Caltrans shut down the Webster Tube from Oakland to Alameda on Sunday evening after a 2-inch-thick chunk of concrete fell into the tunnel.

An 8-foot-by-8-foot chunk of decorative concrete fell onto the roadway in the tunnel under the Oakland Estuary at about 6 p.m. Sunday, said Caltrans spokeswoman Lauren Wonder. Nobody was injured, she said, and the debris was found by a Caltrans worker before any cars hit it.

Engineers were examining the entire tunnel Sunday night and hoped it would be open in time for the Monday morning commute, Wonder said. Although the chunk that fell was heavy, it was not structurally important, Wonder said.

"The integrity of the tube is fine," she said.

The incident came three days after a bus accident shut down the neighboring Posey Tube to Oakland.

Reach Matt Krupnick at 925-943-8246 or mkrupnick@bayareanewsgroup.com.