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May 15, 2008

CSO Tunnel Reaches Halfway Point

     After groundbreaking in September, 2007, the tunnel that is the centerpiece of the Combined Sewage Overflow (CSO) control project is now halfway from the Conley Terminal to its intended terminus near the Boston Teachers Union Hall.  Completion is expected to be on time later this year.


by Rick Winterson

     Not to be outdone by the Big Dig downtown, ground was broken on South Boston’s own tunneling project last September – on Wednesday, September 12, 2007, to be precise.

     The project is officially entitled “The South Boston CSO Storage Tunnel”.  The abbreviation CSO stands for Combined Sewer Outflow.

     When it is completed, our CSO project will clear up a long-term problem with South Boston’s storm drainage and sewer systems.  As of now, severe rainstorms cause the local sewer systems to back up, because the underwater line to the Deer Island treatment plant isn’t large enough to handle combined flows from both sewage and excess storm waters.  As a result, the excess flow has to be dumped into Boston Harbor, contaminating the sea water and forcing beaches to close.  This occurs several times each year.  In addition to being a health hazard, this situation also violates the agreement to clean up Boston Harbor.

     The Court-approved solution was to dig a large, two-mile tunnel from the Conley Terminal to the area directly behind the State Police barracks, near the Boston Teachers Union Hall.  This will intercept and hold excess flow from heavy rainstorms until it can be worked off through the existing pipeline to Deer Island.  At 17 feet in diameter, the tunnel will hold up to 18 million (18,000,000) gallons of combined outflow for later processing.

     The CSO tunnel will actually function as a very large holding tank.  When finished, it will prevent almost all outflows to the Harbor.  These outflows will then occur once every five years or less, instead of several times each summer.  To get an idea of the tunnel’s size, picture a convoy of 4,000-gallon tank trucks parked bumper to bumper along the Mass Pike from Boston to Worcester.  That’s equivalent to the volume the tunnel will hold.

     As of last Friday, May 9, the tunneling portion of the project hit the halfway mark.  One full mile (plus another 70 feet or so) stretches from the Conley Terminal to a point under the greenie where K Street intersects with Columbia Road.  There’s another mile to go, which should be completed around Labor Day.  The $180 million project is “doing fine”, according to Fred Laskey, the Executive Director of the MWRA (Massachusetts Water Resources Authority).

     The MWRA personnel conducted a tour of the mile-long tunnel segment last Friday.  The tour group got a look at the tunnel boring head and associated machinery.  The entire process is highly automated.  Even the large concrete segments that form the tunnel itself are pre-cast and rotated into place with a computerized hydraulic rig.  The tunnel bore is 18 feet in diameter; the finished tunnel has an interior diameter of 17 feet.  In addition to drop shafts at either end of the tunnel, six intermediate shafts, a pump station, and an odor control facility are planned.

     The tunnel is being constructed by members of Local 88 Tunnel Workers, Local 103 Electrical Workers, and Local 4 Operating Engineers.  Design, construction, and field supervision are provided by the following firms:  Parsons Brinckerhoff/Metcalf & Eddy; Fay, Spofford & Thorndike, Inc.; Hatch Mott MacDonald/Shaw Environmental, Inc.; and a Shank/Balfour Beatty/Barletta joint venture.

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CSO below ground shaft
The below ground shaft at the entrance to the 18-foot high CSO tunnel.

The tour members are ready to board the narrow-gauge rail shuttle at the entrance to the CSO tunnel.  City Councilor Michael Flaherty is second from left; Congressman Stephen Lynch, fourth from right.

A view looking back along the mile-long tunnel from the rotary tunneling head.