Work on a repair-access pit being built next to stranded tunnel machine Bertha is likely to take as much as a month longer than planned because it’s taking longer to chisel and grind through portions of the concrete columns that form the walls of the pit, the project director said Monday.
The 120-foot-deep pit, which is being dug to allow the front end of the tunnel machine to be lifted to the surface for repairs, should be completed in August instead of July as first planned, said Chris Dixon of Seattle Tunnel Partners (STP).
The challenge lies in the geometry of the vault. To prevent mud from seeping through, a pair of the 10-foot-diameter concrete columns are poured and left to harden. Then a 2-foot arc is cut in the concrete on the inner sides of the two columns and a third column will be built to fit between them.
Creating those arcs means breaking up concrete, which, says Dixon, isn’t easy. “We thought they would chew through the concrete better than they’ve been able to,” he said
Malcolm Drilling, the pit subcontractor, has been using a pair of chisels for the job. Drillers first carve through soil and adjacent pillars with a hollow circular drill casing, which is similar to the hand-drill attachment you might use to cut the hole for a new door knob, only in this case, the hole is drilled vertically.
However, it’s also necessary to slip a chisel inside the hollow casing to break up the unneeded concrete. Once that concrete is loosened, a clamshell shovel is inserted to pull out the fragments.
“You’re constantly changing the equipment,” Dixon said.
Also, 11 columns have been added to the original design, for a total of 84, the state Department of Transportation (WSDOT) said in an online update Monday.
Bertha is on track to resume digging in March 2015 as announced, WSDOT says, although state program Administrator Todd Trepanier said his position is to remain skeptical.
“They [STP] still believe they can make the overall schedule,” Trepanier said.
Dixon said the repair timeline started with two months of contingency time, or “float,” so the tunnel boring could resume at the start of 2015 if everything went perfectly. There’s now a month of spare time left, he said.
When the access pit is finished, dirt will be removed from inside the ring. Bertha will grind through the south wall and into open air. A giant crane will lift the entire 2,000-ton front end, which will be disassembled and laid on the surface for repairs. The main bearing and leaky seals will be replaced, by parts being assembled in Japan by Hitachi-Zosen.
The more than 57-foot-diameter tunnel machine, which set out from Sodo last July, has advanced only 4 feet since Dec. 6, after the front end overheated and sand was noticed in the bearing grease, indicating leaky seals.
Barring further major problems, STP expects to finish the four-lane highway link from Sodo to South Lake Union in fall 2016, under a $1.44 billion contract. The partnership, led by Dragados USA and Tutor-Perini, has filed a $125 million claim related to repair delays, which WSDOT denied, setting the stage for prolonged negotiations or a legal battle.