Tunnel upkeep records sought

The engineer hired by the state to independently investigate the water leak in the Central Artery's northbound tunnel said yesterday that whether the breach was a one-of-a-kind event or a harbinger of systemic problems in the tunnel walls remains an open question.

Briefed about the leak yesterday by project engineers, Jack Lemley, who has consulted on major tunnel projects worldwide, called for an extensive review of tunnel maintenance records to look for evidence of additional water leaks.

An 8-inch hole opened in the eastern wall of the northbound tunnel Wednesday afternoon, and the water that gushed into the tunnel backed up traffic for miles on the Southeast Expressway.

Yesterday, workers filled the hole with grout and covered the opening with a 6-foot-by-4-foot steel plate, said Sean O'Neill, spokesman for the Massachusetts Turnpike Authority, which oversees the Big Dig. He said the tiled wall panels would not be permanently replaced over the hole until workers are certain the patch is secure.

A top engineer with Bechtel/Parsons Brinckerhoff, the private-sector consortium that manages construction of the Big Dig, has said the hole did not compromise the structural integrity of the tunnel.

Lemley said that to add expertise to the leak investigation, he called in as a consultant George Tamaro, the engineer who designed the foundation walls of the World Trade Center in New York, which held firm even after the towers collapsed in the Sept. 11, 2001, terrorist attacks.

Tamaro was scheduled to arrive in Boston today to join Lemley and others examining the site of the water leak, said Edward M. Ginsburg, a retired state judge appointed by the state last year to head a team reviewing the $14.6 billion project in an attempt to get contractors to refund money to the state for any mistakes they made.

"The leak is not necessarily unexpected in a project as large as this," Lemley said. "But by the same token, you want to be absolutely sure there isn't a systemic problem, and that is what the project team is trying to establish, that it's not an extensive problem."

Asked about his initial impressions, Lemley said: "I know that when it rains, there is more water in the tunnel than when it doesn't rain. When you have an underground facility, you expect some leaks. There is no such thing as a dry tunnel. But one would expect it would be reasonably dry.

"More investigative work needs to be done," he said. "I don't think anyone has reached any conclusions yet."

Engineers for Bechtel/Parsons have said there was an underground layer of sand near the part of the tunnel where the leak occurred. Keith Sibley, director of construction for Bechtel/Parsons, said Wednesday that construction records indicate work crews hit sand when digging the trench for the wall nine or 10 years ago. A few other spots along the 5 miles of tunnels beneath Boston share the geological characteristic, he said.

In a statement about the cause of the leak, Bechtel/Parsons said, "Our preliminary assessment is that soil composition unique to that particular location was a major factor."

Lemley said he agreed with project engineers that the breach was caused by what engineers term "an inclusion," or sand pocket, in the tunnel wall. How that pocket became part of the wall is of great interest to the investigating engineers, he said.

Engineers who build walls as deep as some sections of the Central Artery tunnel, which runs as much as 110 feet below the surface, rely on thick, liquid substances known as slurry. While a crane and clamshell bucket are used to excavate soil, slurry is poured into the trench to hold back the earth on either side. Once the excavation is complete and the trench is filled with slurry, a tube is pushed to the bottom of the trench and concrete is pumped in, gradually displacing the slurry.

In the process, a clump of sand may dislodge from the surrounding earth and become entombed in the wet concrete, according to engineers. Because sand is more permeable than concrete, the pocket attracts water, which then may work its way through the wall. The material stuck in the wall is known as an inclusion.

"Why there was an inclusion, no one is in a position to say definitely," Lemley said. "We are looking at it."

He said project engineers told him they will drill along the outside of the tunnel wall, near the leak area, in the next few days to take readings on the dimensions of the sand layer and to look for indications of other leaks.

"They will probe the outside of the wall to make sure they have a good impression of the implications of whatever the failure was, to set some parameter around it," Lemley said.

He said his investigators would not conduct their own exploratory drilling.

"Our independence will be in the fact that we are watching what they are doing, but not try to duplicate it, because it is too complicated," he said. "We are looking at it independently and will form our own conclusions."

Sean P. Murphy can be reached at smurphy@globe.com.