Seattle Tunnel Partners (STP) – Washington State Department of Transportation’s (WSDOT) design-build contractor for the SR-99 (Alaskan Way Viaduct Replacement) Tunnel Project, in Seattle, US – has identified damage to the main bearing seal system as the underling source of high-temperature readings that, combined with a clogged cutterhead, lead to the shutdown of its 57.5ft (17.52m) Hitachi Zosen EPB machine ‘Bertha’ two months ago.
On December 6, STP proactively stopped tunneling after unanticipated and increasing resistance was experienced, which significantly decreased the advancement rate of the machine and the volume of tunnel muck on the machine’s conveyor belt. At the same time abnormally high temperature readings were recorded.
Having evaluated various strategies to determine what was preventing the machine’s progress, STP decided to drill a series of dewatering wells around the machine – which is currently located about 60ft (18m) below the surface, to the west of the Alaskan Way Viaduct, within the project’s secant pile ‘tunnel in a box’ TBM launch structure – to facilitate a safe working environment for compressed air interventions.
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Workers from Ballard Marine Construction then performed 158 hours of hyperbaric inspections, between January 17 and January 28. Once the hyperbaric work was completed, it was determined that a major obstruction was not the cause of the mining difficulty. The more likely cause was the clogged cutterhead.
On January 29, STP drove the machine forward an additional 2 feet and installed the next ring of concrete segments. However, once again, higher-than-normal heat sensor readings occurred in the same way they did on December 6. In the course of investigating these temperature readings, STP discovered damage to portions of the main bearing seal system.
Speaking to NATJ, the state’s Deputy Program Administrator for the project, Matthew Preedy, confirmed that sand and pieces of seal have made their way into the gearbox, indicating there is a breach of the entire sealing system. However, “the main bearing of course has a different grease system to the gearbox. So the contractor still has to do a full inspection of the main bearing to determine if there has been any contamination there,” says Preedy.
STP and its experts were due to meet with senior managers from Hitachi Zosen yesterday, Wednesday 12 February, to determine the extent of the damage to the 25ft (7.6m) diameter US$5 million main bearing unit and its sealing system and to decide the best path forward.
To date, crews have installed 1,022ft (311m) of tunnel. Back at the beginning of 2013, testing at Hitachi Zosen’s manufacturing plant in Sakai revealed a problem with $80 million machine’s main drive unit, requiring crews to make appropriate repairs to the machine before shipping from Osaka Bay, in March. Bertha passed all further tests following the repairs, and was due to officially become the property of STP once she had mined approximately 1,300ft (396m) without any issues.
Further delays to the schedule occurred shortly after the TBM’s launch, when a labor union dispute led to a blockade that effectively shut down the site for a month.
STP is expected to release further information on the main bearing investigations by the end of this week, along with a plan for moving forward. One scenario would involve significant repairs to the sealing system carried out through the back of the machine. However, STP Director, Chris Dixon, told local press on Monday that his team is also weighing “four or five options” for a front-end mission that would most likely involve sinking a shaft to facilitate the removal of the cutterhead in order to conduct repairs. The machine’s location, approximately 500ft south of the Alaskan Way Viaduct, within the footprint of the now abandoned highway arterial, actually provides a fairly good location for dropping such a shaft – with existing soldier pile walls installed to the left and right of the machine that could potentially be factored into a shoring system design.
Whatever the results of this week’s investigations, months of additional delays are expected while repairs are carried out.
For more information about the SR 99 Tunnel Project, part of the Alaskan Way Viaduct Replacement Program, visit: www.alaskanwayviaduct.org.
The essential problem is the blockage of cutterhead inducing great resistance. Therefore, the easiest and economical way is to reduce the resistance by adding conditioning materials, such as foam or slurry to the front head and adjacent ground.