World’s largest tunnel boring machine halted by a steel pipe
The world’s largest tunnel boring machine (TBM) was brought to a halt under the streets of Seattle, WA by a steel pipe that was buried in 2002 by a research crew for the Highway 99 project, state officials announced.
The TBM, called Bertha, is digging the Highway 99 project. It came to a halt on Dec. 3 when the teeth of the machine hit the steel pipe and knocked a 55-ft fragment of pipe to the surface, the Seattle Times reported.
The Department of Transportation (DOT) could not estimate how much time and money it will take to get the world’s largest drill moving again.
The costs of the delay will be determined later through negotiations between the state and Seattle Tunnel Partners (STP), maybe even a legal dispute.
The $2 billion tunnel budget includes a $40 million risk allowance for repairs and inspections near the front of the rotary cutting face — plus a $105 million general contingency fund to deal with crises. Matt Preedy, deputy Highway 99 administrator for the state Department of Transportation, said some of that money will be consumed, the Seattle Times reported.
The culprit is an 8-in. diameter, 119-ft-long well casing, used to measure ground water for the Alaskan Way Viaduct replacement project, officials said. Back then, a shallow cut-and-cover tunnel was a leading option.
A jagged piece of the pipe appeared after enough groundwater was pumped out of the machine and soil to allow STP crews a view through the cutting face.
The 2002 well site was listed in reference materials provided to construction bidders, as part of the contract specifications.
Chris Dixon, STP’s project director, said the builders presumed the pipe had been removed.
Bertha first encountered the pipe Dec. 3, Dixon said. The clockwise cutting motion pushed part of it above the surface, and fragments showed up in a conveyor system within the machine, he said.
“We saw a pipe come up, out of the ground 6 or 7 ft,” he said. “We continued mining, successfully.”
Workers removed a 55-ft-long piece, he said. For two days, the drill moved along fine, creating what Dixon called “a false sense of security.”
The blockage, which Bertha hit 60 ft below surface, is beneath the area where Seattle settlers dumped fill soil and debris, based on local histories. No metal should be there.
Construction of the four-lane tunnel is three months behind its schedule to be open at the end of 2015.
However, the machine in November was advancing as fast as 50 ft a day, prompting Preedy to say it’s possible to regain time after the steel is removed.
An inspection included only the top 15 feet of the machine because the rest is flooded with mud and groundwater. STP must somehow check the lower 42 ft of the 57-ft diameter face, perhaps with tunnel-trained divers.
Then the steel must be extracted from above, going through unstable soil, or from within the cutter face.
Dixon said contractors are trying to figure out what methods are possible, while protecting worker safety.
