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A US$3.4 billion tunnel, set to replace an elevated highway in Seattle, Washington in the US, has been completed, some 20 years after the project was first mooted.

At a celebratory event, the Washington State Department of Transport’s project administrator, Joe Hedges, said, from a tunnelling perspective, the project to replace the Alaskan Way Viaduct was an engineering feat akin to “going to the moon”.

The new 3.2 km tunnel has an 18.3 m diameter – large enough to house a two-storey two-lane road.

Hedges said the tunnel was “a first of its kind,” adding “a tunnel this diameter has never been bored in soft dirt before”.

With the tunnel complete, 85% of the project has been undertaken, and only the Dearborn Street off-ramp bridge is required before the viaduct highway can be torn down.

Speaking about the bridge, Hedges said, it was “not just designed to stay standing and safe during an earthquake but to survive an earthquake.”

He said new technology and the use of titanium rebar and flexible concrete made the bridge more durable, adding, “This is a prototype that will survive and endure.”

The cost of the entire project is estimated at US3.4 billion. The SR 99 TBM (tunnel boring machine), nicknamed Bertha, will now be demolished.

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