Tall and majestic, the Golden Gate Bridge rises some 200 ft. above the water line separating San Francisco from Marin County, Calif.
One hundred and ten mi. to the northeast, the Mosquito Road Bridge stands twice as high — the roadway is a full 400 ft. over the South Fork of the American River gorge in the Sierra Nevada foothills.
Planned for completion late this summer, the $74 million Mosquito Road Bridge project ($102 million, including overall costs), a 1,200-ft.-long cast-in-place balanced cantilever bridge, will connect Placerville and Swansboro, two communities in rural El Dorado County. It replaces a smaller wooden suspension bridge built in 1939. The old bridge will be retained for foot and bicycle traffic.
In addition to meeting 21st-century transportation needs, the new bridge will accommodate the growing tourism industry in the area and increase the ability of first responders and wildfire teams to reach their communities quickly.
Because the site is so remote, and the bridge is so tall, Irvine-based Shimmick Construction Co. Inc., deployed unique construction techniques and special equipment for the project, Project Manager Charles Marrow told Construction Equipment Guide.
"The unique construction technique on this job is the cast-in-place balance cantilever construction of the superstructure," he said. "A lot of bridges are built traditionally, where you build a complete falsework system to support the construction of the entire bridge. You build your bridge superstructure and then remove the falsework system. The Mosquito Road Bridge, in contrast, is built segmentally in 15-foot increments using a modular traveling formwork/falsework system that starts at the columns and works to the bridge ends."
Designed by SYSTRA International Bridge Technologies in partnership with Quincy Engineering Inc., the Mosquito Road Bridge includes two 12-ft.-wide lanes with 5-ft. shoulders and a 54-in.-high barrier.
The superstructure of the bridge is made up of more than 68 cast-in-place cantilever segments, with the two ends of the box being cast on falsework, and the deck of the bridge receiving a polyester deck overlay, according to Shimmick.
Marrow said Shimmick still has several steps to take before the project is complete.
"We're finishing up the superstructure of the bridge over the next two months," he said. "That would essentially finish the main structure component. Beyond that, it's a dozen different things all at once: We remove our temporary crane trestles. We do some permanent erosion control. There are a couple of final drainage systems, drainpipes that need to be installed in the ground surrounding the structure itself."
The El Dorado County Board of Supervisors awarded the building contract to Shimmick in May 2022. Groundbreaking took place that September.
Remote Location
Of the site's two sides, the south side is the more accessible — about 10 mi. from Placerville, the county seat. For its concrete supply on the south side, Shimmick contracted with Folsom Ready Mix.
The north side was another matter. With no commercial contractors in the area and access only by logging roads, transporting materials from Placerville to the north side would have added an extra hour, Nick Berger, Shimmick's field engineer on the Mosquito Road Bridge project, told Construction Equipment Guide.
"The difficult terrain made us think outside the box," he said.
Added Marrow, "The logistical addition of building in a canyon, in a remote area — it basically adds an extra step of planning and an extra step of troubleshooting. We had to figure out how to get equipment here. How to deploy it."
The solution was to erect a batch plant owned by Shimmick on the project's north side to manufacture concrete.
"It took a couple of weeks to mobilize this thing," Marrow said. "There were some logging roads that went through the mountains. So, we had to find these logging roads and get [the batch plant] in, one big piece at a time."
"It was a pretty intense operation," Berger said, recalling the crew having to pick up a trailer to allow the 100-ft.-by-40-ft. structure to make it around a curve.
The batch plant was 2 or 3 mi. as the crow flies from the project site on the north side, Marrow said.
The new bridge is the largest capital project in El Dorado County, local publication the Mountain Democrat wrote in 2024. "Because of funding acquired through the federal Highway Bridge Program, the full cost of the undertaking is set to be reimbursed to the county."
"The project includes contractor-led design and installation of multiple access roads, along with two corresponding temporary crane trestles on either side of the river," according to Shimmick.
"The bridge has two main piers that are more than 230 feet tall from top of footing to bridge deck. The piers are founded on 36-inch CIDH piles and a set of 9-foot-thick footing slabs approximately 50 feet by 60 feet in dimension. The piers require the footings to be embedded in the mountain side, requiring over 10,000 square feet of soil nail and ground anchor retaining wall," Shimmick continued.
Perfect Geometry
The Mosquito Road Bridge superstructure was built starting at each column and working its way to the center and ends.
"Every time you pour, this structure leans," Berger said.
Added Marrow, "Cantilever segmental construction is one of those methods where you construct your columns and then you build out. We've been working our way out about 15 feet at a time. It's a matter of handing off the construction from one side to the other."
Shimmick worked with Colorado-based McNary Bergeron & Johannesen on the modeling for the bridge.
"They are, essentially, our segmental bridge engineer. They modeled how this structure would react," Marrow said. "They added our construction loading, and they really helped us to understand how this thing would act.
"This thing is moving," he continued. "So, they have to model it in every phase to understand where the bridge needs to be. It's called geometry control. It's like a typical construction survey on steroids.
"Geometry control determines where we're going to pour the next segment," Berger said.
"And we have to alternate, north-south, north-south, because the system has to be balanced," Morrow said. "And when we're getting far out with our cantilever construction and that column is actually flexing, we survey every day. And we track how much that column leans back and forth."
McNary Bergeron and Johannesen analyzed the structure and "provided us with what is called the ‘Geometry Control Plan,'" Marrow said. "This plan adds permanent design camber into the regular structure, ever so slightly raising the structure so in 27 years [of settling] it achieves the ‘perfect' geometry."
"Creep" is the official term for "settling," Marrow said. "Concrete flexes and creeps as it pushes down on itself and, over time, this creep changes the shape, but only by a few inches here and there. Unnoticeable to drivers.
"So, 27 years from now," he added, "this bridge will have settled a little bit and be — quote unquote — perfect."
Project subcontractors included: Alta Engineering Group Inc. of San Francisco; Folsom Ready Mix of Rancho Cordova, Calif.; Tyrrell Resources Inc. of Redding, Calif.; Drill Tech Drilling & Shoring Inc. of Antioch, Calif.; Roll N Rock Construction Inc. of Mt. Shasta, Calif.; Dees Burke Engineering Constructors LLC of Ontario, Calif.; Vintage Paving Co. Inc. of Winters, Calif.; K&G Concrete Pumping of Roseville, Calif.; and Grass Roots Erosion Control, of Lotus, Calif.
Prime contractor consultants included Krew Cos. of El Dorado Hills, Calif.; McNary Bergeron and Johannesen of Broomfield, Colo.; GeoEngineers of Redmond, Wash.; and JTW Consulting Engineers of Sacramento, Calif. CEG
(All photos courtesy of Shimmick Construction Co. Inc.)
