The machine successfully tunnelled through complex soils only 15ft below the viaduct’s foundation – the closest the TBM will come to any structure at any point in its drive. Structural engineers with WSDOT completed a thorough inspection of the viaduct and confirmed continued stability of the ground and the viaduct.

WSDOT said it temporarily closed the highway so crews could more closely monitor the structure. The tunnel team originally planned to keep the highway closed until after the machine had completely cleared the viaduct, but the success of the tunnelling operation and the continued stability of the ground led to discussions of an early opening as work progressed.

Contractor Seattle Tunnel Partners estimated that tunnelling beneath the viaduct would take approximately two weeks.

"Closing a major highway is never easy, and the public deserves a big thank you for their patience and flexibility while this crucial work took place," said Washington governor Jay Inslee. "I would like to thank the WSDOT and STP project teams and construction crews on a job well done. To finish this piece of the project almost a week early is commendable."

On May 18 WSDOT announced the TBM had installed 300 concrete tunnel rings and mined more than 2,000ft from the launch pit, more than 20 per cent of the drive.