By rosebell
The project was delivered by Lane, the US subsidiary of Webuild Group, to reduce combined sewer overflows (CSOs) into the Lake Washington Ship Canal, Salmon Bay and Lake Union.
Commissioned by Seattle Public Utilities through a partnership between the City of Seattle and King County, the scheme provides additional capacity to manage excess wastewater and stormwater during heavy rainfall.
4.25km tunnel provides 112 million litres of storage
The project’s main component is a 4.25km-long storage tunnel with an internal diameter of 5.7m. It can hold more than 112 million litres of excess flow during storms, before conveying the stored water to a treatment facility for processing.
The tunnel drive was completed in 2023, when Herrenknecht earth pressure balance tunnel boring machine (EPBM) Mudhoney broke through into a flooded shaft in Seattle’s Wallingford neighbourhood.
Herrenknecht TBMs constructed conveyance tunnels
The project also includes two conveyance tunnels, with internal diameters of 2.4m and 2.1m. These were excavated using two remote-controlled Herrenknecht tunnel boring machines.
Together, the storage and conveyance infrastructure will help reduce untreated sewage and polluted stormwater discharges, improving water quality and protecting aquatic ecosystems.
According to the project information, the completed system is expected to prevent approximately 284 million litres of polluted stormwater and sewage from entering Seattle’s waterways each year.
