The Science Behind An Engineer’s Dire Warning About Bertha | KUOW News and Information

The Science Behind An Engineer’s Dire Warning About Bertha

Jan 13, 2015

They were not reassuring words.

Engineers hired to rescue Bertha, the deep boring machine stalled under downtown Seattle, wrote to state officials: “If we continue the current ‘repair as we go’ method of excavation, we significantly increase the risk of a catastrophic failure.”

That phrase kicked off fireworks at a Seattle City Council briefing on Monday. City Councilmember Mike O’Brien chastised officials at the Washington State Department of Transportation.

"You have a document from an engineer that calls out the possibility of catastrophic failure, and you have not brought that to our attention?” O’Brien said. “WSDOT needs to do a better job of communicating with SDOT, with council and frankly with the public about what’s going on down there."

State officials were equally offended, frustrated that the city had gone public with its concerns about the Bertha rescue pit. They said the city needlessly alarmed the public by taking the engineer’s phrase – “risk of catastrophic failure” – out of context.

So what does that phrase mean, exactly?

The pit to rescue Bertha is currently 98 feet deep and is lined with steel pilings filled with concrete. The walls of the pit are strong because they form an arch. (Arches are strong by design. Think Roman aqueducts: They’re still standing!)

Said Todd Trepanier of WSDOT: "There is nothing that is in danger.

WSDOT's Todd Trepanier, administrator of the Alaskan Way Viaduct replacement project, faces critical councilmembers Mike O'Brien and Kshama Sawant at a Seattle City Council briefing.
Credit KUOW Photo/Joshua McNichols

The Alaskan Way Viaduct, and the access pit that is being built by Seattle Tunnel Partners to access Bertha to do the repairs is safe – it always has been safe, there has never been any risk of failure."

But arches have a weakness. Remove a segment, and the whole thing collapses. That’s what the engineer was warning about in that letter: The pit walls form an incomplete arch, because there are gaps between the concrete pilings.

As construction crews excavate the pit, they have been filling in those gaps with grout as those gaps are exposed. The engineer calls this technique "repair as we go." It makes the compression ring strong again. 

But as the pit gets deeper, the soil pressure outside the compression ring gets much greater, and those cracks between the pilings become a place where soil can push through.

A diagram shows the wall of the Bertha rescue pit. New pilings are shown in medium gray, while older pilings (from before Bertha was stuck), are shown in lighter tones.
Credit Washington Department of Transportation

And escaping soil is probably not good for Pioneer Square.

The engineers basically said that once you’ve reached a depth of 90 feet, you should fill the gaps between the columns before digging deeper. In fact, don’t dig deeper until you do this. If you do things the way you used to do, that would lead to catastrophic failure.

At the council meeting on Monday, Councilmember O’Brien said he needs time to digest the engineer’s memo, which he saw for the first time when it was practically pried out of Washington State officials’ hands during the city council briefing.

A more in-depth review by the state’s independent panel is due by the end of February.

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