The North Dorchester Combined Sewer Overflow (CSO) Tunnel is a 17-ft-diameter soft ground tunnel for the MWRA to provide storage and pumping capacity for the collection of overflows from seven combined sewer outfall pipes into North Dorchester Bay within the Boston Harbor. This was part of the federally mandated Boston Harbor cleanup project. 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. Eldon Abbott, PE served as the Project Manager for the design of the tunnel. Connections between the existing CSO outfalls and the new tunnel consist 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.