Sinkhole Opens Up Beneath Big Dig Tunnel

State transportation officials say a massive 4-foot-deep hole has opened up about 9 feet below the road surface in a Big Dig tunnel in Boston, but poses no danger to drivers.

The roughly 190-foot-long sinkhole that has filled with water was caused by settling of the clay around the tunnel.

During construction 11 years ago, contractors chemically froze the soil so it wouldn't collapse as they dug the tunnel connecting the Massachusetts Turnpike with the Ted Williams Tunnel under train tracks.

Engineers expected settling, but it has happened faster than expected.

"All of this causes wear and tear that we have to stay on top of," said newly named Highway Administrator Frank DePaola, referring to the fact the structure handles thousands of cars a day. "We have hundreds of people all day long, working seven days a week, maintaining these tunnels in a safe state of good repair."

It's expected to cost $15 million to fix the problem, but not until the ground finishes settling by 2014.

The $15 billion Big Dig highway project has been plagued by cost overruns and construction problems.

With reporting from The Associated Press and the WBUR Newsroom