Downtown streets under close watch as Bertha prepares to make moves

Bertha’s 2,000-ton face lift is finally complete, leaving officials planning for the next step and what Seattleites have expected all along: tunneling.

Bertha is slated to break through the concrete wall of its access pit along Seattle’s waterfront in late November. Before that can happen, the contractor, Seattle Tunnel Partners, has a list of prep work to complete.

Testing

First up is a series of tests to ensure that Bertha is fit to bore.

“Right now they are hooking up hundreds of electrical lines, hydraulic lines, lines that will feed the soil conditioners, welding in more steel,” said Todd Trepanier, program administrator for the Alaskan Way Viaduct Replacement Program.

Trepanier briefed the Seattle City Council on Monday morning on the progress of Bertha’s repairs, and updated the city on the new schedule for tunneling underneath downtown Seattle.

Bertha will soon begin no-load testing; running the boring machine, spinning one way and the other, without any surrounding material.

Tests will be run on each switch and system before the machine is deemed fit to work.

When the machine is spinning properly, crews will fill in the access pit that was built to pull Bertha out of the tunnel for emergency repairs.

Just add water

“Now comes a critical piece where they have to back-fill the shaft,” Trepanier said. “This access pit is about 120 feet deep. They will start filling this in with material. And as they reach the halfway point … they will shut off the de-watering wells.”

Crews will also add even more water into the shaft to bring it up to the appropriate water table pressure.

Those wells are important. They keep water from flooding into the pit. But the wells have to be shut off strategically and slowly, because there is a possibility of the ground rebounding once saturated with water again.

“This is the plan that basically, in a very methodical way, turns off each of the four wells. It isn’t like a light switch that you just turn off,” said David Sowers, deputy program administrator for the Alaskan Way Viaduct Replacement Program. “It has to be done with a lot of thought. You want the ground water to recharge in a slow and methodical way.”

The well shut-offs will happen in early November, before Bertha is expected to tunnel again.

WSDOT will also run a monitoring program to determine any ground shifts that occur in downtown Seattle as a result of the water and soil pressure change.

“There could be some rebound as a result of introducing the water back into that underground layer,” Sowers said. “As you do that, the ground can rebound slightly. We want to see how much of that occurs. Usually, it’s a small fraction of what it was originally.”

Drill, baby, drill

When water and ground pressure is set appropriately, Bertha will burst through the concrete wall of its access pit and continue on.

“They will slowly start moving this machine toward the north wall of the access pit, and they will engage that wall and mine through that wall,” Trepanier said. “Once they are through that wall, they have 400-500 feet of mining … as they move on to Safe Haven 3, right before you go under the viaduct.”

Arrival at Safe Haven 3 is scheduled for January 2016. Bertha will leave that point in February and continue to tunnel underneath the Alaskan Way Viaduct. The crucial arterial &#8212 Highway 99 through downtown Seattle &#8212 will be closed to traffic during that time.