Readers like Creighton Fung are having trouble wrapping their minds about the new Petaluma River bridge that is being built on Highway 101 as part of the Marin-Sonoma Narrows widening project. They’ve heard about it but haven’t seen much construction.

“Are we getting a new 101 bridge over the river, or are they widening the existing bridge?” Fung wrote. “If a new bridge is being built, how do they build that while the old bridge is in place?”

He might be surprised to know that crews have been working on a new bridge since early this year, a project that will take two and half to three years start to finish. From Highway 101, you’d never know they were down there, and until the action surfaces in March, motorists along Petaluma Boulevard South are the only ones watching the work.

“I’ve been anxious to drag people down there to see it all,” said Senior Bridge Engineer Jeffrey Kress,who serves as Caltrans’ eyes and ears on the project.

Kress gave us a tour and described the complicated choreography involved with planning and building the 907-foot bridge, the third longest precast, pre-stressed concrete girder bridge in the United States. When it’s finished in late 2015 or early 2016, it will replace the two existing bridges, with enough extra width to hold shoulders and three lanes of traffic in each direction.

The new bridge has arched girders, wave patterns on the deck and pier caps and fractured fin texture on the columns. (Caltrans rendering)

Building the new bridge without disrupting traffic on the current spans is tricky, as are the rigorous environmental restrictions in place to protect the riparian ecosystem, relocating the utilities that snake through the area (PG&E, AT&T, Comcast, Sonoma County Water and Marin County’s municipal aqueduct) and legal challenges aimed at protecting the cliff swallows that nest beneath the girders.

Even so, Kress said, crews are working at “a furious pace,” putting in six- and seven-day weeks to stay on schedule. “I’ve been building bridges for 23 years, and these contractors are moving faster than ever before.”

Here’s what they’re up to:

* Building the substructure that will hold the new six-lane bridge. The new bridge will be 970 feet long, with a 212-foot main span across the river. Because its support piers are farther apart, it will have a wider footprint, allowing the river beneath it to be widened from 100 feet to about 150 feet.

Five sets of concrete columns have been or will be poured in sets of three, spread between the south side of Petaluma Boulevard South and the northern river bank. They’re in various stages, with those on the north and south sides nearly done. Those in between are closer to the water, and coffer dams are required to build foundations. Once all the columns are done, five concrete pier caps will be installed on top of them.

Kress hopes the substructure will be done in mid- to late March 2014.

New columns are being built between existing pier supports. The middle one is wrapped in a thermal blanket to control the temperature while the concrete sets.

* Building the median. When the substructure is ready, 300-foot cranes will be constructed at river level, a process that will take a couple of weeks, and Highway 101 motorists will begin to see the construction. Starting in mid-April 2014, the cranes will be used to hoist 27 girders onto the pier caps that will have been built in the gap between existing bridges, where they will be spliced together to form a new median. A deck will be built on top of the girders, and by late summer the median will be ready to hold traffic.

Coffer dam on south river bank where new column will be built to replace the one in the center of photo.

*Demolishing the existing bridges, built in the 1950s. This will be done in stages, with southbound traffic shifted to the median in mid- to late August 2014 so the southbound bridge can be demolished. The 300-foot cranes will reach out over the river, lowering pieces of debris onto barges that will take it away to be recycled. Demolition should take a month to a month and a half to finish, ending in late September 2014.

* Building the new bridges. Once the southbound bridge is gone, new abutments can be built and crews can begin building the west half of the new bridge, a process expected to start in October 2014. Girders, each weighing 90 tons, will be flown in from the Central Valley, and the cranes will be used to set them onto the pier caps. Decking will be installed, and southbound traffic will be shifted onto the new southbound lanes. If all goes as planned, that will happen in March 2015.

The process will be repeated for the northbound lanes, with cranes moved to the east side of the bridge, northbound traffic shifted to the median, existing bridge demolished (in April 2015) and construction beginning in May 2015 on the new northbound lanes.

The new bridge is expected to be complete by late 2015 or early 2016.

*Clean-up work. Caltrans contractors will remove the old piers from the river. The Corps of Engineers will dredge the river to take advantage of its newly widened channel, which will make it easier for boats to navigate.

*Adornments. Although it would be possible for crews to personalize the concrete columns while they are being poured, Kress said he knows of no effort to leave a Bay Bridge troll-style keepsake under this new bridge. “I’m the only troll under this bridge,” he said with a laugh.


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