State officials reveal gap under I-90 tunnel
Insist that safety is not in jeopardy
State transportation officials revealed yesterday that soil thawing underneath a Big Dig tunnel has created a gap filled with water about 9 feet below the roadway. But they said they are monitoring the issue and it does not pose any risk to the traveling public.
Work crews chemically froze the soil 11 years ago so that it would not cave in as they dug into the ground and built the tunnel, which connects Interstate 90 to the Ted Williams Tunnel near South Station.
Engineers always anticipated that, as the ground thawed over the years, it would contract. However, it has receded twice as much as initially anticipated and then filled with water because the area is below the water table, officials said.
“We would not be letting cars go through there . . . if there were any [safety] issue whatsoever,’’ said Richard A. Davey, the general manager of the MBTA, who takes over as state transportation secretary next month.
“We have to manage the situation, and that’s what we’re going to do,’’ Davey said, speaking after the monthly Department of Transportation meeting. “So whatever it was that was incorrect or mis-assumed back in 1995 [when the tunnel plans were drafted], we don’t know. But what we are focused on is managing the situation.’’
The issue is the latest problem for the Big Dig, which has been under intense public scrutiny since 2006 when a section of the same tunnel, officially known as the Interstate 90 Connector, collapsed and killed a Jamaica Plain woman.
More recently, engineers have had to battle water leaks throughout the system and have had to secure thousands of tunnel light fixtures after one of them fell from the ceiling of the Thomas P. “Tip’’ O’Neill Jr. Tunnel in February. This most recent problem is unrelated to the water leaks in the tunnels.
State officials have spent $15 million so far to monitor the soil thawing above and below the I-90 Connector, and have budgeted $10 million more for repairs. The money comes from a fund set up by the Big Dig’s contractors.
When the area is completely thawed, at the end of 2013 or beginning of 2014, crews will suck out the water from underneath the tunnel and fill the gap with concrete.
Officials said they do not know the exact size of the space underneath the tunnel, because it is 60 feet below ground and hidden below the roadway. They said it could grow in size as the ground continues to thaw and recede over the next two years.
To assure themselves that the tunnel is safe, they conducted tests that assume the worst-case scenario: that the gap is the size of the tunnel itself. Even if the gap were that big - and engineers do not believe that it is - the tunnel would hold, much like a bridge over a river, state officials said.
Officials had already disclosed in February 2010 that soil above the tunnel was shifting as it thawed, creating potential problems for the commuter rail and Amtrak tracks that run into South Station. But yesterday was the first time officials revealed that ground thawing below the tunnel was also a cause for concern.
That ground has already damaged a 345-foot drainage pipe that runs between the eastbound and westbound sections of the tunnel. Several years ago, the pipe fell 8 feet out of alignment, as the ground below it contracted, officials said yesterday. Workers built a bypass system for that pipe, at a cost of $1.2 million, and will need to replace the pipe itself once the soil is completely thawed.
The ground has been thawing since 2002, when work crews finished building the tunnel and stopped pumping saline coolant into the earth. Officials said they do not know whether engineering errors are to blame for the ground contracting more than initially projected.
“It’s another issue that we have to monitor so that we can react if anything does manifest itself,’’ said Frank DePaola, the state’s acting highway administrator. “So far, there has been no indication of stress or strain within the tunnel sections itself.’’
While the damaged pipe has been the biggest problem to date, the receding ground has caused other problems in the area. Crews had to reinforce a high-voltage electrical duct for the Red Line to prevent the duct from sagging, as the ground below it contracts. They also repaved part of the parking lot for the US Postal Service facility near South Station, because the pavement there had buckled.
Officials pledged to continue monitoring the tunnel by conducting monthly inspections of the ground surface, which give them an indication of how fast the soil below is thawing. Inspectors have also been ordered to regularly check the I-90 Connector’s joints, walls, roof, and concrete for any signs of movement.
State officials are paying for the monitoring and repairs by tapping the $485 million settlement that Bechtel/Parsons Brinckerhoff - the firms that designed and managed the Big Dig - agreed to pay in 2008 to avoid criminal charges and civil liability stemming from leaks, the fatal ceiling collapse, and other flaws that plagued the project in the past.