Bertha: A whole lot of digging ahead
Bertha is going to be partially unearthed. Seattle Tunnel Partners confirmed on Friday that they need to dig an access shaft to replace damaged bearing seals on the Highway 99 tunnel-boring machine.
The repair is expected to take “months,” according to Washington State Department of Transportation. Although the tunnel contractor has said the shaft would likely be needed to fix the seal, Friday was the first time they said it would definitely be required.
Chris Dixon, a project manager for Seattle Tunnel Partners, said that executives and technical staff from Hitachi Zosen, the builder of the machine, would be in town next Thursday to outline plans and a schedule for repairs. The set of five seals that failed protects the 57.5-foot diameter machine’s main bearing, which allows its “cutter-head” to turn.
Seattle Tunnel Partners, a joint venture between Dragados USA and Tutor Perini, has received design proposals for the access shaft, but have not selected a contractor. The rectangular hole will be about 120 feet deep and 80 feet wide; it will extend some 20 feet to 30 feet in front of the machine and will be lined with concrete “secant piles.” Seattle Tunnel Partners may begin to install the piles before the shaft design is completed. Hitachi Zosen is still preparing final specifications.
The contractor is working with Hitachi Zosen to determine whether to build a four-sided, or three-sided shaft. The four-sided shaft, Dixon said, would be easier to “dewater.” Water from Elliott Bay and groundwater is flowing into the gravelly soil around Bertha.
But the four-sided shaft would require driving the machine forward a short distance to break through the wall closest to the cutter-head. Hitachi Zosen has not said yet whether it would be advisable to make that move. Dixon could not say whether that was because of existing problems, or concerns about further damaging the machine.
Archaeologists from WSDOT will be involved with the dig. They are waiting for more details about the shaft before coming up with an archeological monitoring plan. The location where the machine is stopped is above a tidal flat, which was filled with dirt and debris. The state has already conducted extensive archeological tests in the vicinity of the machine.
“The archeological team already knows the current location is not near any areas where natives lived,” WSDOT spokesperson Laura Newborn said in an email.
Dixon also said that Seattle Tunnel Partners does expect to remove the machine's 745-ton cutter-head to access the bearing assembly, even though Hitachi Zosen has not verified that this will be necessary for the repairs. Seattle Tunnel Partners does not believe the main bearing was damaged when the seal failed, but is still asking the component's manufacturer to recertify it as a part of the repair process. The bearing assembly can be accessed from inside the tunneling machine, but it requires the removal of a substantial number of parts.
Seattle Tunnel Partners is taking steps to reduce their workforce due to the current tunneling stoppage. Dixon couldn't say how many workers would be laid off.
The boring machine has been at a near standstill since Dec. 7 about 60 feet under Pioneer Square, near South Main Street.