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Seattle's giant tunneling machine Bertha is off to a slow start.

The machine has traveled just 24 feet in 18 days. Why has it moved so slowly?

First, Bertha only digs during the work week in two 10-hour shifts per day, WSDOT explained on its website. On weekends, Bertha gets some beauty sleep and important maintenance.

Additionally, because Bertha needs strong support to push off of as she tunnels through the earth, crews are building rings in the launch pit and reinforcing them with steel. It's a slow-going process that requires Bertha to stop every two feet.

Finally, before she hit soil, Bertha bored through a 15-foot-thick concrete wall at the north end of the launch pit. She broke through onAugust 13. Digging through a thick headwall is standard for all tunnel projects, and it's not unusual for it to take a while, WSDOT said.

As we head into fall, Bertha's progress will vary -- she may dig two feet one day and 20 the next. Eventually she will hit the 6-foot-a-day average.

Visit Bertha's Twitter page for more updates.

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