Leaks in the Big Dig highway tunnel system are more widespread than state officials have acknowledged, and top construction managers had warned as far back as 1998 about problems that could cause failures in tunnel waterproofing, the Boston Globe reported Wednesday.

Earlier this month, independent engineers hired by the state to investigate a huge September leak in the Interstate 93 tunnel (search) said the $14.6 billion Big Dig tunnels, which sit almost entirely within the salty water table underlying downtown Boston, were riddled with more than 400 leaks. Project managers and state officials have insisted that the tunnels are safe for motorists.

Now, documents obtained by The Boston Globe show there are nearly 700 leaks in a single 1,000-foot section of the I-93 tunnels beneath the South Station train terminal (search).

And records show that since early 2001 project managers have collectively signed off on at least $10 million in cost overruns to repair leaks and water damage in the costliest highway project in U.S. history.

The project — formally known as the Central Artery/Third Harbor Tunnel project (search) — replaced the elevated Central Artery of I-93 with tunnels through downtown Boston, connected the Massachusetts Turnpike to Logan International Airport and added the Ted Williams Tunnel beneath Boston Harbor. The final major leg opened less than a year ago.

In September, an 8-inch leak flooded the northbound I-93 tunnel.

The new documents obtained by the Globe show the Massachusetts Turnpike Authority (search), which oversees the Big Dig (search), and the project's private manager, Bechtel/Parsons Brinckerhoff, established a Leak Task Force early in 2001.

An internal memo circulated by managers within Bechtel/Parsons and obtained by the Boston Herald raised concerns in June 1998 that waterproofing in tunnel walls would fail unless leaks were repaired.

When a leak was discovered in 2001, Bechtel/Parsons managers allowed lead contractor Modern Continental to make repairs without removing the defective material that eventually caused the September leak, a state investigation showed.

"Let's keep the focus on the real culprit in this matter — and that is Bechtel/Parsons Brinckerhoff," state Assistant Inspector General Jack McCarthy said Tuesday.

Officials of Bechtel/Parsons did not immediately respond to calls seeking comment Wednesday morning.

Doug Hanchett, a spokesman for the Turnpike Authority, said the agency has made progress in controlling the leak problem, and is working to recoup costs from contractors.