The head of the Massachusetts Institute of Technology's civil engineering department says he doesn't think the public should be concerned about thawing soil that may have caused a massive hole to form under the Big Dig's I-90 Connector tunnel.

Early in the tunnel's construction process more than a decade ago, engineers froze the soil under railroad tracks outside South Station by pumping super-cold saline through underground pipes. Soil-freezing is a common process in construction — although not typically at the 130-foot depths used in this project — because it makes ground more stable for digging. In the case of the Big Dig, it was also done to save money and prevent disruption to rail travel.

But as the soil continues to thaw nine years later, that has led to more settling of the earth than engineers had calculated. The settling has caused rail tracks to sink several feet and a water main to break, according to state Department of Transportation documents.

Transportation officials have not yet been able to measure the exact size of the potential hole. But Andrew Whittle, who heads the civil engineering department at MIT and serves on the board of directors for the state transportation department, says drivers should not be alarmed.

"You don't need continuous solid support for the tunnel to function just fine," Whittle told WBUR's All Things Considered host Sacha Pfeiffer, referring to the possibility that a sinkhole may have developed in the ground beneath the tunnel. "The tunnel is very strongly reinforced with steel. It's got reserves of strength and capacity to handle having voids under it.

"I don't think the public should be concerned about it, to be honest," he added. "I think if there were a concern, the DOT would be acting on it very, very promptly."

Whittle acknowledged that public confidence in the state's transportation leaders was shaken by a fatal 2006 tunnel ceiling collapse and their recent failure to notify the public about problems with corroded light fixtures. But he noted that a "stem-to-stern" safety review of the tunnels was done following the ceiling collapse, and he said the soil-settling issue does not rise to that level of concern.

Still, Whittle said transportation officials should do a thorough review of how engineers arrived at their projections for the rate at which the ground would thaw and shift.

He said the transportation department's board of directors first learned about the settling problem in late winter or early spring of 2010. Since then, the state has hired an independent engineering firm, STV Parsons, to study the situation.

The firm has issued a report that concludes that the tunnel sections in question "still have a capacity greater than the stresses they are experiencing," but that some sections have "factors of safety lower than as specified" by the American Association of State Highway and Transportation Officials.

Whittle reconciled those conflicting findings by noting that there are "margins of safety" built into design code recommendations, and that the Big Dig tunnel design is within those margins. Still, he said he expects the state transportation department to bring the tunnel back up to code.

Whittle said it's unclear whether the greater-than-expected rate of soil-shifting reflects an engineering error made during construction.

"It was an unusual project, a completely unique project, and it had an innovative solution to enable the construction," he said, referring to the unusually deep freezing of the soil.

"I mean, you could turn around and say that you learn from experience when you do new things for the first time, so you could turn this around and say this is the price we're paying for an innovative construction technique."

So far, about $15 million has been spent on monitoring and mitigating the effects of the settling soil, and another $10 million has been budgeted for anticipated repairs, according to state transportation department documents.