Bertha rescue efforts suspected in sinking of 30 Seattle businesses
Dec 9, 2014, 7:41 AM | Updated: 2:01 pm
WSDOT is investigating whether pumping of groundwater around a rescue pit for Bertha the boring machine is causing nearby businesses to sink. (WSDOT image)
(WSDOT image)
It sounds like the State Route 99 tunnel project has entered the improv stage. Engineers remain unsure about what exactly is causing settling in the area surrounding the huge access pit being dug for Bertha the boring machine.
It may have caused the Alaskan Way Viaduct to sink at least one inch around areas of King Street and Alaskan Way. At least 30 businesses in nearby Pioneer Square have been sinking, too.
At Monday’s Seattle City Council meeting, the Washington State Department of Transportation’s tunnel project manager, Todd Trepanier, again said the viaduct is still safe, but they have to proceed very carefully.
“There is a water aquifer beneath this pit that has pressure in it. It had pressure to such an extent that they believed it could jeopardize the design of the shaft by that pressure forcing the bottom of that shaft up, once they excavated to a certain depth,” explained Trepanier.
That’s why they started digging additional wells to relieve the pressure. In relieving the pressure, however, they may have cause more settling. But no one is really sure.
“They’ve been pumping those wells for approximately three weeks and that seems to coincide when this was found out,” said Trepanier.
Members of the council asked why crews couldn’t just shut the wells down.
That’s a bad idea, according to Trepanier.
“If you just shut them off in a non-very-engineering-style-method, you could damage buildings or you could damage the pit itself. Then, if you have to decide if you’re going to turn those back on – what’s going to happen there, as well.”
Even more worrisome is that in addition to the viaduct, there are about 30 buildings in the settlement area.
“Roughly about a third of those were in our preconditioned survey zone,” Trapanier said.
He said the WSDOT has implemented protocols to immediately go back out and check those buildings.
Councilmember Mike O’Brien pointed out that the engineers have yet to say how much settlement would be too much.
“WSDOT has failed to articulate what is the threshold other than to say, trust us, we will let you know when we cross it.”
To which the project manager said there’s just no easy answer.
