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Seattle tunnel progress clears way for viaduct demolition

Demolition of the SR99 Alaskan Highway Viaduct in Seattle, US is set to start this evening as work to open the long-awaited replacement tunnelled route nears completion.

Work to build the 3.2km tunnel by Seattle Tunnel Partners – a joint venture of Dragados and Tutor Perini – was hit by a number of technical issues starting in December 2013 when the tunnel boring machine (TBM) was halted for two years.

The project was hailed as a landmark scheme with the tunnel allowing traffic to be diverted away from surface and demolition of the seismically substandard viaduct clearing the way for new open space on Seattle’s coast.

The viaduct was built in the 1950s and seismic design is now light years ahead of what we knew back then,” said Washington State Department of Transport deputy administrator of the Alaskan Way Viaduct Replacement Program David Sowers. “Ultimately, replacing the viaduct is all about keeping people safe and this new tunnel meets the gold-standard for safety.”

The viaduct’s vulnerability to earthquakes was the biggest motivation for its replacement. The new tunnel is designed to withstand a 2,500 year earthquake – roughly a magnitude 9 off the coast of Washington state.

Work on the tunnel started in June 2013 and was due to open in spring 2018 but the TBM issues resulted in delays. Nonetheless, Seattle Tunnel Partners said that the delay was partly mitigated by design changes that reduced the originally planned construction period.

The Hitachi Zosen TBM – named Bertha – had to be brought to surface from a rescue shaft to allow modifications to both seals and the cutterhead to be carried out and did not restart work until December 2015.

The tunnel itself is set to open in early February but work on demolishing the viaduct it replaces is now set to start over several phases.

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