Included in the US$570m SCWQP contract is the construction of a 4.32km-long storage tunnel with an internal diameter of 5.74m. It is designed to capture and temporarily store around 132bn litres (29m gallons) annually of combined stormwater and sewage overflows from flowing into and polluting local water bodies, including the Lake Washington Ship Canal, Salmon Bay and Lake Union. Flowing westwards, the tunnel will convey stored flows to the pumping station and from there to the Westpoint Treatment Plant which will finally discharge treated water into Puget Sound.
MudHoney is an earth pressure balance (EPB) TBM with a cutterhead featuring 18 double-disc cutters, 48 scrapers and 16 bucket cutters. It will line the tunnel as it advances with six concrete segments to a ring. Before, during and after excavation, crews will monitor over 200 structures above the tunnel path. The project also includes two smaller tunnels, one to go under the Ship Canal and one in Ballard (to be constructed in 2023).
Lane Construction (the US subsidiary of Italy-based Webuild) is building the US$255m tunnel which is expected to be completed by 2025.
