Sinkhole discovered under Big Dig tunnel

Officials say until they can fill the void, the tunnel will span it without danger

August 11, 2011

A void has formed under the I-90 tunnel in Boston, the Boston Herald reported, caused by the clay soil settling more than expected. The sinkhole is at least 4 ft deep and up to 190 ft long, formed about 9 ft under the tunnel’s roadway and has filled with water.

The tunnel is still safe, according to Frank DePaola, acting highway administrator for the Massachusetts Department of Transportation (MassDOT), and Richard Davey, the newly appointed transportation secretary.

“We would not be letting cars go through there, or more specifically trains over those tracks, if there were any issues whatsoever,” Davey said.

DePaola said the problem was actually discovered four years ago. He explained the situation to the MassDOT board of directors yesterday as part of his effort to evaluate all of the many problems the Big Dig has experienced and disclose them all to the public.

The sinkhole formed in an area where MassDOT’s contractor performed the world’s largest “frozen-ground” construction operation. Chilled saltwater was pumped into the ground to make it hard enough to hold its shape while crews dug the tunnel. MassDOT’s consultants are now studying the settlement problem, DePaola said, but so far are at a loss to explain why the ground is settling so much.

Unexpectedly large settlement also has taken place under a 345-ft drainage pipe under the tunnel’s ramp, according to the Herald, and under eight sets of commuter rail and Amtrak tracks. The drainage pipe was damaged and had to be repaired at a cost of $1.2 million. The rail tracks have had to be repeatedly restored to their intended height with more ballast underneath.

Massachusetts expects to solve the sinkhole problem by drilling under the tunnel and filling the void with concrete after the ground finishes settling in 2014. Until then, officials said, the tunnel will span the void like a bridge without any danger.