Crews are about to enter a pressurized workspace in front of the massive boring machine now at a standstill under downtown Seattle. When they do, the clock will start ticking on a contract provision that has the potential to drive up the state’s costs for the Highway 99 tunnel project.
Seattle Tunnel Partners (STP), the contractor leading the project, announced on Wednesday that it was staging a “hyperbaric intervention” to investigate parts at the front of the hamstrung rig, nicknamed Bertha. To do that, STP will inject bentonite — a clay material that's often used to stabilize underground construction — around the top half of the machine's roughly five-story-tall cutting face. The contractor will then try to pressurize that space with air, creating a place for workers to safely inspect the cutterhead.
According to the project's contract, the state’s lump sum payment to STP covers the cost of 1,440 hours of hyperbaric intervention work. After that time is used up, the contract states: "WSDOT shall share in the cost and schedule risk of [hyperbaric] Intervention Work".
Once the 1,440-hour threshold is exceeded, $40 million that has been set aside will be available to cover any additional hyperbaric costs. The $40 million (which is part of a much-discussed $205 million risk contingency fund) will also pay for any costs incurred due to site conditions that are different from the ones the state told STP about in the project's planning documents. If any of the money set aside for hyperbaric work is left over at the end of the project, STP and WSDOT will split the total 75-25, with the contractor getting the larger share, according to a WSDOT spokesperson. If there are hyperbaric-intervention costs beyond $40 million, then the state will pick up the tab. For now, though, the $40 million portion of the budget remains untapped.
The giant tunnel boring machine is submerged amidst "non-cohesive," gravel-like earth and muck, which creates pressures similar to those experienced under water. The machine is about 60 feet below ground and entering a section of the dig where pressures are expected to increase significantly. Water from Elliott Bay and other groundwater is flowing into the area around the front of the machine. Since boring stopped, STP has drilled 10 wells and installed pumps to lower the water level and thus the pressure in this area.
Removing water, while essential to getting Bertha boring again, is not without risk. The ground around the stalled machine could sink. WSDOT program administrator Todd Trepanier said on Wednesday that the ground had settled between one-quarter and one-third of an inch, so far.
At a press conference on Wednesday, Seattle Tunnel Partner’s project manager Chris Dixon said that during the planned inspection, workers will attempt to remove enough water and muck to expose about half of Bertha’s face. After they inspect the upper exposed portion of the cutter head, they'll rotate the head 180 degrees and look at the other half.
Cutting discs, mixing arms and the machine’s central hub are among the parts that workers will examine. The hyperbaric work will involve three shifts a day with crews who will work up to four hours at a time. After each shift, the five-member crews will decompress in Bertha's high-pressure chambers.
At a state Senate transportation committee meeting on Thursday afternoon, Trepanier said workers were preparing to inject air into the Bentonite-sealed area at the front of the machine and that crews would probably enter the area on Friday.
“Hopefully in the next few days we’ll be at a point where we’ve assessed what the situation is,” STP project manager Chris Dixon said on Wednesday. “And then we can determine what we need to do to resume tunneling.”
The progress of Seattle's 57-foot wide tunnel-boring machine slowed on Dec. 3 when Bertha hit an 8-inch diameter, 119-foot long steel well pipe. The resistance increased in the days that followed. A WSDOT contractor had placed the pipe in the ground back in 2002. WSDOT alerted the tunnel contractor about the location of the well pipe in planning documents, but Dixon said that STP thought the pipe had been removed.
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