Crews are digging a launch pit for the enormous drilling machine that will bore a hole from Sodo toward South Lake Union in the Highway 99 tunnel project.

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Rotting wood, leftover highway columns and tidal surges are just some of the obstacles workers are tackling before a giant drill arrives in Sodo next summer to build the Highway 99 tunnel.

Crews are scooping away soil to form what will become an 80-foot deep launch pit for the drilling machine. The $2 billion tunnel, to open by the start of 2016, will pass under the old Alaskan Way Viaduct and downtown, emerging near South Lake Union.

Earlier, construction crews drilled vertically into the soft soil to install giant concrete shafts to form the four 110-foot-deep walls of the launch pit. Now that walls are in place, the team is pumping out groundwater — which can rise at high tide — and removing dirt to reach a depth of 80 feet.

The Seattle project would use a tunnel machine with a world-record 57 ½-foot diameter, though Russia is attempting to build a 63-foot tunnel in St. Petersburg.

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More than 1,400 deep pilings are required in the Sodo pit and nearby to prevent soil collapse and to protect buildings and some still-used viaduct spans from ground settling.

Still buried within the pit are the stubs of several 1959-vintage viaduct columns that were severed last year, while the southern mile was demolished. As dirt around them is removed, conventional wrecking machines will bash the old columns.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom.

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