Japan-based Hitachi has signed a letter of intent worth $80m with Seattle Tunnel Partners and the Washington State Department of Transportation in the US to supply tunnel boring machines for the $1.35bn SR 99 tunnel project beneath downtown Seattle.

The tunnel boring machines will be 60ft in diameter and more than 300ft long and it will be barged to downtown Seattle and assembled at the south end in early 2013.

The tunnel is being built as an alternative for replacing the seismically unsafe Alaskan Way Viaduct along Seattle’s waterfront.

The project is expected to open to drivers in late 2015.