Last fall, the world’s largest diameter TBM was making significant progress mining below Seattle. It was quite possibly a mere week away from installing the 200th ring, marking the final manufacturer’s hand-off, in December when tunnelling stopped, and has yet to properly resume.

March 2015 is the most recent deadline for the TBM to start mining again. Stalled underground for six months, by the time this issue goes to print, there are still many questions about damage to the machine, repair work and the cost in both time and dollars to the project.

Japan’s Hitachi Zosen is supplying the EPB TBM, 57ft (17.5m) in diameter, that will build the world’s largest diameter deep bore, a highway tunnel accommodating four lanes of traffic in a double-decker formation.

The tunnel is 1.7 miles (2.7km) long and reaches a depth of 120ft (36.5m) from sea level, while maximum cover is 215ft (65.5m). The maximum horizontal curve is 2,000ft (650m) and the maximum vertical gradient is 4 per cent.

The Washington State Department of Transportation (WSDOT) is building the tunnel to replace the Alaskan Way Viaduct, which sustained damage after a 2001 earthquake and was deemed unlikely to survive another one.

WSDOT awarded a design-build contract worth USD 1.35bn to Seattle Tunnel Partners (STP), a joint venture of Dragados USA and Tutor Perini. The agency said in January, "We were drawn to one component of their proposal, which involved building a protected underground area along the first 1,500ft (460m) of the tunnel drive.

"In addition to protecting the viaduct and other structures in this very shallow, challenging section of the route, this protected area allows the contractor to test all functions of the tunnellng machine and its operations prior to tunnelling under downtown Seattle."

STP launched the TBM at the southern portal on July 30, 2013. The alignment descends at four per cent gradient to attain maximum depth prior to the TBM passing beneath the existing viaduct foundations, approximately 1,750ft (533m) after the launch site.

On Saturday, December 7, 2013, STP stopped tunnelling "after unanticipated and increasing resistance was experienced, possibly due to an obstruction, which significantly decreased the advancement rate of the machine and the volume of tunnel muck on the machine’s conveyor belt," WSDOT would announce the next week.

About 60ft (18.3m) underground, the TBM had mined more than 1,000ft (305m) but was still in the section of the tunnel drive with difficult ground due to the historic fill soils. Shallow depth and the high groundwater table also had to be factored into any decisions for moving forward on the drive.

STP attempted to identify and remove any potential obstructions by installing wells approximately 120ft (36.6m) deep near the TBM to lower water pressure in the ground, allowing workers to enter the excavation chamber at the front of the machine. At the same time crews drilled 17 exploratory probes in front of the machine to look for objects that might be blocking the TBM’s path, encountering obstructions in four of the probes.

Over the month of December more than 300,000 gallons of water were removed and on Wednesday, December 20, crews were able to turn the machine’s screw conveyor and remove some muck from the excavation chamber. "As the muck lowered, a crew member was able to take a quick peek through a hatch near the top of the machine. What they saw inside was dirt, sand and cobbles – nothing extraordinary. Because water quickly filled the part of the chamber that was previously occupied by muck, they were only able to look around for a few minutes before they had to close the hatch," according to information released by WSDOT.

Casing concerns
On January 2, crews successfully reduced water pressure enough to inspect the top 15ft (4.6m) of the excavation chamber. A piece of an 8in (203.2mm) diameter steel pipe could be seen protruding through an opening in the machine’s cutterhead. Probes from the surface also detected metal in front of the machine.

The steel pipe is a well casing installed in 2002, after the 2001 earthquake, to help geologists better understand how groundwater moves in the area. The location of this pipe was included in reference materials in the contract, WSDOT says.

Once the face was accessible STP crews carried out maintenance, inspecting and replacing damaged cutter tools on the face of the machine while further investigation work continued. On January 5, Malcolm Drilling, as a subcontractor to STP, started drilling four, 5ft- (1.5m-) diameter shafts in front of the TBM where earlier probing detected metal.

It was hoped the shafts would further identify the limits of any metal in front of the machine and remove as much of it as possible. However, if no objects were detected or removed from the shafts, they would be filled to form part of an underground barrier that would create a safe environment for workers to enter the machine’s excavation chamber.

Drilling to a depth of approximately 118ft (36m) starting on January 7, investigations in the first three shafts produced inconclusive results. By January 13 the third shaft was being refilled, and drilling for the fourth was put on hold.

Hyperbaric intervention
On January 17, crews entered the excavation chamber to inspect the cutterhead under hyperbaric conditions. Within the first 35 hours, they cleaned the spokes and removed a bent piece of metal well casing and plastic PVC pipe and identified a large boulder or piece of concrete material in a cutterhead opening.

"We changed a significant number of tools going in through the spokes. When we did the hyperbaric interventions we changed more cutting tools on the cutterhead," Dixon would later explain in a press briefing on February 11.

Hyperbaric intervention finished on Tuesday, January 28, with crews performing 158 hours of inspection work, including removing clogs of dirt and other material in the cutterhead openings, and replacing cutting tools. No major obstructions were found inside or in front of the machine.

That same day, after the hyperbaric work completed, STP moved the TBM forward by about 2ft (0.61m) to build the next tunnel lining ring (ring number 150), which was in progress before the machine was stopped on December 6, followed by an evaluation of the machine and its operating systems.

