Highway 99 tunnel machine Bertha has completed most of the dig to South Lake Union, making room to assemble the double-decker road behind the drill.

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Nearly one-third of the Highway 99 tunnel’s upper road deck has taken shape, now that Highway 99 tunnel-boring machine Bertha has managed to consistently churn ahead since spring.

The rotary machine is 170 feet below Blanchard Street in Belltown and climbing gradually toward the surface. It is expected to emerge at South Lake Union in June, followed by a few months to remove the parts from a vault.

Workers at Seattle Tunnel Partners (STP) assembled the giant tube’s 1,000th concrete ring Tuesday morning, passing the 70 percent mark in the 1.7-mile route, while crews in the rear pour concrete for the roadway.

The Washington State Department of Transportation (WSDOT) showed off a mile-long tunnel segment to news reporters, and posted videos of ring and deck construction.

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

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“It’s a good day!” said administrator Joe Hedges, gesturing toward tunnel segments in a storage yard, the Sodo operations building and a flexible, earthquake-proof Sodo exit ramp.

The upper, southbound roadway will eventually be flanked by walls on either side. The arc of the tunnel will remain visible overhead, with light fixtures and lane-control signs hanging from the tube — unlike Boston’s Big Dig, where four concrete panels of a tunnel’s “false ceiling” broke loose and killed a 38-year-old woman in 2006.

Drivers using the lower, northbound deck will be surrounded by boxy walls, and the upper deck provides a flat ceiling.

Chris Dixon, STP project manager, said Bertha has been traveling at least 50 feet on a normal day, and if there aren’t any interruptions, such as a clog in the grout lines, it can move about 75 feet. Grout is routinely sprayed as the machine advances, to fill an 8-inch gap between Bertha’s shell and the surrounding soil, and prevent ground settlement.

The lower deck, yet to be installed, will proceed quickly because it’s made of precast concrete panels, and because the ledges to support them are halfway done already. After the giant tunnel drill is removed, Dixon said, supplies will be carried in from the north, instead of from the southern Sodo end as they are now.

Except for a couple stops to exchange eroded cutting bits, Bertha has worked dependably since it passed beneath the old Alaskan Way Viaduct at Yesler Way on April 29, without causing damage.

The record-breaking 57- foot, 4-inch-diameter tunnel will bypass downtown and replace the viaduct in early 2019, three years late following a December 2013 machine breakdown, repairs and difficulty moving through wet waterfront soil. STP is seeking up to $480 million in compensation from the state or insurers, beyond its $1.44 billion state contract.

The tube will be tolled and it lacks a Western Avenue exit, so thousands of cars and trucks must exit to a slow surface boulevard to reach Belltown, Magnolia or Interbay.