Todd Herman

Renowned architect says sinkhole is the second big mistake for Bertha

By MyNorthwest.com, Staff report | January 19, 2016 @ 8:24 am

The sinkhole that developed behind Bertha could have destroyed a portion of the Alaskan Way Viaduct, or a building, former Seattle City Council member and architect Peter Steinbrueck says. (WSDOT)

A sinkhole at the Seattle waterfront isn’t good, but it could have been much worse, former city council member and architect Peter Steinbrueck says.

If the hole that developed behind Bertha the boring machine was closer to the Alaskan Way Viaduct’s foundation, it could have toppled a portion of the aging infrastructure, Steinbrueck told KTTH’s Todd Herman.

“This was a massive collapse due to over-excavating,” Steinbrueck explained.

On Jan. 13, the Washington State Department of Transportation announced the sinkhole developed shortly after Bertha began digging again. The hole was spotted about 100 feet south of Bertha’s current position.

Noting how “seriously disappointed” he was, Gov. Jay Inslee announced he was suspending the work until Seattle Tunnel Partners could prove it to be safe.

The sinkhole was caused by human error, Steinbrueck believes. Crews weren’t back-filling enough as Bertha moved.

It took the equivalent of 22 cement trucks worth of concrete to fill the hole, Steinbrueck said, “and it’s still sinking.”

“It’s a pretty big hole,” he added.

The sinkhole is the project’s second big mistake, according to Steinbrueck. The first could be traced all the way back to Bertha’s design. Steinbrueck believes the approximate two-year shut down of Bertha was due to the machine not being up to the task at hand; it wasn’t just a pipe that forced work to stop.

When Bertha begins its journey again, the machine needs to dig at least 1,500 feet to enter a “less risky area,” in terms of soil, according to Steinbrueck. It needs to reach harder, more compact soil.

But will the tunnel be safe when it’s complete? Steinbrueck says yes. In fact, it could become one of the safest places in the city. But that is a ways out.

Before the latest hiccup in the project, Seattle Tunnel Partners estimated the tunnel would open to traffic in April 2018. Before that can happen, Bertha still has nearly 8,000 feet to go, and that work could include drilling through more unstable soil at the other end of the tunnel near Lake Union.