The double-deck interior of the Highway 99 tunnel is taking shape quickly and could be finished by March, on track for traffic in early 2019.

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The Highway 99 tunnel construction project moved into a new direction this week.

Northbound.

The upper, southbound deck is almost finished, which clears enough room beneath for assembly to begin for the lower deck that will carry traffic north from Sodo to South Lake Union.

A total 1,152 concrete panels, each as heavy as a transit bus, will be trucked in from Tacoma and fastened atop internal ledges, 7 feet above the bottom of the tube that former tunnel boring machine Bertha left behind.

All the decks and walls of the 2-mile project are on track to be completed by March, said Joe Hedges, program administrator for the Washington State Department of Transportation.

“What a great day today, isn’t it?” he told reporters after a group tour. “Two of the elements are complete, that’s the tunneling and the extraction, and the third element, the interior structures, is well on its way.”

Media toured the Bertha tunnel on Tuesday as construction continues. (Steve Ringman / The Seattle Times)

Bertha broke through to daylight near Seattle Center on April 4, more than two years behind schedule. Its final parts were extracted Aug. 23.

Complete installation and tests of lights, signals, ventilation, fire sprinklers and other utilities will take months more. Finally, outside ramps must be connected to the north and south tunnel portals — for a few weeks, the old Alaskan Way Viaduct will be closed before the new tunnel is ready, in early 2019.

The $2.1 billion tunnel reached another milestone this week as the first road-legal vehicles, including a concrete mixer truck, drove the entire way. Riding on a contractor’s tram Thursday, it was impossible to escape the novelty of a traffic-free trip through downtown.

Sprinklers, overhead lights and spray-on fireproof minerals have all been applied to the northbound ceiling from the stadiums to mid-downtown. Workers on scaffolds are installing the last of the upper decks in and adjacent to the north portal.

In Sodo, groundwater still flows into Bertha’s launch pit, in which roadway entrances are now built.

The area will dry out when walls or possible grouting are completed, said Chris Dixon, project manager for contractor Seattle Tunnel Partners (STP).

About 300 construction workers are still building the tunnel.

Lawsuits continue over how STP, insurers, tunnel machine builder Hitachi Zosen, and the state might divide a half-billion dollars in claims for cost overruns.

With the most suspenseful chapter finished, Dixon has moved back to Los Angeles, where he’s acquired a tan and spends most workdays leading Tutor-Perini Corp’s Purple Line subway construction, through Westwood and Beverly Hills.

WSDOT will soon choose another contractor to demolish the viaduct in 2019.

The old Battery Street Tunnel will be filled and decommissioned, Hedges said. Demolition teams have the option to fill it with viaduct rubble. A citizen coalition called Recharge the Battery is advocating reuses, such as bikeways and public art.