Thursday, December 12, 2013

Bertha Hits a Wall!

It's been a chuckle-fest watching local prognosticators speculate as to what has brought the world's largest tunnel boring machine (TBM), digging a highway tunnel under Seattle, to a screeching halt near Pioneer Square.

A week ago - December 6th - having just achieved the 1,000 foot mark, Bertha, so named to honor the female mayor of Seattle, Bertha Landes, has nosed into a mysterious circumstance, unable to continue forward movement.

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Everything from a buried locomotive to outgoing Mayor McGinn's buried bicycle has been suspected to be the cause for the shut down. This, despite numerous reports from Tunnel Partners and Washington State Department of Transportation spokesmen, that Bertha has progressed beyond former land fills, and is well into "native" ground.

Bertha is an Earth Pressure Balance Shield. (EPB). EPB's are shield machines designed  to burrow through soft under ground conditions containing water under pressure, such as loose sedimentary deposits with large boulders and a high water table.

Because the cutting face is pressurized, divers from Ballard Diving & Hyperbaric Services may be deployed into the cutting face.   Bertha can only back up about 18", allowing a crew to enter the void and determine was is causing the blockage.

(As an example, pressure measured in a South African gold mine, two miles below the surface, approached 12,000 pounds per square inch!)

Cick to open; click magnifier glass for full view

It is riveting to see the results of Bertha's progress thus far!

See Also:  
•  Some Assembly Required
•  Some Assembly Required - Part Two
•  Some Assembly Required - Part Three
•  Off in a Cloud of Dust!

2 Comments - Click here:

GeoLurking said...

Since "Bertha" is designed for soft sediment, it is entirely possible that it hit something deposited by a glacial outburst flood similar to the Missoula Floods. After all, the Willamette Meteorite was placed in that manner.

Robert in Port Townsend said...

I, too, believe it to be leftovers from the glacial age. Despite the announcement by Seattle Tunnel Partners that the TBM is in native soils, the snooze media insist on the "locomotive Theory." I remember our house in West Seattle required several truck loads of top soil. Our back yard was nothing but boulders and gravel...

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