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North Dorchester Bay Combined Sewer Overflow Tunnel

In South Boston, the North Dorchester Bay Combined Sewer Overflow (CSO) Tunnel, a 17-foot-diameter storage tunnel of reinforced concrete segmental lining about 2.1 miles long, will result in the elimination of CSO discharges to North Dorchester Bay.

In addition to tunneling by a tunnel boring machine, there were more than 5,000 feet of new storm water piping installed to separate sanitary and storm water flows into the new storage tunnel.

WSP USA designed the tunnel as part of a joint venture team. We also assisted the client, Massachusetts Water Resources Authority, during construction by reviewing shop drawing submittals and saw through the construction of the tunnel.

The CSO runs 17 to 35 feet under Day Boulevard, the main road accessing Carson Beach. To avoid interrupting traffic, a cut-and-cover tunnel was ruled out in favor of a bored tunnel.

The project team was worried about running into abandoned seawalls, wells and timber piling supporting existing outfall pipes crossing the tunnel alignment. Extensive probe drilling and historical plan searches helped to set the tunnel profile to avoid these potential obstacles.