North Dorchester CSO Tunnel

Location: Boston, Massachusetts

Owner: MWRA

Completed: 2011

ND CSO Tunnel 2008

Eldon Abbott served as Project Manager for the design of a 17-ft diameter soft ground tunnel for the MWRA for storage and pumping capacity to enable the permanent closure of seven combined sewer outflows (CSO) into North Dorchester Bay within the Boston Harbor.  The CSO tunnel was driven in soft ground consisting of mostly glacial outwash sands and gravel and glacial marine clays, primarily Boston Blue Clay. This 11,100-ft long, $150 million tunnel has a crown depth of 25 to 55 feet below ground surface and was constructed using an earth pressure balance (EPB) tunnel boring machine (TBM) and precast concrete segmental gasketed, bolted linings. The proximity of the Atlantic Ocean and the presence of hydrocarbon pollutants from neighboring petroleum storage facilities required limiting drawdown and water infiltration. Connections between the existing CSO outfalls and the new tunnel consists of helicoidal-ramp type drop shafts to minimize both the need for deaeration chambers as well as the length of connecting pipes between the tunnels and drop shafts.