Milestone Hit in Boston CSO TunnelThe Massachusetts Water Resources Authority (MWRA) has notched another successful tunnel boring operation, its fifth in 20 years, as it completed the North Dorchester Bay storage tunnel on 13th August, 2008. This is a 3.4 km-long storage tunnel, part of a 15-year combined sewer overflow (CSO) programme totalling USD850 million. The MWRA previously has dug three deep rock tunnels but this is the first tunnel of this size the authority has dug through soft rock. The 5.860 mm-wide, 5.2 m-finished diameter, USD148 million North Dorchester Bay CSO tunnel will virtually eliminate closures at Carson Beach in south Boston. The tunnel runs from a 17 m-long, 15.2 m-diameter mining shaft at the downstream end of the tunnel in Massport's Conley Terminal to a 2.2 m-long, 10.4 m-diameter removal shaft at the upstream end of the tunnel near the Bayside Exposition Center.Crews used a soft ground custom-made Hitachi TBM, mostly through Boston blue clay. The 350-tonne machine includes a lengthy train of supporting equipment that followed in back of the tunnel drill face as the boring progressed. The TBM is driven by a power unit that is approximately 12 m long. The end of the boring unit is serviced by two trains of rail-mounted gondolas that stretches for 98 metres underground. The machine erected a 1.22 m-wide, six piece, universal ring. An average of 25 people worked underground and 30 people above ground at any given point during construction. Visit www.hitachi-c-m.com/global/pdf/brochure/current/tunnel/B_Tunnel.pdf The tunnel was designed by Parsons Brinckerhoff/Metcalf & Eddy and built by the joint venture of Shank/Balfour Beatty/Barletta. A team of Hatch Mott MacDonald and Shaw Environmental provides construction management services. Visit www.pbworld.com, www.m-e.aecom.com, www.hatchmott.com and www.shawgrp.comUnder normal conditions, rain and sewage are typically piped to Boston's water treatment plant. But during heavy storms, the rain can overwhelm the sewer system, forcing a mix of runoff and sewage into relief valves that empty into the harbour. High bacteria levels after storms have forced the closing of beaches in South Boston an average of 21 times a year. The new tunnel and pumping station will hold overflows from all but the largest storms. When the new system is in place, engineers will open gates when heavy rainfall approaches that will direct the CSOs to the new North Dorchester Bay CSO tunnel, where it will sit until the storm ends, before being pumped out. The tunnel will hold up to 72 million litres of water during a storm and send it to a new pumping station at Conley Terminal. The pumping station will then push storm water to the Deer Island sewer treatment plant. When completed, the tunnel will virtually eliminate CSOs and stormwater discharges into the harbour and dramatically reduce beach closings in south Boston. The one-year tunnelling job started on 12th September, 2007. On 9th May, the tunnelling portion of the project hit the halfway mark. Tunnel work will conclude this September. Phase 2 (a pump station and an odour control facility estimated at USD35 million) will be let in November, for May 2011 completion. Click us/52. Visit www.mwra.state.ma.us/cso/southboston.htm 36/08.