The North Dorchester Bay and Reserved Channel CSO Consolidation conduits are being constructed for the Massachusetts Water Resources Authority, along with the Reserved Channel Pumping facility to provide the conveyance and pumping capacity to enable the permanent closure of eight combined sewer outflow into the North Dorchester Bay within the Boston Harbor and sufficient capacity to convey flows currently tributary to the Reserved Channel from three CSO outfalls under a 2 year, 24-hour storm. The CSO conduits will be a minimum of 13-foot diameter tunnels constructed in soft ground consisting of mostly glacial outwash sands and gravel and glacial marine clays, primarily Boston Blue Clay. This almost $100 million tunnel project will be constructed using a pressurized face tunnel boring machine and pre-cast concrete segmental gasketed, bolted linings. The proximity of the Atlantic Ocean and the presence of manmade hydrocarbon pollutants from neighboring petroleum storage facilities present the need to limit drawdown and water infiltration quantities. The North Dorchester Bay CSO Tunnel will be approximately 13,000-feet long with tunnel crown depth varying from 25 to 55 below ground surface. The 2,600-foot long Reserved Channel conduit will be the same diameter to allow use of a common machine and will be approximately the same depth below ground surface. Connections between the existing CSO outfalls and the new tunnels are made through dropshafts. The dropshafts will be a truncated helicoidial-ramp type to minimize need for deaeration chambers and long connecting pipe between the tunnels and dropshafts. Tunnels will be constructed from large diameter construction shafts. The upstream ends of the Bay and Channel tunnels will terminate into large-diameter, multi-use access shafts which will be large enough to allow removal of the tunneling machine, and will also function as permanent inspection and maintenance points and provide air venting and odor control during exhaust of air during tunnel filling in a storm event.
