Seattle tunnels see mixed progress

Seattle has seen good progress on its Sound Transit project with the first TBM run completed through Beacon Hill station and mined excavation at the site now finished. But, elsewhere in the city, support was inconclusive in a recent public vote to replace Alaskan Way Viaduct with a tunnel at the waterfront.

Last month, both the station dig and first drive by the 6m diameter Mitsubishi EPBM at Beacon Hill were finished, and this month the TBM is to start the parallel drive. The drive was interrupted several times, initially to await some platform excavation and then due to investigations into supply train derailments (T&TI, February, p11).

On the Alaskan Way Viaduct, there was a hiatus in planning activity following the public vote in March on the replacement option. Technically, therefore, both options presented to voters - pull down the viaduct and build a tunnel instead, or replace it with a new one - were defeated.

The tunnel was the more expensive option by a notable margin, but late on the city tried to explore a cheaper slimmed-down tunnel design.

The city council and state government have agreed to push on with traffic safety and mobility projects while a decision on how to replace the viaduct is postponed. The bridge has only a limited useful life as it previously suffered seismic damage and remains a quake risk.