Ten Mile River Bridge Construction

The Ten Mile River Bridge near Fort Bragg, California, presents some construction challenges, beginning with high-profile environmental issues under close scrutiny by various entities. Having to build a trestle over protected land just to drive a crane to a pile driving site is one thing; not being allowed to walk on the ground without it first being "protected" with tarps is another, as is not being allowed to lay pipe pile waiting to be driven on bare ground. Entering the river for construction was delayed until mid-June as a condition of various permits to protect the fish and marine habitat. Construction progress tends to slow down when these kinds of restrictions are imposed but, "Protection of the natural resources is a positive result of these things, and GSB is working closely with Caltrans and the permitting agencies to assure that the resources are protected," said David Riccitiello, president of Golden State Bridge, Inc., of Martinez, Calif., the contractor on the $44-million-plus new bridge,

Beyond that, Will Reames, project manager for Golden State Bridge, has been dealing with another construction challenge: saturated soils. The site is located within the tidal zone of the Pacific Ocean, so footing excavations require cofferdams.

Thirty-inch diameter pile pipes are assembled to 100-foot or 120-foot lengths.

"Water levels reach up to 30 feet within the cofferdams," said Reames, "so we use a 29-foot-long Pileco-manufactured follower to drive the underwater piles."

"After initially using a vibratory hammer (HPSI 500) through the soft layers at about 30-60 feet, we use a Delmag D42 diesel impact hammer for the strikes through the harder layers, to final depth," Reames said. "We had planned on about 40 strikes per foot at this depth, but we've been getting as few as 25 per foot, and have had to extend our pipes to 120 feet in order to reach bearing values, but Caltrans is doing more testing in this regard," he said. That's because, according to Reames, flooding during the 1960s shifted top soils over positions where the new bridge's foundations are being set. In some areas piles are having to be extended because of unexpected layers of this flood-caused soft earth and mud.

"Since cofferdams for footing construction are in a tidal zone," he said, "hydrostatic pressure is such that dewatering is not an option; the bottom excavation would collapse, and more water would 'boil up,' destabilizing the soil further." Golden State is using three sets of whalers on the cofferdams. The lowest one, underwater, is a 6-foot poured concrete seal course.

Work began in January, and a completion date is scheduled on or before December 2009.

Project Description:

The existing concrete two-lane bridge is 1,381 feet long, 31 feet wide and was constructed in 1954. The bridge is located in Mendocino County on Route 1, about six miles north of Fort Bragg. The concrete replacement bridge is 1,479 feet long and 44.9 feet wide with two, 12-foot lanes and 6-foot shoulders. A 5-foot-wide, ADA-compliant sidewalk will be constructed on the West Side of the bridge to accommodate users of the Coastal Trail. The replacement bridge will be constructed on a parallel alignment approximately 60 feet east of the existing structure. The existing bridge will be dismantled and removed.

Project Purpose and Need:

Replacing the existing bridge will meet the goals of the Statewide Seismic Safety Program, mandated by the governor and state Legislature. The Ten Mile River Bridge has been identified as susceptible to collapse during a Maximum Credible Earthquake (MCE) of moment magnitude 8.0. The San Andreas Fault is the nearest active seismic source, located approximately 11 miles west of the bridge and is capable of generating an MCE of magnitude 8.0. The bridge replacement project is essential to ensure public safety, maintain the local economy, enhance coastal access, and protect environmental resources.