January 8, 2016

Moving about one inch a minute, Bertha tunnels out of rescue pit

Photo by Benjamin Minnick [enlarge]

Crews from Mammoet are taking down the giant gantry crane that was used to repair Bertha.

On Wednesday, the machine boring the replacement tunnel for the Alaskan Way Viaduct mined through the north wall of its concrete repair pit and then on toward a check point about 400 feet away.

Bertha has been mostly silent for two years while crews from Seattle Tunnel Partners toiled to fix the gigantic machine. By yesterday afternoon, the front of the machine was about 40 feet out of the pit and 80 feet underground.

“(Wednesday) was a very good day,” said STP project manager Chris Dixon. “We mined five rings.”

Dixon said the machine placed the 170th ring Thursday morning near Pier 48 and South Main Street. The 1.7-mile tunnel will be made of 1,450 rings, each with 10 curved concrete segments.

Dixon said crews were checking Bertha's operations after placing ring number 170 and were expected to resume tunneling late yesterday.

“The TBM is performing very well,” he said. “Temperatures are good, tolerances are good.”

More good news for Bertha: the machine is mining deeper in native soils instead of the fill that was along the first 1,000 feet of the route. Dixon said the mix of sandy soils and clay is easier to mine, and any ground settlement is easier to control.

Dixon said the machine is mining about 1 inch per minute. It takes about 78 inches of mining to install a tunnel ring behind Bertha. At the current speed, that's just over an hour. Dixon said the machine was designed to mine a ring in half an hour.

He said crews expect to pick up the pace before stopping at the check point by the end of the month. Mining and placing rings will take about 60 to 70 percent of each day, and the rest of the time will be spent doing maintenance.

Crews will take about a month to fine-tune Bertha at the check point near Yesler Way. In March, the machine will tunnel under the viaduct, and that requires closing the highway for two weeks.

Todd Trepanier of Washington State Department of Transportation said closing the viaduct will allow crews to better monitor the structure and ground for settlement and cracks.

Dixon said while Bertha is idle at the checkpoint the team plans to add another 1,000 feet to the conveyor belt that carries mining debris. This will allow crews to get Bertha well past the viaduct before they have to stop again and add more belt.

STP has been using two 25-person crews, each working six 12-hour shifts per week. They will work continuously while under the viaduct.

Dixon said continuous mining below the viaduct will minimize the likelihood of ground settlement. Crews will inject grout if any soil is lost or voids appear.

Tunneling is expected to wrap up Jan. 3, 2017. Dixon said there still will be 15-16 months of work to do after that.

In a sign of confidence about Bertha's repair, crews from Mammoet started dismantling the giant red gantry crane that was used to pull the TBM's broken parts to the surface. The operation is expected to take about a month.

A video of STP crews at work this week can be found below.