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Crews had dug down nearly 20 feet as of Wednesday afternoon, as they make a 120-foot-deep vault where tunnel machine Bertha’s front end can be cut open for repairs.

Digging started late last week and will continue through November, until about 20,000 cubic yards of soil is removed from within a deep concrete ring that was built this summer.  A thick concrete slab will then be poured at the bottom, to support the mammoth tunnel-boring machine. While the concrete cures, the machine will grind forward about 43 feet, breaking through the south wall of the vault into open air.

By December, a crane will lift the 2,000-ton cutting drive to the surface, where it can be disassembled and repaired, said Chris Dixon, project director for Seattle Tunnel Partners.

This image from a state Department of Transportation pit cam shows the operation:

pitOne the depth reaches 40 feet or so, crane-mounted scoops will be lowered to carry out the dirt, Dixon said.  Digging will be interrupted sometimes, to adjust five wells that are removing groundwater. People and loading machines need a firm footing.

“If we don’t de-water at all, it would get to the point where it’s very soupy.” Dixon said.

The 57-foot-diameter machine is buried near South Main Street, where it’s moved only a few feet since the cutter overheated and failed to grab soil in early December.  The main bearing will be replaced and surrounded by stiffer seals.

Dixon said he’s primarily concerned with the safety of the workers, and with protecting nearby buildings and the Alaskan Way Viaduct from soil settlement.  STP is scheduled to resume boring the four-lane highway tube from Sodo to South Lake Union in March 2015. The $2 billion toll tunnel was supposed to open at the start of 2016 but has fallen close to a year behind schedule. Work continues on the Sodo-end highway ramps as well as the north portal opening near Seattle Center.