Construction will begin late next month on a pit that will be used to access and repair damage to the machine, which stopped tunnelling in December. The TBM, Bertha, is the world's largest.
The US$2bn (£1.2bn) project is being carried out by Seattle Tunnel Partners, a joint venture of Dragados USA and Tutor Perini Corporation. HNTB Corporation is lead designer for the Washington State Department of Transportation (WSDOT) scheme.
Building the pit is the first of several steps STP has laid out to resume tunnelling:
- Late May: Begin building the access pit’s underground walls.
- Late July through to September: Excavate the pit.
- October: Remove the machine’s cutterhead and begin repairing damage to the seal system and main bearing.
- February 2015: Test the machine to ensure it is ready to tunnel beneath downtown.
- Late March 2015: Resume tunnelling.
While this timeline delays tunnel boring by up to 16 months, 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 opening the tunnel in late 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 Chris Dixon, Seattle Tunnel Partners project manager. “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.”
STP has informed WSDOT that crews 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.
The repair schedule will include additional time to accommodate potential improvements to the machine that STP or the machine’s manufacturer, Hitachi Zosen, might choose to make after the cutterhead is removed and crews are able to perform a full inspection. WSDOT will work with its strategic technical advisory team, made up of international and national tunneling experts, as well as consultants, to review the plan.