Seattle mobilizes for sewer upgrade
Sep 2011
Paula Wallis, TunnelTalk
- Contractor J.W. Fowler of Oregon is mobilizing to start work on a 2,000ft long tunnel under Seattle's Salmon Bay Bridge in Ballard, linking Lake Washington to Puget Sound (Fig 1). Running below the existing dual siphons, the new tunnel will reduce potential sewer overflows into the Lake Washington Ship Canal.
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Fig 1. Ballard Siphon replacement alignment
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Fig 2. Conceptual profile of the tunnel
- Initial designs called for replacing the existing dual wood-stave siphons, built in 1935, with dual tunnels: one to carry a similar-sized 36in i.d. pipe to handle daily flows and a larger tunnel with an 84in i.d. pipe to accommodate larger storm events. However, following extensive evaluation designers TetraTech & Staheli determined the existing siphons could be slip-lined and remain in service, thereby limiting construction to the single larger CSO tunnel for a potential US$3.1 million cost saving.
- A 2.6m diameter Herrenknecht EPBM is on order to excavate the new siphon, about 60ft beneath Salmon Bay. The new pipe will run from Seattle's Ballard Regulator Station to the North Interceptor in the Interbay.
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Construction of existing wood-stave siphons in 1935
- Owner, King County specified EPBM over horizontal directional drilling and microtunneling methods, because it was deemed the least risky and would have the least impact on local neighborhoods and businesses. In addition to the technical challenges, space is limited by underground utilities, railroad tracks and businesses on both sides of Salmon Bay. Jacobs Associates is providing construction management services on the project.
- The tunnel will be constructed between two shafts, the 140ft deep north shore shaft with a 40ft i.d. and the 100ft deep south shore shaft with a 70ft i.d. and 140ft deep, which is also the launch shaft for the TBM (Fig 2).
- The $32.7 million tunneling contact includes the rehabilitation of the existing siphons. Additional equipment supporting the new pipe will also be installed at the Ballard Regulator.
- The tunneling is scheduled to begin in the summer of 2012, with a projected completion date in late 2013.
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