Construction crews have plugged hundreds of Big Dig leaks, but the tunnel system is still on pace to collect and pump out the same amount of water this year as it did in 2006, according to a study presented to the Massachusetts Turnpike Authority board yesterday.
The number of leaks is down to about 650 from a high of several thousand a few years ago, and officials expect to finish plugging the rest by the summer. But the continued flow of water - about 24 million gallons in each of the last two years - points to the complexity of the leak problem in the $15 billion road project, according to a key outside consultant.
"It's an ongoing, evolutionary thing, where you just keep peeling down the onion," said Robert R. Rooney, state deputy secretary for public works, who headed a review of the tunnels initiated after a fatal ceiling collapse in 2006.
Leaks in the Big Dig have been a prominent public concern since a September 2004 breach in the Thomas P. O'Neill Jr. Tunnel wall sent water gushing into northbound lanes at a rate of 300 gallons per minute for several hours. The Globe reported in July that the tunnel continues to collect about 2 million gallons per month.
Yesterday's report is part of a "stem to stern" review initiated after the July 2006 ceiling collapse, which killed 38-year-old Milena Del Valle of Jamaica Plain. The authors plan to release a final report in July. The contract with Bechtel/Parsons Brinckerhoff to build the Big Dig required that the tunnels have no leaks.
But Rooney and consultant Andrew E.N. Osborn, an engineer with Wiss, Janney, Elstner Associates Inc., say there will always be leaks in this type of tunnel system - hopefully no more than a few dozen at a time once the situation is under better control. The key is finding them and plugging them, making sure they are not near electrical rooms or where they might produce dangerous icing conditions.
If the leaks are not regularly found and plugged, "it could have long-term degradation on the steel," leading to a safety risk, Rooney said.
Osborn said one ramp that is part of the Turnpike Interchange with Interstate 93 is seeing deterioration problems as a result of old leaks. The report says leaks are responsible for 40 percent or more of the water that must be pumped out. Rain water is the second-biggest factor, leading to a seasonal difference in the amount of water that needs pumping.
Osborn said the fact that water volumes are equal to last year demonstrates progress because this was a rainier year. He said comparisons between dry months, not listed in the report, show the amount of water collected from leaks is "clearly going down."
Guidelines developed for other tunnel systems would allow the Big Dig to handle four times as much water as it collects without risking public safety, according to the report. But Rooney and Osborn point out that the Big Dig is required to meet higher standards because of the contract with Bechtel/Parsons Brinckerhoff.
Rooney recommended that the Turnpike Authority board adopt a "water management program" that would include continued spotting and plugging of leaks and better measurements of the water being flushed out by the Big Dig's pumps.
Bernard Cohen, the state transportation secretary and chairman of the Turnpike Authority board, said the leak situation is improving "but we do have some significant risks to both structure and electrical systems, long-term."
Noah Bierman can be reached at nbierman@globe.com.
