The US state of California has begun tunnelling works on the fourth bore of the Caldecott Tunnel.

Tunnelling work on the $420m project, which is located on State Route 24 between Oakland and Orinda, is being delivered by main contractor Tutor-Saliba.

The new 3,389ft bore will add two freeway lanes north of the existing three Caldecott Tunnels to relieve congestion at the traffic choke points.

The project is expected to be complete by 2013 and will also include two smaller projects to meet the traffic flow.

The Bore Project is a partnership between the California Department of Transportation, the Contra Costa Transportation Authority and the Alameda County Congestion Management Agency.