When the machine moved forward, crews saw indications of above normal temperature readings in part of the machinery, similar to readings encountered before crews initially decided to stop mining on December 6. The next day, Wednesday, STP made adjustments and mined an additional 2ft. The above-normal temperatures persisted, and STP made the decision to stop and continue investigations, which would then reveal damage to the seal system protecting the TBM’s main bearing.

"Portions of the seal system have been damaged and need to be repaired or replaced," WSDOT announced on February 7. "STP and its tunneling experts are working with the machine’s manufacturer to determine the best fix for this issue."

Tunnels and Tunnelling contacted both STP and WSDOT for this article to discuss the investigation and potential causes for the machine’s damage. STP responded via WSDOT that it is still actively investigating the root cause of the machine’s issues and could not answer questions about the investigation work or potential causes of the damage at the time of the magazine going to print.

Repair Plans
Todd Trepanier, WSDOT’s administrator for the Alaskan Way Viaduct Replacement Program, and Chris Dixon, project manager for STP spoke with reporters about the TBM Tuesday, February 11.

At the same time Hitachi Zosen staff flew in from Japan to help develop repair plans and assess the situation. STP considered two options — digging an access shaft in front of the stalled machine or accessing the machine from the back, performing repair work under hyperbaric conditions.

On February 21, STP announced it had decided to access the seal system through the front of the machine by digging a vertical shaft 100ft-deep by 80ft-wide (30.5m by 24m) shaft in front of the TBM, driving the machine forward into the shaft and then making the needed repairs. Dixon initially estimated that this work will take at least six months.

Two months later, STP released more information about the repairs and confirmed the TBM won’t start up again until March 2015. At the time of publication, STP confirmed it will replace the machine’s main bearing and install a more robust seal system, "which could include strengthening the seals, installing redundant systems, and adding monitoring equipment." Additional details will be included in a plan to be submitted to WSDOT for review by June 16.

WSDOT’s contract with STP mandated an extra bearing be manufactured for the project.

"We are disappointed by this delay, but we believe the new schedule is moving in the right direction," Trepanier said. He adds, "We’re also focused on the rest of the program, which includes more than USD 750M worth of work at the tunnel portals and elsewhere along the SR 99 corridor. That construction is not affected by the tunneling stoppage and continues full speed ahead."

By September the shaft should be fully excavated and repair work on the seal system and main bearing is expected to start by October. Testing for the machine is scheduled for February 2015. "Resuming tunnelling will take longer than any of us would have liked, but making these repairs is a significant engineering challenge that must be done safely," said Dixon on April 21. "We are committed to this project, and to taking the necessary steps to recover time and open the tunnel to drivers by WSDOT’s original target date."

Recovering Time
WSDOT’s expert review panel (ERP) for the project released its 2014 report on February 27. It states: "the TBM position on December 6 when it stopped was within two feet of its originally scheduled location. This indicates significantly better than expected performance of the TBM while it was mining." The ERP is chaired by Patricia Galloway, and includes Robert Goodfellow and John Rose.

In the February 11 press briefing Dixon explains, STP planned on a 14-month tunnel drive, starting in July 2013 going to September of this year. Its original construction schedule included 339 working days to mine the tunnel, including time for maintenance and interventions. This was based on a planned production rate that was slower at first, but increased to nearly 40ft (12m) per day – a pace that STP believes is conservative when the machine is functioning properly. "We planned the work on a five-day per week, two shifts per day, 10-hours per shift, he said. "If we went to a 7-24 that’d be 168 hours a week so that’d be a 68 per cent increase in the available work hours per week.

"When you go to an around the clock operation you’re not quite as efficient, so each hour you’re working over the 100 hours you’re not necessarily picking up an hour on the schedule but you have the ability to recover some time."

He also points out that originally STP planned to start tunnel interior structures, electrical and mechanical and other systems work once the TBM had mined 2,000ft (610m). And that there was also downtime planned in safe haven three (the last stop before mining below the actual viaduct foundations and deeper underground) for work in the launch pit, some of which can be performed during the current downtime, which will shorten the time spent in safe haven three when the TBM ultimately gets there.

"The fact that the machine performed very well in its initial tunnelling…achieved the same footage in the first 135 days that we planned even though we were stopped for a month with the Longshoremen picket line. So the machine performed and advanced much faster than we were anticipating."

The newly announced timeline delays tunnel boring by up to 16 months, however STP hopes to recover as much as four months of schedule to meet WSDOT’s original tunnel opening date of November 2016. STP had proposed a contract completion date for December 31, 2015.

Cause of the clog
WSDOT says it does not believe that the well casing was a major contributor to the current stoppage.

In the February 11 press briefing Dixon says there are a variety of reasons that the seals could have been compromised, but STP will not be speculating. "We believe that there was pretty significant damage to the cutting tools," Dixon says.

He adds, "We believe the cutting tools were damaged by the pipe."

In reference to the well-casing pipe causing damage to the cutterhead, he later says, "We think that created friction, which created heat, which could have resulted in damage to the seals.

"But again it’s something we have to prove to WSDOT. Tight now it’s just a theory that we have. All I’m really saying is we haven’t ruled out the pipe as a contributing factor.

"How it contributed, we’re still looking at that. That’s just one of the theories we have as to how it could have contributed to the problem".