Design of Seismic Joint for San Francisco Bay Tunnel

A prefabricated tunnel was constructed under San Francisco Bay for the Bay Area Rapid Transit System. The tunnel is in an area subject to earthquakes, and a flexible sliding joint was developed to permit differential movements between the tunnel sections and the ventilation buildings they were joined to. The joint permitted motions up to 6 in. in any direction along a vertical plane and 3 in. in a longitudinal direction. A research and development program of the joint was instituted prior to construction to insure the design assumptions were valid. Four separate joints were constructed for the project. The joints enable the tunnels to function properly in an area subject to earthquakes and also allows for differential settlement between the tunnel and the building.