Borings begin to study Big Dig sinkhole near South Station

State contractors are burrowing the first of four 135-foot-deep holes near South Station to remove earth that will provide clues about the cause and size of a possible sinkhole beneath one of the Big Dig tunnels. The earth samples should also provide information about how to plug the cavity.

During the summer, state highway officials said the void could possibly be as large as the tunnel itself — but that does not mean they believe it is anywhere near that large, the state’s top highway official said today.

“There’s been a little bit of an exaggeration,’’ said Frank DePaola, administrator of the Department of Transportation’s Highway Division, standing on land above the tunnel. “We don’t even know that there is a hole. We know that the ground has settled.’’

To construct the Interstate 90 Connector Tunnel linking the end of the old I-90 through South Boston to the Ted Williams Tunnel, Big Dig contractors a decade ago chemically froze the earth on the downtown side of Fort Point Channel, to keep the ground from caving and leaking, and to allow MBTA and Amtrak trains to continue running overhead.

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Engineers expected the ground to settle slightly from thawing but were startled to discover that the ground between the tunnel and the railroad tracks was settling at about an inch a month, or about eight feet over eight years, which they filled with gravel.

Additional study suggested the thawing was even more widespread and may mean the soil has thawed all the way to the bedrock, well below the highway tunnel. The Department of Transportation publicly disclosed that issue over the summer, amid controversy over the speed and thoroughness with which it had revealed the extent of a separate problem afflicting tunnel light fixtures.

“We decided we would just make public all the information that we have on the latent issues remaining from the Central Artery/Tunnel,’’ said DePaola, referring to the formal name for the Big Dig.

The thawing could mean that a gap has opened deep below ground running the length of the tunnel — that is unlikely, but even then the tunnel would hold, because it is built like an underground bridge, DePaola said. A state consultant and a separate federal review affirmed the tunnel’s safety, he said.

Now the state is boring four five-inch wide core samples 135 feet down to try to determine the cause and extent of any settling as well as how much grout should be pumped in to solidify the thin or missing soil below and around the tunnel.

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Contractor New Hampshire Boring must first dig through more than 50 feet of surface fill before reaching the layer of Boston Blue Clay soil, where continuous two-foot samples will be extracted until striking bedrock – technically, Cambridge Argillite – another 80 feet later.

The contractor is digging one hole along the outside of the eastbound barrel of the I-90 connector tunnel and three holes between the eastbound and westbound barrels of the connector. Work on the first hole, in the parking lot behind Boston’s main post office, began last week, and all four borings should be done by mid-November. Laboratory analysis and conclusions should be ready this winter, DePaola said.

He called public confidence the department’s primary concern. “We want to make sure that people are confident, so we will find out what the issue is, and we will treat the soil to give everybody an assurance that there are no latent concerns underground,’’ he said.

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