The last road decks for the Highway 520 floating bridge will be lifted into place Monday, a milestone toward finishing that segment by spring.

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A red crane will lift the final two precast pieces of road deck onto the floating part of the new Highway 520 bridge Monday, a significant achievement in the $4.6 billion project.

Once the 12-foot gap is filled, trucks can drive the length of the new bridge, to pour concrete for the western high-rise, said Dave Becher, Highway 520 construction director for the Washington State Department of Transportation. Workers in the floating section will continue to tighten the deck parts, build expansion joints and install the traffic-information systems.

The project progresses despite a year’s delay caused by cracks in the first four large pontoons, cost increases near $400 million and a worker’s fatal fall at the east high rise in March.

State lawmakers this year allotted $1.3 billion from gas-tax increases to cover a funding gap, and eventually build the Montlake landings and Portage Bay crossing in Seattle.

Currently, 400 to 500 people are working at the pontoons and the six-lane deck, as well as a 14-foot-wide trail to walk and bike across Lake Washington.

When the floating bridge and high-rise are finished in spring 2016, traffic there will connect to the old four-lane bridge near Foster Island, until future segments are done.