Highway 99 tunnel-boring machine Bertha will stop Wednesday for replacement of eroding cutter bits, just beyond the midpoint of its 1.7-mile route. And FYI, drivers, the viaduct will close this weekend for inspection.

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If tunnel-boring machine Bertha continues its recent pace, it could break into daylight near the Space Needle by May of next year.

Seattle Tunnel Partners (STP) boss Chris Dixon offered that estimate Monday morning after the giant Highway 99 tunnel drill passed the halfway mark of its trip from Sodo to South Lake Union on Friday.

The top of the 57-foot-4-inch-diameter machine is now 190 feet below First Avenue downtown, passing Pike Place Market on its left.

Projected Highway 99 timeline

According to Seattle Tunnel Partners’ schedule, as of August:

May or June 2017

Bertha breaks out*

November 2018

Tunnel road decks completed

March 2019

Tunnel road connections completed and tunnel would be open to traffic.

* STP project director Chris Dixon said Monday if progress continues at the current rate, it could be ahead of schedule, in May.Source: Seattle Tunnel Partners

Meanwhile, the Washington State Department of Transportation (WSDOT) warned drivers about “double trouble” from two weekend highway closures:

• The Alaskan Way Viaduct will close from 6 a.m. to 6 p.m. Saturday and Sunday for routine inspections. The viaduct will be replaced by the new tunnel, possibly by early 2019, three years late.

• The Highway 520 bridge and some lanes of Montlake Boulevard are to close from 11 p.m. Friday until 5 a.m. Monday for paving, striping and toll-equipment installation. Eastbound travel will completely close from Interstate 5 to 92nd Avenue Northeast, while westbound lanes will be available only from Montlake to I-5.

WSDOT said Monday that Bertha has gone 4,662 feet of its 9,270-foot route.

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It will stop Wednesday for a few weeks so crews can inspect and replace many of its 750 or so steel cutting bits, which are constantly being worn down by loose, abrasive soils, Dixon said. These weigh 75 pounds and must be installed in front of the cylindrical drill by divers working at 2½ times atmospheric pressure.

That stoppage was expected, and another one is likely when the drill has 2,000 or so feet left to dig, Dixon said.

Except for the time it’s taken to respond to cutter erosion, the job has gone smoothly since April 29, when Bertha dived under the viaduct at Yesler Way, following two years of repairs and a waterfront sinkhole that caused the state to suspend drilling for a month.

Since then, buried sensing deviceshave shown only one- or two-tenths of an inch of soil settlement deep in the ground, which returned to normal after the drill passed, Dixon said. The soil near Bertha now is a stiff mix of clays, cobbles, gravel and silt with a chance of 2- to 8-foot-wide boulders, according to geotechnical studies. At the bottom of the machine, dense sand and gravel are expected.

As Bertha ascends, soil will become looser and more abrasive, Dixon said. Abrasive soil is a serious risk in the Seattle area, where glacial till can scour steel — badly enough to damage two machines in 2009 at the Brightwater sewer project north of Seattle. Before Bertha’s dig began, then-WSDOT Secretary Lynn Peterson said 9 tons of steel were likely to flake off the machine during the 1.7-mile dig.

Cutting parts needed replacing during June and September maintenance stops, and a few 600-pound disc scrapers were checked or replaced each weekend.

The four-lane, double-decked road tube was supposed to open in December 2015. It’s now expected to be three years late, mainly because of damage that required an unprecedented repair mission.

That job came after Bertha overheated to 140 degrees and stalled in December 2013, causing grit to penetrate into the main bearing assembly, where a rotating ring and shaft turn the cutter face. STP has estimated total costs and delays at $143 million to dig the 125-foot-deep access vault, raise the 4 million-pound front end, install stiffer bearing seals, perform repairs and make modifications. Machine maker Hitachi Zosen funded the new parts and engineering, while STP, led by Tutor-Perini of California and Dragados of Spain, paid for the vault.

Future motorists who use the tunnel will be charged a yet-undetermined toll.

The $2.1 billion tunnel is the most-complicated piece of a $3.14 billion program to replace the 63-year-old viaduct, which was damaged in a 2001 earthquake and might collapse if another one hits. Politicians and developers also are eager to open up the waterfront by adding promenades, parkways and lucrative bay views.

In other tunnel progress:

• Traffic signals were recently erected for a future Harrison Street junction with Aurora Avenue North, where city transportation leaders seek to create a future hub for RapidRide E Line bus riders, and a proposed Sound Transit 3 tunnel station. After the tunnel is done, with its portal near Mercer Street, the Battery Street tunnel will be sealed and Aurora turned into a surface street between Denny Way and Mercer.

• Construction has begun for the stadium-area exit at South Dearborn Street — which includes a bus-exit lane to serve some 15,000 daily passengers. What happens farther north is still unclear, as Seattle hasn’t committed yet to providing bus lanes in the crowded surface corridor from Dearborn to Columbia Street.

• The state has paid STP $1.2 billion, as of June, to cover finished work, out of the $1.4 billion construction contract. But a more comprehensive view of tunnel finances, in a yearly report to the Federal Highway Administration, isn’t available yet.

• This summer, WSDOT Secretary Roger Millarasked lawmakers for an extra $60 million over two years to cover the state’s own Highway 99 tunnel costs, such as engineering oversight, administration, outreach and rent for office space through 2019. Up to $223 million may be needed to close out the project.

• The state agreed to pay $50 million worth of change orders to STP, says a June 30 listing released to The Seattle Times under the public-records act. Their negotiations resolved about 100 issues, include delays related to a 2013 labor dispute over whether longshore union workers would load muck onto barges (they wound up getting a share of that work), and for a delay in digging the repair vault while archaeologists investigated a layer of oyster shells that turned out to have been discarded by settlers. Bigger disputes lie ahead, including STP’s nine-figure claim that a leftover steel pipe in the soil triggered the catastrophic damage to Bertha in 2013.