattle to points east across Lake Washington is part of a larger $4.13-billion project.
Joint-venture prime contractor Kiewit-General-Mason is following a sequence of six pontoon cycles. Tugboats began towing the first two cycle 3 pontoons to the lake in mid-October, with others right behind. Cycle 3 addresses a post-tensioning design issue that led to cracking (ENR 6/4/12 p. 10).
The flaw cost $71.2 million in change orders, charged to the risk-reserve fund, and at least six months' delay, say Washington State Dept. of Transportation officials. Final costs are in negotiation, WSDOT says.
In all, there are 21 360-ft-long longitudinal pontoons, two cross pontoons and 54 supplemental stability pontoons cast in Aberdeen, Wash., and Tacoma, Wash.
In cycle 1, the contractor returned pontoons W and T to drydock to add post-tensioning to the end-bolt-beam zone, says Larry Kyle, WSDOT SR520 engineering manager. Kiewit-General will repair pontoons U and V— already attached to supplemental pontoons—in a coffer cell on the lake. That cell should be ready this year, and the fix will take "several months," Kyle says.
WSDOT expects the cycle 4 float-out to begin late this year or early next year.
"To date, the work we've seen to modify the pontoons and implementation has been a success," says Julie Meredith, SR 520 program director. "What we saw out of cycle 2 and 3 met our expectations."
Pontoons will be moored in the lake in two staging areas—one on the west end and one on the east end—until needed for construction. WSDOT says the bridge will open to traffic in late 2015 or early 2016.