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Soil Sinking Around Big Dig Tunnel

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File Image (Photo by Darren McCollester/Getty Images)

File Image (Photo by Darren McCollester/Getty Images)

BOSTON (CBS) – Engineers say a space has opened up under one section of a Big Dig tunnel, and they can’t fix it.

When engineers built the tunnel system, they froze the soil in some places, to keep it from collapsing.

But now the soil under one section of the tunnel is thawing, and a space is opening underneath.

In parking lot along the South Station train tracks you can see can see the buckling, an impact of the sinking soil around the Big Dig tunnel below.

WBZ-TV’s Karen Anderson reports

Wednesday, the Acting Highway Chief Frank DePaola briefed the MassDOT board on the problem.

DePaola said, “While some ground settlement was expected, the thawing of the frozen soil during the past nine years, has resulted in greater settlement than we originally predicted.”

When the I-90 tunnel was built, engineers froze the ground around South Station to stabilize it. They froze it 200 feed wide by 160 feet long by 130 feet deep.

Now, as it’s melting, the clay soil is settling twice as much as engineers expected, leaving a void below the I-90 connector tunnel. It’s a hole filled with water, and they don’t know how big.

“We don’t think it’s serious at all, it’s another issue that we have to monitor,” says DePaola. “So far there has been no indications of stress or strain within the tunnel sections itself.”

DePaola says the tunnel has not been affected by soil problems, in part because it was built to act as a bridge.

But a major water main pipe was affected. It sunk eight feet and had to be replaced at a cost of 1.2 million dollars. That happened several years ago.

Incoming Transportation Secretary Richard Davey says the tunnels are safe.

Davey says, “We would not be letting cars go through there or more specifically trains over those tracks if there was any issue whatsoever.”

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