Inside the construction of Folsom Dam's second dam
September 30, 2013
FOLSOM, Calif. -- In California's Sacramento Valley, residents are watching Folsom Dam's new auxiliary spillway project transform from rock and dirt into a massive, life-saving flood risk reduction project. It's one of the largest construction jobs in the nation, and crews are working around the clock to get this critical infrastructure project done by fall of 2017.
The heart of the project is the spillway's new control structure, essentially a second dam. It's comprised of six large columns of concrete called monoliths, with six bays where water will pass through gates that are each 30 feet wide by 45 feet tall. The dam will be 367 feet wide and 146 feet tall when completed. Including its 45-foot deep foundation, it will be slightly shorter than the Statue of Liberty.
Just about everything needed for construction is on the project site; including a concrete plant and material testing lab, a carpentry shop and a water treatment system. In total, it will take 300,000 cubic yards of concrete to build both the spillway and the control structure--enough to fill a line of concrete trucks 200 miles long.
Concrete production and placement is done mostly at night, and a major challenge is proper temperature and timing.
"We have some very specific requirements in terms of temperature," said Josh Wagner, a civil engineer on the project. "Basically, the concrete at the placement location has to be between 40 and 55 degrees, so one of the means of achieving that is to inject liquid nitrogen into the mix."
Each layer of concrete is placed in 5-feet-deep layers called lifts. Each lift is placed on top of the lift below it before it has dried, so the layers bind together.
"We don't want the concrete to begin to harden before the entire lift is in place," Wagner said. "This could cause it to cool unevenly, and possibly not bond correctly. If that happened, we might have to pull that lift out, which could cost us time and money."
But before concrete can be placed, workers in the project's carpentry shop build forms to help the concrete keep its shape. Steel rebar cages are assembled above the construction pit and hoisted into place in the control structure by crane. In all, more than 30 million pounds of rebar will be used--enough to build two-and-a-half Eiffel Towers.
"The workers bind or tie the rebar panels together," said Wagner. "When they get those in place, they'll put the containment or form in place around it, and then they'll place the concrete into that."
Before, after and during construction the Corps follows strict environmental guidelines to minimize construction impacts. For instance, rainwater and water used for construction processes is purified by treatment systems at the construction site, to avoid contaminating Folsom Lake or the American River downstream of the project.
"Any water that comes into contact with fresh concrete or is involved with the creation of concrete is industrial water and has to be collected, contained and disposed of separate from the other water collected on the site," said Wagner. "Industrial water is a lot different than storm water because it can't be disposed of in the lake," Wagner continued. "The storm water is an entirely different process; we can release that into the lake after it's treated and we do."
Together with ongoing improvements to levees downstream of the dam, the Folsom Dam auxiliary spillway project is a key part of modernizing Sacramento's flood control system so that it will continue to reduce our flood risk for decades to come.
The Folsom Dam auxiliary spillway project is a joint effort of the U.S. Army Corps of Engineers Sacramento and the U.S. Bureau of Reclamation to reduce flood risk for the Sacramento area and improve Folsom Dam's safety. Construction of the control structure is scheduled to be complete in 2015; the spillway is scheduled for completion in late 2017.