TBM breaks through on Seattle water storage tunnel

The 5.4m diameter EPBM has tunnelled 4.25km from Ballard to Wallingford, where it emerged in a flooded shaft to prevent groundwater flowing in. Once MudHoney has installed the last few tunnel segments, the shaft will be emptied and crews will dismantle the TBM.

When the 2.4m diameter remote-controlled TBM Bertha, which dug the conveyance tunnel, completed work earlier this year, concrete was injected around the shaft to prevent the flow of groundwater but for TBM MudHoney the limited working space meant flooding the shaft was a better solution.

Excavation of a second conveyance tunnel is due to start later this year.

Seattle Public Utilities and King County Wastewater Treatment Division’s Ship Canal Water Quality Project is designed to stop 284 million litres of stormwater and sewage flowing into the Lake Washington Ship Canal, Salmon Bay, and Lake Union each year. During a heavy storm, the new tunnel will capture and temporarily store more than 110 million litres of untreated stormwater and sewage until the treatment plant can accept it.

Along the 4.25km tunnel path, there are five vertical shafts at Ballard, East Ballard, Fremont, Queen Anne, and Wallingford that will collect stormwater and sewage flows from each basin and send them approximately 12-25m below ground into the new storage tunnel.

The conveyance tunnel under the Ship Canal connects the vertical shafts in Queen Anne and Fremont. For the final piece of the project , new pipes will be installed to connect the existing sewer systems to the new shafts.