After drilling four deep shafts and sending a man down to look last week, Seattle Tunnel Partners and the state Department of Transportation don’t fully know yet what’s blocking tunnel machine Bertha, some 60 feet deep next to Pioneer Square.
So the DOT announced Tuesday it is “going hyperbaric,” by sending workers to the pressurized cutting face. Teams of five or six workers would take turns inspecting the huge area.
Bertha is equipped with two hatches for men and one for equipment, to reach a 5-foot wide chamber where excavated muck enters the conveyor system, right behind the 57-foot rotary cutter face. The workers don’t use scuba gear, but breathe freely. However, they need to spend an hour in a pressurized chamber afterward, so their bodies can gradually re-adapt to normal atmospheric pressure after each three hour shift.
High pressure exists because ground water permeates the soil here. But workers won’t be sent underwater. Instead, a brown slurry of bentonite clay will be sprayed into the 57-foot wide cutter, and then compressed air would be pumped in, to push the bentonite against the soil walls, sealing them. Additional compressed air would then create a “bubble” where work can take place.
Pre-construction forecasts said pressures might be more than double that of the atmosphere. But DOT and STP plan to significantly reduce the pressure — making the job safer and quicker — by pumping away groundwater through 10 temporary wells that were installed last month.
In other tunnel projects, builders have sent actual divers at much higher pressures, beneath the Elbe River in Hamburg, Germany, in Copenhagen, Denmark, under the Yangtze River in Nanjing, China, and for at the Brightwater sewer tunnel north of Seattle. For this next task at Highway 99, Ballard Marine Construction is supervising STP workers, who’ve received additional training.