Big Dig workers froze a portion of soil so it wouldn't cave in during construction of the Boston tunnel in 2002, but now that soil has thawed -- and settled almost twice as much as engineers had expected. The thawing should be complete in a few years, when the residual water will be pumped out and concrete poured in. "So far, there has been no indications of stress or strain within the tunnel sections itself," said Acting Highway Chief Frank DePaola.

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