Bailing Bertha: Water rushes in ahead of cutting face, behind it
Groundwater is now flowing into Bertha’s front end, hindering workers from getting a view of whatever is blocking the Highway 99 tunnel machine’s progress.
Seattle Times transportation reporter
So much groundwater is flowing into the front end of Highway 99 tunnel machine Bertha that officials said Friday it will take two more weeks for workers to dry the area enough to view and measure an object that’s blocking the route.
The world’s largest rotary drill, at 57 feet, 4 inches diameter, has been stuck near Terminal 46 since Dec. 6.
The clog is thought to be a loose boulder, or perhaps a giant “glacial erratic” rock that migrated to Seattle atop an ice sheet in prehistoric times. The piece isn’t secured enough by surrounding soil for Bertha’s cutting tools to dig in and crack the object apart.
Meanwhile, a sealed 5-foot-thick chamber behind the cutter face, where the churned dirt is supposed to collect before entering a conveyor system out the back end, is practically filled by water, mud and rock — which, considering the dimensions of the machine, could be more than 90,000 gallons.
Water flows underground toward the tunnel route both from rainfall on nearby hills and from Elliott Bay.
Six wells have been installed this month to pump groundwater away. Despite removing more than 300,000 gallons to date, the effort barely keeps pace with the deluge still flowing in. Four more wells will be installed.
The state Department of Transportation hopes a 10-well effort can lower the water level inside Bertha, enough for workers to look out at the clog, and maybe even break it.
Failing that, the team would need to pump in compressed air at extreme pressure, to create a bubble that can fend off the incursion of water and wet sand.
Specially trained divers would be sent in, using handheld power hammers and drills to attack the stone, or whatever’s there.
The slow pace increases the possibility of cost overruns in the $1.4 billion contract, to open the four-lane Highway 99 tunnel to traffic by December 2015.
The contract includes a $40 million fund to handle surprises, and asked if this was a $40 million-plus slowdown, project director Chris Dixon of Seattle Tunnel Partners said: “Well, we don’t know yet. We don’t know what the extent of the delay’s going to be, or what the cost associated with that delay is going to be.”
Matt Preedy, deputy tunnel administrator for the state Department of Transportation, said it’s too early to predict whether the project can still be done on time and on budget.
Workers got a quick look through a hatch a couple days ago, Preedy said. “We didn’t see much. We didn’t expect to. Then we had to close it.”
Water was pouring in, and pressure was building in the remaining air near the top of the mixing chamber.
Preconstruction soil studies estimate that where the machine bottom sits, about 110 feet deep, groundwater produces a pressure of up to 2.5 times the normal air pressure up at the surface.
Dixon said he’s confident the team can remove the blockage using handheld tools, built-in drills on the machine and cutting blades on the huge rotary drill. Obstructions occur and are broken apart in tunnel projects all over the world, he said.
Dixon did not rule out the possibility that some object is stuck inside the mixing chamber.
Meanwhile, many workers on the project are taking holiday breaks or finishing other tasks, Dixon said, such as dismantling temporary frames and concrete rings inside the launch pit in Sodo. Bertha used those rings to make an initial push using 56 hydraulic thrusters, out of the pit and into shallow soil during August.
The machine has gone just over 1,000 feet on its 1.7 mile path from Sodo to South Lake Union.
Among other effects, the blockage will postpone an expected two-week closure of the Alaskan Way Viaduct to car traffic, which was to occur in January.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org. On Twitter @mikelindblom