Tunnel machine Bertha gets a breather over Memorial Day weekend after another stretch of productive 40-foot days.

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Highway 99 tunnel-boring machine Bertha is now about one-quarter of the way to South Lake Union, having made a steady forward churn of almost 40 feet per day for about the past month.

The giant drill is currently under Western Avenue, 100 feet deep in stable clay soil. It passed beneath the Alaskan Way Viaduct about a month ago on its way into the downtown hillside. No soil settlement or viaduct movement has appeared, said spokeswoman Laura Newborn of the Washington State Department of Transportation.

As of Thursday afternoon, 2,255 of the needed 9,270 feet had been excavated, or 24 percent of the future Highway 99 tunnel. Two or three barges per day have carried dirt to a quarry across Puget Sound.

The path continues toward the Pike Place Market, but workers will break for Memorial Day weekend. Seattle Tunnel Partners (STP) typically digs for 10 hours per day, followed by overnight maintenance.

The machine pushed ahead 110 feet between Monday and Thursday of this week. If such progress continues, STP hopes to finish drilling by December.

That would allow decks and utilities to be completed in time for traffic to shift underground in spring 2018. Then would come demolition of the 63-year-old waterfront viaduct.

Bertha began in Sodo on July 30, 2013, but was delayed two years by damage to its main bearing, and an elaborate operation to fix and strengthen the 4 million-pound front end.