The Big Dig highway tunnels under downtown Boston are structurally sound and safe despite recent leaks, the Federal Highway Administration said Monday.
The agency began the investigation into the Interstate 93 tunnels after water broke though a faulty wall panel in September, backing up traffic for miles and setting off a new round of criticism of the much-maligned, multibillion-dollar road project. Studies also revealed hundreds of smaller leaks.
The report said the most serious breach "appears to be isolated to a discrete section of the tunnel and primarily the result of poor quality control during construction."
But overall, the Big Dig itself "is structurally sound and remains safe for traffic," it said.
The $14.6 billion Big Dig, formally known as the Central Artery/Third Harbor Tunnel project, is the most expensive highway project in U.S. history.
Its cost was estimated at $2.6 billion when Congress first approved funding for it in the mid-1980s.