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Bertha’s route corrected after it was off alignment

Bertha’s route under Seattle had to be corrected last week after it was discovered that the boring machine was veering off its alignment.

It was off by 6 inches.

Related: Bertha enters final stretch in its journey under Seattle

The Washington State Department of Transportation reported Monday — after Bertha was stopped and surveys were conducted — that crews have compensated for the 6 inches.

STP designers made a slight change to the tunnel alignment between the machine’s current location and the end of the tunnel drive.

The machine is now moving forward once again, and is expected to emerge at the surface sometime in May. It is less than 1,000 feet from its finish line.

So far, Bertha has mined 8,310 feet, and has installed 1,270 rings (out of 1,426). Cutterhead tools were also replaced while the machine was stopped last week.

Bertha is able to “steer” under Seattle as it bores forward. It does this in a few different ways. It increases or decreases pressure on parts of the cutterhead and uses the tunnel rings to adjust its direction.

WSDOT reports that the latest stop is, at least, the second time the machine has had to correct its path under Seattle and that adjustments are common. Bertha altered its course shortly after it left its repair pit near the Seattle waterfront.

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