North Dorchester Bay CSO Tunnel Project

Location: South Boston, Massachusetts
Contract Value: $147,531,347
Project Dates: August 2006 – August 2009


Barletta was a joint venture partner on the Massachusetts Water Resources Authority’s $148 million CSO Storage Tunnel project.

The North Dorchester Bay CSO Tunnel is designed to temporarily hold up to 19 million gallons of combined sewer and storm water overflows caused by heavy rainfall.  For decades, combined sewer overflows have discharged at six outfalls along South Boston’s public beaches.  This project eliminated CSOs to the Bay, protecting swimming beaches, shell fish beds and other sensitive waters.

A 350 ton custom-built tunnel boring machine drilled and lined a two mile long, 20 foot diameter tunnel through glacial outwash sands and Boston Blue Clay at depths of 30 to 50 feet below K Street, Columbia Road, Day Boulevard, and the M Street, L Street, and Carson public beaches in South Boston.

The machine was lowered into place through a 55-foot-deep, 50-foot-diameter mining shaft at the starting point alongside the Massachusetts Port Authority’s Conley Container Terminal near Castle Island.  It was removed through a 40-foot-deep, 34-foot-diameter receiving shaft drilled at the end point of the tunnel, behind the Massachusetts State Police barracks.

The TBM typically traveled 88 feet a day, and completed the mining and lining of the tunnel 6 months ahead of schedule.  Installation of the five horizontally drilled connections between the tunnel and the diversion system drop shafts at the existing outfalls was done, and the concrete lining and roof structures at the mining and receiving shafts were completed.

An average of 25 people worked underground and 30 people above ground at any given point during the construction.