SAN FRANCISCO (KGO) --The federal government is now helping Caltrans find weak spots in the lower level of the Bay Bridge's Yerba Buena Island Tunnel.
Engineers from the Federal Highway Administration used ground penetrating radar to examine the walls and ceilings inside the tunnel. Infrared imaging and Impact-Echo are also being used to test the tunnel.
RELATED: Caltrans finds series of weak spots in Yerba Buena Tunnel
Impact-Echo is when sound waves are used to find voids in the concrete. The work started Tuesday morning. Caltrans says this is the first time engineers are using radar penetration to examine the tunnel since 1936.
Back in February, a chunk of concrete fell damaging a passing car, narrowly missing the driver. Caltrans blamed the weak spots on corrosion caused by rainwater. The agency says there is no immediate danger to drivers.