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A section of the Dornan Drive Tunnel wall with damage is seen in Richmond, Calif. on Monday, Feb. 11, 2013. Crews recently discovered significant concrete failure in the tunnel wall, prompting the closure of the tunnel. (Kristopher Skinner/Staff)

RICHMOND -- A project to shore up a tunnel that links Point Richmond with the shoreline hit new snags last week when rock and other debris tumbled down, halting work to bolster the tunnel's concrete lining.

This the second setback in plans to reopen the Dornan Tunnel. City officials initially hoped to reopen in April, then pushed the target date back to June 30 after crews in February discovered large voids above the century-old passageway.

Interim City Engineering Services Director Alan Wolken said Friday he is unsure what the latest setback means for costs or the amount of time the project will take.

"We'll know where we stand with our costs next week, after we complete our analysis," Wolken said.

The City Council in January narrowly approved the project to shore up the tunnel. At first, the estimated cost was $776,000, mostly paid for by federal transportation grants.

But the cost has ballooned to about $1.1 million, Wolken said, since workers discovered hollow areas, including one 12 feet long, 10 feet wide and 4 to 6 feet deep, above the concrete shell. The tunnel lining had deteriorated to as narrow as 4 inches in some spots.

Wolken said $913,000 of the cost is covered by federal grants, with the city picking up about $250,000.

Last week, while contractors were preparing to repair portions of the voids with rebar and concrete, 30 to 40 cubic yards of rock and other debris poured down to the street below, Wolken said. No one was injured.

Wolken sent an email explaining the delay to city officials.

"We have a geologist analyzing the debris to assist with our plans to stabilize the soil movement in order to finish the repair in a safe manner for our workers," Wolken wrote, adding that the rock fall was of an "alarming rate and volume."

Councilman Tom Butt, who lives in Point Richmond, said he regards the tunnel's continuing structural challenges as "not a big deal," but added that residents in Point Richmond routinely inquire about when they can use the tunnel again.

"We've been living with it like this for a long time," Butt said. "Life goes on."

Work teams have been on the site since January. Workers initially reduced the two-lane tunnel to one lane during construction, but the tunnel was closed in early February after concrete at the top began crumbling to the ground.

The tunnel links the commercial and residential areas of historic Point Richmond with the shoreline areas of Keller Beach Park and the Brickyard Cove and Seacliff neighborhoods.

The original project aimed to repair cracked areas of the tunnel lining, add carbon fiber strips to strengthen the lining and install a new drainage system.

While the tunnel is closed, motorists and cyclists must detour via Washington Avenue to Western Drive or via Canal Boulevard and Seacliff Drive, which run through the Brickyard Cove and Seacliff neighborhoods. Point Richmond residents have for months expressed concerns over increased traffic on Washington Avenue, a narrow residential street through the quaint historic district.

Contact Robert Rogers at 510-262-2726 and follow him at Twitter.com/roberthrogers.