Crews in Seattle are now expected to finish digging a 12-story pit to reach stalled tunnel-boring machine Big Bertha one month later than planned due to concrete pilings proving more difficult to drill through than expected, according to a report from the Seattle Times.
Bertha, the world’s largest tunnel-boring machine went down from overheating in late January, one-tenth of the way through digging the 1.7-mile State Route 99 tunnel which will carry a double-deck highway and replace the SR 99 Alaskan Way Viaduct.
Water and sand clogged the machine’s cutterhead openings and penetrated seven rubber seals meant to protect the main bearing. In repairing the machine, crews will install 216 steel ribs and plates that will add 86 tons of reinforcement to the 7,000-ton machine’s drive block and cutter drive.
But to make those repairs, workers are in the process of digging a pit 120 feet deep to access the machine before pulling its 630-ton cutterhead, drive axle and bearing to the surface in order to replace the bearing and add the steel reinforcements. The pit was initially expected to be completed by the end of this month, but construction is now expected to last into August.
The lead contractors on the project, Seattle Tunnel Partners, brought in Malcolm Drilling as the subcontractor to build the repair pit, according to the Seattle Times. STP spokesman Chris Dixon told the paper that Malcolm is having trouble with construction of the pit wall which is being formed through the installation of 84 interlocking concrete pilings.
First, crews install the first set of pilings by drilling a hole 120 feet deep by 10 feet wide and then filling it with concrete. Those pilings are set six feet apart. Then in the space between the pilings, another 10-foot-wide piling is installed requiring crews to drill two feet into both of the existing pilings on either side. The Times has put together a great graphic that illustrates construction of the repair pit perfectly.
And that drilling is what has slowed crews down, Dixon said. “We thought (the drills) would chew through the concrete better than they’ve been able to,” he explained.
To fix the problem, Malcolm has been forced to add a step to the installation of each piling by using gravity-driven chisels to soften up the concrete before drilling.
Despite the delay to the pit, Big Bertha’s repairs are still scheduled to be completed with drilling resumed by March 2015. The tunnel was originally scheduled to be completely drilled by the fall of 2014 and open to traffic by the end of 2015. With the delay, the tunnel could open by November 2016—the state’s completion deadline.