The 2,000t section was lifted from an access shaft and placed on a repair platform nearby. Crews have since removed the machine’s bearing block and, as Tunnels & Tunnelling goes to print, are expected to remove the main bearing. Manufacturer Hitachi Zosen will then begin making repairs. The lifts were performed using a Modular Lift Tower, supplied by Mammoet.

A new main bearing will be installed, along with repairs to the outer seal ring, among other work to create a more robust system. As well, steel plates are being added to the shield for reinforcement. After the TBM is reassembled in the access shaft, there will be a period of testing and commissioning before the TBM can relaunch.

The TBM stopped mining in December 2013, some 1,000ft from the launch shaft, with another 8,000ft left of the drive. Following hyperbaric inspections during early 2014, WSDOT and STP identified two contributing factors: a clogged cutterhead and high-temperature readings that led STP to discover damage to the seal system that protects main bearing. To make necessary repairs and replace the main bearing, STP chose to excavate an access shaft along the alignment rather than performing the work in the tunnel.

Crews completed excavation on the 83ft-wide circular shaft, 120ft (37m) deep on January 30, and then built a concrete cradle on the bottom to support the TBM upon reception. STP advanced the TBM in late February into the access pit for disassembly.

"The industry keeps moving forward with larger and larger tunnels," said Chris Dixon, STP’s project manager. "The tunnelling industry is watching this project very closely so everybody is doing their part to make all of this happen as quickly and safely as possible, so that we can resume tunnelling as soon as possible."

STP comprises a joint venture between Dragados USA and Tutor Perini Corp. STP’s team includes several firms local to Seattle, among them Frank Coluccio Construction and HNTB Corp. WSDOT awarded the contract in January 2011.