Fed report casts doubt on Big Dig 'sinkhole'

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 and was caused by the settlement below a portion of the tunnels where deep ground freezing occurred," the report said.

Even if there is a void under the box, it poses no danger to drivers, according to state and federal officials who 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.

State Highway Administrator Frank DePaola said several borings have been taken adjacent to the concrete box housing the tunnel and none has indicated a gap in the soil that might point to a bigger void.

DePaola said he launched the investigation because the settling of soil adjacent to the tunnel was greater than engineers had anticipated when the tunnels were built.

The soil, mostly clay, expanded by about a foot when it was frozen but in one area has sunk by about 8 feet, he said.

"We had been noticing a lot of settlement at the ground surface around those tunnels," DePaola said.

To be sure the tunnels weren't under stress, DePaola said he ordered his own investigation. He said he's waiting for a final report, which he hopes may have answers to why the clay settled more than anticipated. But he also said there's no evidence of a void under the tunnel.

The Federal Highway Administration report also concluded that the tunnels are safe.

"The team found that the tunnels are not showing any signs of excessive stress or other indications that might suggest structural damage or progression toward reduced motorist safety or performance," the report said.