Thawing land has some unsettled

For Big Dig, soil was frozen; as it warms, costly steps required

By David Abel

Globe Staff / February 5, 2010  Article courtesy of The Boston Globe

It was among the more daunting feats required to complete the Big Dig.Fourteen years ago, as engineers sought to extend the Massachusetts Turnpike to the Ted Williams Tunnel, they froze large swaths of the clay soil that surrounds South Station, allowing tens of thousands of rail riders to continue using the trains running through the area.

Since they completed the project in 1998 and contractors stopped pumping a chilling solution through 1,600 vertical pipes in the ground, the soil has steadily thawed. As a result, the ground has been shifting, and railroad officials have spent millions of dollars ensuring that the tracks do not become misaligned, an expenditure they said they anticipated.

This week, MBTA board officials voted to extend for two years a $346,000 contract that will allow contractors to closely monitor how the ground shifts and whether it moves the tracks. The MBTA has already spent more than $4 million on the project.

“There are no safety concerns now,’’ said Frank DePaola, the MBTA’s assistant general manager of design and construction. “It would only be a concern if we left it unaddressed and didn’t pay attention to it. If we left it unaddressed, the track would settle and go out of alignment, and it could disrupt railroad operations coming in and out of South Station.’’

To guard against buckling rails or depressions beneath the tracks, inspectors walk the tracks daily and take measurements of the ground and the rails monthly. They fix problems as they arise, including repairing tracks and grading the ground.

“It’s not something you come and discover one morning,’’ DePaola said. “It adds up cumulatively over time.’’

He said surveys have shown that the ground in the tunnels has moved several inches since the project ended. He said it is likely to move several more inches before the thawing stops sometime in the next decade.

“Ninety percent of the expected settlement has already occurred,’’ DePaola said. “We still have 10 percent of the settlement left, and we’ll continue to monitor the tracks.’’

But members of the board that oversees the MBTA said they did not expect that the ground would continue to move after so many years.

“It was just a surprise to me that this remains a persistent problem,’’ said Andrew J. Whittle, a professor of geotechnical engineering and one of five MBTA board members. “This certainly is a cost that wasn’t anticipated. I would say it’s a problem associated with construction methods.’’

Andrew Paven, a spokesman for Bechtel/Parsons Brinckerhoff, which oversaw the project, said last night that responsibility lies with other contractors who performed the work.

Whittle said he would have expected the thawing issues to continue for no longer than a decade, rather than the two decades MBTA officials anticipate.

“It’s gone on longer than anyone expected,’’ he said.

MBTA officials said the money being used to pay for the continuing maintenance is coming from the $350 million Big Dig settlement fund, which the state attorney general’s office negotiated with contractors responsible for the persistent cost overruns and repairs.

Joe Pesaturo, an MBTA spokesman, said the maintenance has not affected service.