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Below the Alaskan Way Viaduct, workers are preparing to dig a 420-foot-long pit where a giant boring machine will be launched in June 2013 to drill the Highway 99 tunnel through downtown Seattle.

At the moment when the accompanying photograph was taken Wednesday afternoon, a crane crew was spinning an auger (foreground) to remove dirt and water, while another crew was inserting a temporary steel tube (background).

Steel rebar is to be inserted into each of 226 shafts, and then cement poured down the hole to form a buried pillar. Each is about 5 feet in diameter and about 90 feet deep.

Two rows of these pillars will form the walls of the pit after the dirt between them is scooped away. So far, one-fourth of the 226 pit pillars have been poured.

The 57 ½-foot tunnel-boring machine will travel under the viaduct near Yesler Way. At that site, scores of steel-and-concrete rods are being inserted to reduce risks of soil settlement that might damage the old highway or Pioneer Square buildings.

The new four-lane, $2 billion tunnel to South Lake Union is scheduled to open at the end of 2015.