Stretch of tunnel called full of defects

An inspection by Big Dig officials has found a 1,500-foot stretch of the tunnel near the North End to be the most problem-plagued area of the project, with weaknesses in the tunnel walls that exceed even those in the section of tunnel that erupted in a gushing leak last September.

The North End stretch of the tunnel is so pocked with construction defects -- weaknesses in the walls -- that project officials have brought in a new engineering firm to conduct an independent evaluation.

Big Dig officials late yesterday stressed again that the tunnel is structurally sound. But the discovery has raised fears that new leaks could develop. Even if no leaks occur, repairing the wall defects in the new section will likely mean additional inconvenience for drivers who will have to be detoured onto city streets while the work is done.

The problem area, in the stretch that runs beneath the ramps to the Sumner and Callahan tunnels, is one of the deepest parts of the tunnel, with the roadway located about 90 feet below ground.

''It appears the deepest sections of the Big Dig have problems, and there needs to be an accurate accounting and full disclosure on the problems that are being discovered," state Representative Joseph Wagner, cochairman of the Legislature's Joint Committee on Transportation, said yesterday. ''And, the folks who are responsible for the defects have to be held accountable."

Big Dig officials decided to look more closely at the North End tunnel area in May, after a five-month inspection of the entire Interstate 93 tunnel found that the number of weaknesses in tunnel walls totaled less than 1 percent of the panels in all but two sections of the Big Dig: the stretch under South Station, where the leak erupted last September, and the area near the North End.

The new results appear to have taken Big Dig officials by surprise, with concern focused on the high number of construction defects, which are typically caused by a failure to remove debris when the concrete for the walls was poured. That creates soft spots in the walls vulnerable to leaking from groundwater pressure.

On May 5, a top project official said he believed all the defects in the 1,500-foot North End section had been located. But since then, 25 new defects were found on top of the 35 previously discovered.

''They were genuinely taken aback," a construction official close to the project said.

Big Dig officials want the new engineering evaluation to determine if there was a design or construction lapse that caused so many defects in the North End section, and which contractor or contractors should be held responsible. As part of the evaluation, engineers are expected to pore over thousands of pages of construction logs and field reports made since 1997, when construction of this section of the tunnels began.

''They'll find out if someone should have known something before this," one official said.

The engineering firm will also likely recommend a repair plan.

Records indicate that of the wall panels inspected so far, 17 percent of those in the North End area contain defects, compared to 11 percent in the section of the tunnel beneath South Station, where the leak sprang last September. The records were posted yesterday on the Big Dig website.

Peter White, president of J. F. White, the managing partner of the joint venture that constructed the North End section of the tunnel, said: ''We are participating with the subcontractor in having an independent structural engineering firm to make a full assessment of the nature of the panel irregularities. We stand by the quality of our work, but would like to point out that slurry walls at that depth and under the conditions we worked in can have deficiencies that must be corrected."

Andrew Paven, a spokesman for Bechtel Parsons/Brinckerhoff, the private contractor that manages the project, said in a statement, ''We are of course fully participating in the full review of the slurry walls on the contract and would never shy from our responsibility."

After the Sept. 15 leak, both Bechtel Parsons/Brinckerhoff and Modern Continental Construction Co., which built the tunnel section where the breach occurred, publicly accepted responsibility for the construction flaw and pledged to pay the costs of repairing the area.

The Massachusetts Turnpike Authority, with the approval of the Federal Highway Administration, has settled on a plan to repair that section which involves patching the wall with a concrete encrusted steel plate, a process expected to take months to complete and involve overnight closures of the tunnel for about 90 days.

It's not clear whether the defects in the North End area will require a similar plan, or something more elaborate.

Mariellen Burns, spokeswoman for the Turnpike Authority, declined to make state project director Michael P. Lewis available to discuss the findings yesterday. But Burns, in an e-mail to the Globe, confirmed the number of defects in the North End section of the tunnel. Burns said the defects ''have no impact on the structural integrity of the tunnel." She also said the newly discovered problems will not affect the project's schedule.

Last week, project officials, after long insisting the project would be ''substantially complete" in January, acknowledged that significant work will continue into January 2006.

Big Dig officials and construction officials stress that drivers should not worry about the structural soundness of the tunnel, because of the huge steel beams that form its skeleton.

But the wall problems indicate that something apparently went wrong when workers poured the concrete between those beams.

Defects occur when workers fail to excavate extraneous materials -- sand, gravel, or debris -- from the trench before the concrete is poured.

That extraneous matter becomes embedded in the 3-foot-thick walls. Groundwater pushing on the outside of the walls can force its way through the weakest sections of the wall, those compromised by extraneous materials.

Such a defect, called an inclusion, caused the Sept. 15 breach and another much smaller one in January 2004.