Seattle’s Big Bertha has been lowered and ready to start tunneling again next month.

As reported in a recent ENR blog in August by Tim Newcomb, the damaged front end of the TBM (tunnel boring machine) has been repaired and lowered back into the tunnel.  After assembly and testing, tunneling on the SR99 -- Alaskan Way Viaduct replacement project will resume after a 20-month delay.

Additional info on the SR99 project can be found here. While the WashDot site includes an animation of how TBMs work, we liked the one found for the London Crossrail project. Crossrail employs eight TBMs which will connect 28 stations to the West End of London by 2018. The video additionally shows the difference in operations between the six earth pressure balance and two mixed-shield machines used on the project. The mixed-shield TBMs are used to drive through the chalk under the Thames River.  Click here for the Crossrail TBM video.