Constructing the pit needed to fix Bertha, the machine digging the Highway 99 tunnel in downtown Seattle, is taking longer than expected, the megaproject's main contractor said on Monday.
The contractor, Seattle Tunnel Partners, had anticipated completing the concrete walls of the pit this month, but now says that work will continue into late August. When completed, the access shaft will be roughly 11-stories deep.
Despite the delay, Seattle Tunnel Partners project manager Chris Dixon said that leeway in the repair schedule means it is still possible to resume tunneling at the end of March 2015, as originally planned.
"It was an aggressive schedule," Dixon said referring to the repair timeline. "We essentially had three months on the tail end for testing and commissioning, there is some float in there."
Before the approximately 83-foot wide, 120-foot deep circular shaft can be excavated, its perimeter needs to be lined with an underground wall of concrete “secant piles.” The cylindrical piles are up to 10-feet in diameter. Building the wall involves sinking a series of “primary” piles, spaced a few feet apart from one another. After the concrete on the primary piles hardens, a second overlapping set of piles is put in place to fill in each of the gaps.
A conceptual illustration of the repair shaft, with the modular lift tower that will be used to raise parts of the front of the machine. Image: WSDOT.
According to Dixon, the delay is arising because the subcontractor constructing the wall, Malcolm Drilling Company, Inc., has had to spend extra time busting away concrete on the primary piles in order to make space to construct the secondary piles. To create room for the secondary piles, Malcolm is using a crane to drop a giant-sized chisel attachment onto the concrete.
"It's adding a step to the operation," Dixon said.
So far, he said, 53 of the 84 piles that will form the wall are in place.
Dixon emphasized that there are no problems with the engineering or the design of the shaft, which is being constructed between the Alaskan Way Viaduct and Pier 48, just south of South Main Street. Groundwater and water from Elliott Bay flows into the soil in the area, which includes glacial till and soft fill.
Washington State Department of Transportation program administrator, Todd Trepanier said the agency had not seen signs that Seattle Tunnel Partners would miss the March restart date, but still expressed some skepticism about the timeline for the repairs.
"We know that STP is attempting to build something here that is very sophisticated and very complicated, but that is what they signed up for," he said "Our concern is slightly elevated whenever you start using the float in the schedule."
Bertha is currently stopped 1,023 feet along the planned tunnel path, which runs about 9,000 feet between Sodo and South Lake Union.
The machine began to experience problems early last December. Subsequent inspections revealed that a set of seals that protects the machine's main bearing had failed. The main bearing allows Bertha's 57.5-foot cutter-head to spin and mine through the earth.
Seattle Tunnel Partners and Hitachi Zosen Corp., the machine's manufacturer, plan to replace the seals and the main bearing. They are also going to strengthen the front of the machine with 86 tons of steel ribs and other reinforcement parts. To complete the work, they will drive Bertha forward into the access shaft and then remove the cutter-head and other components, which all together weigh nearly 2,000 tons.
Like what you just read? Support high quality local journalism. Become a member of Crosscut today!