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Largest US double-deck highway tunnel opens after two-year delay

seattle tunnel

The largest double-deck highway tunnel in the United States has opened after almost two decades of planning and construction.

The opening of the £3.3bn SR99 tunnel in Seattle was delayed by two years after the tunnel boring machine (TBM) became stuck after overheating.

Despite initial expectations that work undertaken by the TBM would take just two and a half years, the machine had to be stopped in December 2013 after only 305m of work had been completed.

The main bearing seal had to be replaced, and it was decided to excavate to 37m depth to the TBM, raise the cutter head to the surface, and carry out repairs. The entire process took  two years, with tunnelling recommencing in December 2015. 

double decker tunnel hitachi zosen

Source: Hitachi Zosen

The TBM had a 17m diameter and was 99m long.

The tunnel is 2.8km long and tunnelling finished exactly 64 years to the day since the Alaskan Way Viaduct it is replacing was first opened to traffic.

Seattle Tunnel Partners, a joint venture of Dragados USA and Tutor Perini, was responsible for building the tunnel. Hitachi Zosen Sakai Works in Japan built the TBM, known as Bertha.

The SR99 tunnel in Seattle, Washington, carries two traffic lanes plus a safety shoulder in each direction.

The decision to build the £3.3bn tunnel followed the Nisqually earthquake which almost triggered the collapse of the Alaskan Way Viaduct in 2001.

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