Doubt on cause of Big Dig gap
Some engineers don’t agree with state
As state officials rushed to reassure drivers unnerved by yet another problem in the Big Dig tunnels and Representative Stephen F. Lynch called for a federal investigation, engineers said yesterday they found it hard to believe that thawing soil alone could have caused a sinkhole to open beneath the Interstate 90 Connector.
Joseph Sopko has frozen ground for the construction of the Silver Line tunnel under Russia Wharf in Boston, a giant gold mine in Ontario, and sewage tunnels in Milwaukee. He said the earth around those projects has never settled by 8 feet, as it has around the I-90 tunnel.
“Those numbers are just way out of line,’’ said Sopko, an engineer who was not personally involved in the I-90 project but who works for Moretrench, the company that did the freezing there. “I did my doctoral dissertation on ground-freezing, and I just can’t see, mathematically, how you can get that kind of compression.’’
David K. Mueller, an engineer and vice president at Moretrench who worked directly on the I-90 project, said 8 feet of settlement was more than he had seen in the 30 frozen-ground projects he has worked on.
“On most of the ground-freezing projects we’ve done, we’ve never seen any settlement,’’ he said.
Workers used chemicals to freeze the ground 11 years ago, allowing them to build the tunnel without the soil caving in. They completed the freezing process in 2002 and said they expected some settlement to occur as the ground thawed. But state officials said the ground has receded twice as much as they anticipated.
Though they say they are not sure what caused the problem, they disclosed Wednesday that the thawing soil has left a sinkhole below the roadway that has filled with water because the area is below the water table.
Engineers have not been able to see the sinkhole because of the tunnel’s remote location, 60 feet below ground, but have estimated that it could be 4 feet deep and up to 190 feet long. They say it poses no threat to drivers because tests show the tunnel could span the gap like a bridge over a river.
They plan to fill the space with concrete once the area is completely thawed, sometime in late 2013 or early 2014.
The state has spent $15 million to date to monitor the situation and has budgeted $10 million more for repairs. The money comes from a $485 million fund set up by the Big Dig’s contractors to avoid liability for the fatal tunnel collapse in 2006, leaks, and other problems on the project.
“We’re confident the tunnels are safe, and they can withstand any of these stresses for as long as necessary and well beyond their service life,’’ said Frank DePaola, the state highway administrator. “We’re not going to let the void stay there for that long just for durability and longevity. But it could. Very similar to a bridge spanning a river, it stays there for 50 to 75 years.’’
Mel Levy, a retired tunnel engineer for the New York City Transit Authority, speculated that the tunnel may not be resting evenly on its base. That, he said, could cause the tunnel to bend and stress.
“The problems are more serious than they’re talking about,’’ he argued.
Lynch said he has asked the Federal Highway Administration to join the state in conducting a thorough inspection.
“Due to the high volume of commuter traffic using this roadway and the public safety issues involved, due diligence requires that we obtain a structural assessment to reassure the public that the situation is safe,’’ the congressman from South Boston said in a statement. “The safety of the public must be our top priority.’’
Engineers have been freezing the earth for excavation projects for more than a century, but the I-90 Connector was one of the largest such projects ever. Officials chose to use ground freezing so they could build the tunnel under the train tracks into South Station without disrupting rail service.
“Ground-freezing is, without a doubt, the most expensive approach,’’ Sopko said. “But it’s for projects that can’t be done any other way.’’
Andrew J. Whittle, a soil engineer at MIT and member of the state Department of Transportation board, said the state needs to find out why the ground has settled that much, so it can hold the proper contractor responsible.
“I can’t see how thawing would produce this movement alone,’’ he said. “There must be something else at play, and it probably is worth investigating.’’
Whittle offered one theory. He noted that workers have had to place 8 feet of rock under the train tracks into South Station to keep the tracks level as the ground there has settled. He said those rocks could be pressing the soil down.
“The fact that there are eight feet of stone on the surface is the piece that people have missed,’’ he said. “Eight feet of stone is a lot of stone and, for sure, that could drive this. “
DePaola, however, dismissed the idea that those stones weigh enough to depress the ground.
Thomas C. Sheahan, a civil engineer at Northeastern, said the soil itself could be a factor because it is mostly Boston Blue Clay.
“From a soil point of view, it loses some of its strength’’ when it is dug up during construction, he said. “This phenomenon of the sensitivity of the clay could be leading to additional settlement than would normally be predicted.’’