Greg McCorkle
The tunnel boring machine- Bertha- is the largest in the world. At 57 feet in diameter and 326 long, slightly longer than a football field, nothing else comes close.

'Bertha' The not so boring machine


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With art viewers walking the streets of Pioneer Square on a sunny Thursday evening 20 or so people gathered in the projects public relations office, called Milepost 31, to listen to Chris Bambridge, tunnel specialist and consultant for WSDOT, explain how this incredible machine being assemble on Seattle’ waterfront will do its’ job.

Built in Japan the tunnel boring machine, referred to as a TBM, is the largest of its type in the world. You might think okay largest in the world means what? To give you a sense of how massive this machine is the cutting head is 57 feet in diameter. Orient the head to its vertical position and it’s almost the height of a six story building.

393 individual cutting “tools” on the massive head can chew through almost anything it will encounter. Rocks up to four feet in diameter are no problem for Bertha. I wanted to know what the cutting force that Bertha is capable of but the answer is more complicated than I thought. KaDeena Yerkan, manager of Communications and Public Involvement gave me this answer, “It is difficult to relate the installed thrust force to the force being applied to each cutter. The cutter head has a designed total propulsion force of 392,000kN which is an 88,125,104 pound force! It is not as simple as dividing the cutter force by the cutter number to yield the answer that you requested. The issue is complicated and is affected by geometry and also the way in which the machine is intended to be operated. Bertha only needs to operate at a small fraction of its installed total thrust capability to effectively excavate the soil”.

To put this in layman's terms it’s extremely powerful.

Behind the business end of Bertha is the muck extraction device, a large corkscrew auger that funnels the muck onto a conveyer system that will take the muck (muck is the term for the dirt that’s removed) off to barges waiting at terminal 46. (see video) The muck will then be hauled off to an abandoned quarry near Port Ludlow, the same location that was used for the Brightwater project. The next several sections are called “trailers”. They include giant the hydraulic arms, 56 of them, that make minute adjustments to direct the course of the machine. There is the control room where one man guides Bertha on its course. Other trailers house tool rooms, the electric motors that provide the power for the TBM, and other related facilities. Because the TBM works 24 hours a day there are also bathrooms and a mess facility. All in all Bertha is the length of a football field. 326 feet to be exact.

Guiding the TBM is done by lasers that will create waypoints for the operators to follow using a laser as a reference as it moves forward through the earth. Projected from a fixed point behind the machine, the laser is received by a guidance system at the front of the machine that is precisely calibrated to the tunnel’s predetermined path. The guidance system is referenced by the machine’s operator to ensure the machine remains on course. The operator steers the machine with the hydraulic arms by making minute adjustments with each push forward. The contractor operating the TBM believes they can hit the finish point within six inches. This is quite amazing when you think about the lack of GPS or normal surveying techniques.

Tsunamis and earthquakes are of some concern for us in Puget Sound since much of downtown is barely above water level and built on fill. I asked Chad Schuster of Communications and Public Involvement Alaskan Way Viaduct Replacement Program, What measures have been to counter these possibilities?

“The Seattle Department of Transportation is designing the Elliott Bay seawall replacement to protect waterfront facilities, such as the SR 99 tunnel, against sea level rise and moderate storm surges. In the event of a tsunami, it is highly unlikely that a wave would overtop the seawall and reach the tunnel. WSDOT and the City of Seattle found this could only happen during a very high tide - a combination of events estimated to occur only once every 6,000 to 24,000 years. Washington’s early warning system and the tunnel’s real-time traffic technology would allow us to restrict traffic from entering the tunnel during a tsunami. If necessary, drains and pump systems would help to quickly remove any water from the tunnel”.

In reference to the issue of earthquakes Chad went on to say “Geotechnical and structural engineers agree that tunnels can be designed as one of the safest places to be during an earthquake. The SR 99 tunnel is being designed to withstand an earthquake that only happens every 2,500 years on average (in the range of a 9.0 on the Richter scale) without collapsing."

The TBM will exit the bore near 6th & Harrison to be disassembled at the end of the project. Contrary to the common belief that the TBM will be left abandoned underground much of the machine will be hauled away as scrap however a significant portion will be salvaged for resale in a used market for boring equipment.
If the project proceeds as planned the TBM and its components will be lowered into the launch pit in late June or early July. Updates on the progress of the tunnel project will be forthcoming.

For those of you that want an up close look at the project there are tours available to the public. Go to You can also follow Bertha's progress on the WSDOT site here

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