Bertha begins final drive into recovery shaft 18 Feb 2015

Within the last few hours TBM Bertha has begun mining through the southern wall of the recovery shaft in Seattle. As of 2pm local time yesterday (February 18), the 17.5m diameter Hitachi Zosen machine had advanced more than 6ft. Launch for the short 20ft drive through the concrete pile perimeter of the south wall started at 10.15pm local time the previous evening.

Crews have now stopped mining to begin building the 151st concrete ring. A spokeswoman for the project owner, WSDOT, said: “Seattle Tunnel Partners and tunneling machine manufacturer Hitachi Zosen are continuing to follow strict protocols as they mine through the access pit’s southern wall. These protocols are designed to prevent overheating or further damage to the machine as it mines through 20ft of unreinforced concrete to reach the interior of the pit.”

This is only the second time Bertha has been able to advance since being shut down in December 2013. In September last year a 3ft advance was completed to enable engineers from machine manufacturer Hitachi Zosen of Japan to carry out a systems evaluation of the stricken world record diameter EPBM. Earlier inspections carried out in December 2013, followed up by hyperbaric inspections in February 2014 from within the machine's excavation chamber, diagnosed bearing seal failure as the likely cause of a loss of power; as well as possible, though unconfirmed, damage to the main bearing assembly. It is thought that a steel pipe from an old dewatering well may have caused the damage after passing through the cutterhead during the early stages of the drive.

Bertha must now mine through another 14ft of unreinforced concrete to reach daylight and the beginning of a process of heavy lifting and repairs. A WSDOT spokeswoman said: “The duration of this effort will depend on the machine's ability to mine through the concrete while operating with a damaged seal system. STP anitcipates the machine may overheat, as it did during their most recent attempts at mining. If the machine becomes too hot they will take a break for it to cool down before resuming again.”

A contingency plan is in place to mine through the south wall from the inside of the shaft should Bertha be unable to break through under her own power. Meanwhile, STP crews have been using heavy drilling equipment to score a niche around the circumference of the cutterhead on the inside of the recovery shaft to ensure a cleaner breakthrough.

Heavy drilling preparations for Bertha breakthrough

Once Bertha has been driven through into the recovery shaft she will be partially disassembled and her cutterhead lifted out by heavy duty crane before the damaged components can be accessed and raised to the surface. The repair process, for which Gomez International is appointed by Hitachi to provide technical assistance, will take “a significant amount of time”, according to project owner WSDOT.

Under the original recovery schedule submitted to WSDOT by the Seattle SR99 Viaduct Replacement Project construction joint venture partners – Dragados and Tutor Perini (STP) – completion of the recovery shaft should have occurred by the beginning of October last year (2014). This was then pushed out to November 2014, and finally, earlier this month (February 2015), the shaft was completed by STP appointed sub-contractor, Malcolm. The delays were put down to construction complications, and the need to amend the design of the 120ft x 80ft shaft to incorporate an interlocking pile system.

As well as replacement of the seal system that protects the main bearing, the repairs program will also include replacement of the main bearing itself as a precautionary measure – even though it is possible the original remains undamaged. A spare main bearing, manufactured at the same time as the original that was fitted to Bertha and as specified in the design-build contract between STP and WSDOT, is on site in Seattle. It was shipped to the jobsite from Japan in 2014. In addition to the repairs a number of modifications are to be carried out to improve the “rigidity” of the machine prior to resumption of the 2.7km drive.

Check back with TunnelTalk for updates as they are received.