Tunnel machine Bertha is churning ahead, now passing the halfway point of its 1.7-mile dig.

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Bertha on Friday reached the halfway mark of its monumental dig toward South Lake Union, the Washington Department of Transportation (WSDOT) announced on Monday.

Bertha has been on the rise during its hidden voyage from Sodo. The giant drill on Monday was about 190 feet beneath First Avenue between Pike and Pine streets, just north of the future Highway 99 tunnel’s deepest point.

As of Monday morning, the dig had covered 4,662 feet on the future Highway 99 tunnel’s 9,270-foot route, according to WSDOT.

This summer, the project traded the challenges of groundwater and collapsing fill soil for the task of churning through deeper, more-abrasive dirt.

The current location in downtown Seattle presents a stiff mix of clays, cobbles, gravel and silt with a chance of 2- to 8-foot wide boulders, the geotechnical studies say. At the bottom of the 57-foot-diameter machine, dense sand and gravel are expected.

The machine has traveled 40 to 60 feet on good days since spring, though Seattle Tunnel Partners (STP) has taken maintenance stops at times to protect the steel machine from abrasion. During a stop in late June, STP replaced 33 of 750 scraping blades, a surprisingly low number.

[Drone footage reveals Bertha’s exit point as the boring machine reaches halfway point]

In early September, workers crawled into the hollow cutting-disc spokes, using chains and pulleys to replace 14 severely eroded 600-pound blades. Crews also have replaced cutter teeth during routine weekend stops that include road-building work, said WSDOT spokeswoman Laura Newborn said.

The four-lane, double-decked tube was supposed to open in December 2015 and is now expected to be three years late, mainly because of a mechanical breakdown and unprecedented mission to strengthen the 4-million-pound front end.

Tunnel tolls will be charged at a yet-undetermined amount.

The $2.1 billion tunnel is the toughest piece of a $3.14 billion program to replace the 63-year-old Alaskan Way Viaduct, which was damaged in a 2001 quake and might collapse if another one strikes. Politicians and developers have also been eager to open up the waterfront by adding promenades, parkways and lucrative bay views.

In other tunnel progress:

• Traffic poles were recently erected, along with signals, for a future Harrison Street junction with Aurora Avenue North. Seattle transportation leaders seek to create a future hub for RapidRide E Line bus riders, and a proposed Sound Transit 3 tunnel station. After the Highway 99 tunnel is done, with its portal near Mercer Street, the Battery Street tunnel will be sealed off and Aurora transformed 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 that serves 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, of the $1.4 billion construction contract. But a more comprehensive view of tunnel finances, in a yearly report due in July to the Federal Highway Administration, isn’t available yet. That document is still under federal review, said highway-administration spokeswoman Nancy Singer.

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

• The state settled claims by paying more than $50 million in change orders to STP, says a June 30 listing, released to The Seattle Times under the public-records act.

Among these are more than $11 million in payments for what is titled “labor harmony,” to resolve a dispute over whether longshore union workers would be on crews that load muck onto barges. That 2013 episode, which drew in Gov. Jay Inslee to mediate, is blamed for 36 days of delay, and work was ultimately divided between operating engineers and the longshore union.

The state also paid STP $2.7 million for a 15-day delay in digging a repair vault to reach Bertha’s front end, while archaeologists investigated a layer of oyster shells that turned out to have been discarded by white settlers in saloons.

Bigger disputes lie ahead, including STP’s $125 million claim that a leftover steel pipe in the soil triggered catastrophic damage to Bertha in 2013, as well as tussles involving insurance companies and machine maker Hitachi Zosen.