Using a column design that incorporates seismic elements, Pre-Con Products offers a precast solution to help the County of Los Angeles tackle its longtime stormwater runoff issues.
By Bridget McCrea
Every time the skies open up in Los Angeles, the city sends billions of gallons of “free liquid gold” down the drain. Now, instead of flushing that precious liquid out to sea – as it has done since the 1930s – Los Angeles and surrounding cities have put time and effort into learning how to capture and use runoff to replenish local groundwater supplies.i
In a state where water is nearly more precious than gold, brainstorming sessions among city engineers have segued into new opportunities, one of them being the Gates Canyon Stormwater Improvement Project. Located near Calabasas, the project is one of the most innovative stormwater projects to date in Southern California.
“With our frequent droughts and flooding, California is pushing to develop a water resiliency plan that’s climate change-proof,” said David Zarraonandia, president at Simi Valley-based Pre-Con Products. During the last drought cycle, for example, the water tables dropped dramatically, and wells stopped producing water. The state realized it needs to replenish the water table when it rains, which doesn’t happen too often during the winter and not at all during the summer.
“As a result, any rain that does come basically flows down the creeks and out into the ocean and is lost forever,” he said.
Meeting seismic requirements
Known for its strict seismic requirements, California is one state where earthquakes have to be factored into the design. This is critical in an area of the country that experiences about 10,000 earthquakes annually, according to the United States Geological Survey (USGS).ii Most of the earthquakes are so small they are not felt, and several hundred are greater than magnitude 3.0 and only about 15-20 are greater than magnitude 4.0.
“Seismic is a big deal here,” Zarraonandia said. “Underground concrete structures don’t generally have to meet seismic requirements because the ground just moves, and the concrete is in it, moving right along with it.”
The American Concrete Institute (ACI) code for underground water storage structures includes seismic requirements. According to Joe Dietz, SE, president at DSC Engineering in Laguna Hills, Calif., ACI’s specific code requirements cover both environmental engineering concrete structures (ACI 350) and seismic design of liquid-containing concrete structures (ACI 350.3).
A shift in design
For the Gates Canyon Park project, the underground structures had to be designed to accommodate both the seismic loading in accordance with ACI codes and the vehicle loading specifications from the American Association of State Highway and Transportation Officials (AASHTO).
“The ACI code requires that column elements of walls be provided with confinement reinforcement, spiral or similar,” Dietz explained. “Fitting this required confinement reinforcement within the wall section required a thicker wall section for concrete placement and cover requirements than would otherwise be needed.”
The stringent seismic requirements came about after several government agency engineers realized they weren’t dealing with typical precast concrete products such as manholes, which have less stringent seismic design requirements.
“These are large, multi-segment structures,” he pointed out. “If they don’t meet seismic requirements, the failures can be catastrophic.”
Knowing this, Pre-Con reviewed its original designs for the stormwater project, looking for ways to and ensure it would accommodate the seismic requirements. The project incorporates Pre-Con’s patent-pending StormPrism EQ system, which features a column system to support the structure and resist applied loads. The cantilevered column system is efficient, while also providing area for the required confinement steel in the vertical reinforced core of the column element.
Why columns work
Using a column design – which incorporated roughly 274 precast concrete pieces – was beneficial for several reasons. For starters, the cross-section of each column is large enough to be reinforced with confinement steel. This reinforcement is positioned around the vertical reinforcement extending the height of the column, and helps prevent bending.
“If there’s an earthquake, then the column structure stays in place and won’t collapse on itself,” Zarraonandia explained. He noted it also includes adjustable walls whose thicknesses can be changed based on the magnitude of the load. The column approach also allowed the precaster to adjust the thickness of the floor or “deck,” making the design itself flexible.
“The column diameters and the footprint stay the same, but our deck thickness can be changed based on varying earth covers,” Zarraonandia said.
Reduced movement and sloshing
To provide the increased rigidity needed to handle the site transportation issues and exceed the required moments from the seismic loading conditions, the column system also incorporates a flared cross-section at the top and bottom of each respective modular unit.
Designed to capture, treat, filter, and re-use local stormwater runoff, the project’s cistern component required up to 3.5 acre-feet of storage in an underground structure with up to 14 feet of fill above.
“Precast concrete quickly became the preferred material for the construction of underground stormwater storage cisterns due to quick construction time and high load capacity,” Dietz added.
Once in place, Pre-Con’s StormPrism system created a completely watertight stormwater storage system. The speed of construction met the expectations associated with the use of precast concrete, Dietz noted, while also providing quicker installation.
“The cantilevered column system allowed the rigid frames to easily survive transport to the site and not a single unit was rejected once it arrived on-site,” Dietz said. “The StormPrism precast components also have potential uses beyond the use in stormwater capture projects, as the reduction in internal walls and increased open space allows it to be used for many other applications, including underground storage, shelters and agriculture.”
Reyes Construction served as the prime contractor for the project and also touted the speed of using precast concrete.
“Precon’s precast storage system was a vital component to meeting the major project milestones,” said Project Manager Scott Mothershed. “The ability to manufacture a system off-site while excavation simultaneously occurred on the project saved nearly half the time it would have taken to construct the system as a cast-in-place alternative.”
Making and installing
Pre-Con handled both the precasting and the installation processes for the project. Zarraonandia said being involved with the installation on such a large, significant project helps the company to better understand the intricacies of the work and what it can do to facilitate future initiatives.
For example, Pre-Con has already come up with a number of changes it wants to make on the next go-round, mostly in terms of the process for assembling the precast pieces on-site.
“We’ve come up with some tooling that will make installation even easier,” he added.
There’s More to Come
According to Dietz, the Gates Canyon Park project wasn’t the first to require the use of a modular precast concrete system for underground stormwater storage requirements.
“There are many other examples within the Southern California area of completed and constructed projects utilizing modular precast concrete units to build underground stormwater storage structures of varying sizes and shapes,” he explained. “California Proposition 1 (a stormwater grant program) and the recently-passed Los Angeles County Measure W (a parcel tax for stormwater recycling) will provide a large funding source for regional and municipal stormwater quality improvement projects going forward.
“It’s expected that precast concrete will continue to be used on these future projects as a proven solution.”
According to Zarraonandia, the project’s owner, Los Angeles County Department of Public Works, was pleased with the outcome. Looking ahead, Zarraonandia also sees more opportunities to use Pre-Con’s StormPrism system to support California’s ongoing commitment to water resiliency.
“Stormwater systems lend themselves to precast because most of them are modular, and include repetitive pieces that are large, yet relatively simple to put together – kind of like Legos,” he concluded. “This is a very good niche for precast concrete, especially for public works and other large projects.”
Bridget McCrea is a freelance writer who covers manufacturing, industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.