© The Associated Press / Michael DwyerMass DOT officials say the problem is in the same connector tunnel where Milena Del Valle was killed in 2006 when a three-ton slab of concrete fell on top of her car.
A massive "sinkhole" at least 4 feet deep and up to 190 feet long has created a "void" under the Interstate 90 connector tunnel in the problem-plagued Big Dig, adding to a growing list of engineering failures that include a deadly ceiling collapse, dangerously unstable light fixtures and lethal railings, state transportation officials revealed yesterday.
Transportation officials insist there is no risk of a new catastrophic collapse in the tunnel - the same one where Milena Del Valle was killed by a 3-ton slab of concrete five years ago - but some critics said they aren't convinced.
"You hear this and think, 'What's coming next?' " asked Rep. Peter Durant (R-Spencer), a Transportation Committee member calling for a probe of the latest Big Dig fiasco. "This is just another in a long line of problems for this project."
The gap 9 feet under the tunnel's roadway, filled with water and resembling a subterranean pond, was caused by an unexpected degree of settling of the clay soil around the tunnel as a result of a pioneering technique called "ground freezing" while contractors dug under the train tracks serving South Station.
"It's the settling of the dirt, so it's a sinkhole," Frank DePaola, acting highway administrator for MassDOT, told the Herald.
But he and Richard Davey, Gov. Deval Patrick's newly appointed transportation secretary, insisted the tunnel is safe.
"We would not be letting cars go through there, or more specifically trains over those tracks, if there were any issues whatsoever," Davey said.
DePaola said he outlined the problem, discovered four years ago, to the MassDOT board yesterday because in his new job he is evaluating all the Big Dig issues and wants to share them with the public.
The state expects to spend at least $15 million to repair the problem by drilling the area and filling it with concrete, once the ground finishes settling by 2014. It already has spent $1.2 million on a new pumping system after a tunnel ramp's sagging, 345-foot drainage pipe was damaged after sinking 8 feet.
Eight sets of commuter rail and Amtrak tracks also have had to be repeatedly raised with ballast as the ground below has dropped 8 feet - twice what engineers originally predicted it would sink when they proposed in 2000 to stabilize the ground above the tunnel while they excavated, by freezing it to a depth of 130 feet, DePaola said.
It was the world's largest frozen-ground construction project at the time, what one engineer involved called "uncharted territory." The general contractor built a "freeze plant" to pump chilled saltwater into the ground to fill "the fresh water in pores of the soil and essentially create a big iceberg," DePaola said, adding that consultants evaluating the problem are at a loss to explain why the area has settled so much.
"That is a very good scientific question and I am going to leave it up to the PhDs from MIT to conclude that. Essentially, the clay is retracting more than it swelled when it was frozen," DePaola said. He said engineers took settling into account when designing the tunnel's steel-reinforced concrete tubes.
"The tunnel itself acts like a bridge when it passes over that void, no matter how big it is," DePaola said, asserting the tunnel poses no threat of collapse.
DePaola said consultants assessing the void won't know exactly how large it is until they dig under the tunnel, but they believe it is 4 feet deep and could extend the entire 60-by- 190-foot section of frozen ground. "The consultants concluded that the tunnel can span that with no concern at all," he said.
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