To build an underground highway

I guess you can do anything, if you spend billions and billions of dollars.  Bertha’s cutter head is now about 3,088 feet into the tunnel, not quite half way through the 9,000 feet yet to go. She is paused for complicated maintenance work, 120 feet below Spring Street.

All the work on the the cutter head has to be done under pressurized (hyperbaric) conditions, similar to a deep water dive. 

Since workers can only stay in the pressurized environment for 30 to 45 minutes, seven teams of five rotate around the clock. 

To get access to the space behind Bertha’s cutter head, workers first remove the soil and then fill the space with a slurry of clay and water called bentonite. This turns to a gel-like substance. Once that happens, workers push compressed air into the space, forcing the bentonite into the surrounding soil to make a sort of seal that keeps water and soil from getting back into the area.

This latest maintenance stop has been going on since July 5, and crews have made 44 hyperbaric “dives” replacing 32 cutting tools on the drill head.   

The most recent estimate from Seattle Tunnel Partners says the digging will be complete this December, with the highway opening up to traffic in spring 2018.