With the groundwater wells off during Bertha’s latest shutdown, a tense time may be ahead for nearby building owners and state engineers who baby-sit the seismically vulnerable Alaskan Way Viaduct. If water arrives too suddenly, the ground could rise in places.
Seattle Tunnel Partners (STP) will use the current shutdown of tunnel boring-machine Bertha as a chance to gradually turn off a network of groundwater-removal wells, state officials said Thursday.
As the full water flows return to the soil along Elliott Bay, this could be a tense time for nearby building owners and for state bridge engineers who baby-sit the fragile Alaskan Way Viaduct. If water arrives too suddenly, there’s a risk that the ground would rise in places.
Washington State Department of Transportation (WSDOT) managers have previously said any pump turnoff would be done gradually, and the surface intensively measured.
These wells, including some 185 feet below ground, were activated in fall 2014 to reduce stress on Bertha’s concrete repair-access vault. The vault otherwise would be squeezed by four times atmospheric pressure.
Left on its own, groundwater might have blown mud up through the bottom of the 120-foot-deep vault, endangering workers. The Highway 99 tunnel contractors managed to solve that problem, lift out the 4-million pound front end last year and repair the 57-foot diameter machine.
But a section of the viaduct and some streets in Pioneer Square sank 1¼ inches, prompting Seattle Public Utilities to replace an old steel water main on Western Avenue.
The changes coincided with the start of groundwater pumping, though state-hired experts cited other possible culprits, including building construction and long-term sagging in weak fill soil. Cracks appeared in some buildings, but consultants found no structural damage, the state reported.
STP had planned to leave the pumps on until Bertha reached a concrete-lined inspection point 200 feet ahead of its current position, between South Main and Washington Street.
But on Jan. 14, the state ordered drilling suspended, one day after a sinkhole formed near Bertha’s cutting face — and separately, a barge spilled tunnel muck into Elliott Bay. Gov. Jay Inslee demanded a root-cause analysis report from contractors to explain why the sinkhole formed, after contractors said they hadn’t followed earlier protocols for measuring the amount of soil Bertha was removing.
No restart date has been determined.
Given the new delay, the contractors decided to turn off the groundwater wells now, WSDOT announced Thursday.
Earlier, contractors didn’t want to change the groundwater flow while Bertha was drilling. The machine has broken out of the repair vault and traveled 190 feet since Dec. 22.
STP’s view, according to state tunnel spokeswoman Laura Newborn, is, “In order to better monitor possible ground movement related to shutting off the wells, it is best to shut off these wells when tunneling is not being performed.”
Vibrations from a tunneling machine could loosen the ground and cause soil settlement and complicate efforts to carefully restore groundwater.