The Big Dig (search) highway tunnels under downtown Boston are structurally sound and safe for traffic despite recent leaks, the Federal Highway Administration (search) said Monday.

The agency launched the investigation into the Interstate 93 (search) 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 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 the Big Dig itself "is structurally sound and remains safe for traffic," it said.

The report also urges the Turnpike Authority to work quickly to complete an inspection of the tunnel walls and create an inspection program to detect any future leaks.

Representatives of the agency met this morning with officials from the state Highway Department and the Massachusetts Turnpike Authority (search) to brief them on the findings, which were later posted on the agency's Web site.

The report also looked at drips near the ceiling joints in the tunnel. Those leaks are of a lesser magnitude than the problems with the tunnel walls but are still unacceptable, according to the report, which praised what it said was the project's "methodical approach" to sealing all ceiling leaks by the end of September 2005.

The report also looked at different permanent repair proposals for the wall panel where the September breach occurred. Those proposals include cutting away and replacing the damaged concrete sections of the wall or injecting grout into the damaged sections and welding steel panels over them.

The report said it was "confident that any of the proposals would provide an adequate repair" provided that it is structurally sound, is a durable and effective barrier against moisture and will be relatively maintenance free.

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, and the project will not be finished until later this year, seven years behind schedule.