BOSTON – A new report by federal highway officials is casting doubt on whether there’s a sinkhole under the Big Dig.

State officials had expressed concerns this summer that a sinkhole might have opened under the massive concrete box that forms the outer casing of the Interstate 90 Connector Tunnel, a key element of the $15 billion network of bridges, highways and tunnels in downtown Boston. But a report by the Federal Highway Administration released Wednesday says investigators couldn’t determine whether a void exists.

The report — based on a review of documents detailing the construction of the tunnel, post-construction monitoring results and on-site inspections — concluded that there was a large amount of settling of soil near the tunnel. That’s due to the fact that the soil around the concrete box housing the tunnel had to be frozen during construction. Some of that freezing reached down as much as 140 feet. As the soil thawed, it contracted.

What officials don’t know is if there is any significant settling of soil underneath the concrete box. “The team could not discern whether a void, which has been inaccurately referred to as a ‘sinkhole,’ exists,” the report said.

Even if there is a void under the box, it poses no danger to drivers, according to state and federal officials. They said the construction of the tunnel was intended to withstand the potential settling of soil underneath the tunnel.

The state is conducting its own investigation.

 

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