By Simon Wong, P.E., S.E., M.ASCE, and Zeenat Chandrasekhar, S.E.
About 35 miles north of downtown San Diego, the city of Carlsbad presents a classic southern California backdrop for joggers, cyclists, and surfers bent on enjoying the sun, breaking waves and sapphire-blue Pacific Ocean.
Look to the north, however, and another scene is playing out, a scene that could significantly influence the future of water sourcing in California: the construction of the Carlsbad Desalination plant.
The Carlsbad Desalination Project is, as of this writing, about 50 percent complete. Engineered by ARCADIS and Simon Wong Engineering (acquired by San Diego-based Kleinfelder in January 2013), the plant is a critical milestone for proponents who view seawater desalination as part of the solution to California’s long-term water supply needs.
In December 2012, Poseidon Water and the San Diego County Water Authority finalized a 30-year water agreement for the purchase of up to 56,000 acre-feet of desalinated seawater from the project and secured financing for the $1 billion Carlsbad Desalination project — soon to be the largest ocean desalination plant in the Western hemisphere, producing on average 50 million gallons per day of drinking water for the San Diego region. The plant is expected to be on-line as soon as the fall of 2015. By 2020, the plant is expected to provide 7 percent of San Diego’s total water supply.
Brief survey of conditions
Located nearly 500 feet from the Agua Hedionda lagoon, the plant is being constructed within the footprint of the existing Encina Power Station, a large natural gas-fueled electricity generating plant. The desalination facility will occupy a 6-acre brownfield site that previously held the plant’s aboveground storage tanks.
The heart of the desalination plant is a reverse-osmosis system designed by Israeli firm, IDE Technologies. Under IDE’s design, the existing power plant’s discharge water will be captured by a 72-inch diameter pipe connected to an intake pump station extending 40 feet below grade. From there, the water will be pumped by the intake pumps to the desalination plant where it will undergo a sand/anthracite scrub within the plant’s innovative filtration system.
Once the filtration process is complete, water is pumped through the 1,960 reverse-osmosis pressure vessels under high pressure, thus stripping most all of the salt from the water. After the addition of essential minerals and the addition of a disinfection residual (prior to entering the distribution system) in the 2.5 million gallon product water tank, the clean, drinkable water will be pumped through a new, underground 10-mile pipeline that will run through the cities of Carlsbad, Vista and San Marcos, ultimately connecting to the Water Authority’s regional aqueduct system where the water will be distributed to residents and businesses across the San Diego region.
Although efficient, the system’s design created challenges for the structural team, particularly related to relatively small available space and corrosion control.
In addition to the complex site conditions, more than 10 groups are involved in this project, making it a complicated collaboration effort. Additional stakeholders include Poseidon (the project owner), the San Diego Water Authority (the water purchaser), Kiewit Shea Desalination (a joint venture of Kiewit Infrastructure West Company and J.F. Shea Construction), the city of Carlsbad, California Coastal Commission and NRG Energy Inc. (the power plant’s owner).
Finally, the structural component of the project had to comply with an aggressive 16-month deadline.
Solving the structural
As a member of the structural engineering team, Kleinfelder coordinated several design charrettes with stakeholders immediately following the project kickoff. (Editor’s note: a charrette is a meeting in which all stakeholders in a project attempt to resolve conflicts and map solutions.) The focus of these meetings was to fully understand the project’s needs — from a process, environmental and construction standpoint — and design structures that would best address those needs.
One of the first concerns was that the project’s relatively small, 6-acre site didn’t seem to match the robust needs of the plant. To mitigate these constraints, several belowgrade structures were placed immediately adjacent to large abovegrade structures. For example, the underground product water storage tank is located directly next to an aboveground, roughly 20-foot-tall buffer tank. To accommodate the potential surcharge that would be imposed by the aboveground tank on the wall of the underground tank, cement slurry mixture was used to strengthen the sloping section directly underneath the buffer tank and against the product water tank wall. By replacing the soil with a stiffer slurry backfill material, the load of the buffer tank goes directly to the soil below the product water tank.
This solution is used on many of the project’s other structures, including the Backwash Pit adjacent to the Pretreatment Building.
Rust never sleeps
The second design concern was corrosion. Only about 500 feet from the Pacific Ocean to the west and Highway 101 to the east, the plant location made corrosion control extremely challenging, as the combination of vehicle emissions, salty sea air and the thick marine layer that blankets Southern California’s coasts every night creates a corrosive "witches’ brew" that could affect the structures.
To combat corrosion, Kleinfelder recommended that the project’s structures be constructed using cast-in-place reinforced concrete. The concrete used on the site was amended with admixtures to reduce permeability and inhibit corrosion.
For steel, the rate of corrosion is dependent on the availability of water, oxygen and chloride ions, electrical resistivity of the concrete and temperature. The availability of oxygen is a function of its rate of diffusion through the concrete, which is affected by how saturated the concrete is with water. When totally submerged, the diffusion rate is slowed because oxygen must diffuse through the pore water. When the concrete is dry, the oxygen can freely move through the pores. Alternating wet-dry cycles — like incoming and outgoing tides — accelerates the corrosion process.
Admixtures used on the project include colloidal silica, hydrophobic pore blockers and supplementary cementitious materials (SCMs).
Further, all structures adjacent to or in contact with salt or corrosive water will have additional concrete cover and/or protective liners.
Other challenges include designing structures for large uplift pressures. For example, the foundation slab for the intake pump station — which will extend approximately 40 feet below grade — is 4-feet thick with walls nearly 3.5-feet thick. The thick concrete resists the high soil and water pressures and also adds mass, which circumvents buoyancy uplift caused by tidally-influenced groundwater.
Blessings and curses
The concrete solution was both a blessing and a curse. As concrete sets it undergoes a process called hydration, an exothermic reaction caused by the interaction of water and cement. The large forces on the structure resulted in greater concrete thicknesses thus giving rise to concerns related with mass concreting. High cement content and large concrete thickness result in high temperatures within the concrete. To avoid cracking and other temperature related damage to the concrete, the maximum temperature and temperature difference between the interior and the surface of the concrete needs to be controlled. Generally, any placement of structural concrete with a minimum dimension equal to or greater than 36 inches can be considered mass concrete.
For this project, the initial pours at the 42-inch thick "SuckBack" tank foundation were monitored for internal temperatures and temperature gradients with thermocouples.
Mix designs were selected to reduce temperature rise and temperature differences after placement. The temperature rise of concrete is directly related to the types and quantities of cementitious materials within it. To reduce heat of hydration, Class F fly ash was used to replace a portion of the cement.
One other critical element of the desalination plant’s design was seismicity. The plant’s Southern California location demanded that the team consider all potential effects of seismic activity. The plant is located 4.3 miles from the Rose Canyon Fault Zone, an active fault known to have generated a 6.5 magnitude earthquake in 1800, and as such had to be designed to stringent seismic codes. All water-bearing structures were designed for hydrodynamic forces in addition to hydrostatic forces.
Expert coordination, upgraded materials, and innovative site use helped the Kleinfelder team circumvent challenges associated with corrosion and the small site footprint and allowed this groundbreaking project to begin construction in November 2012, nearly one month ahead of schedule. More importantly, however, the successful design is a milestone for a project will help to secure the future of desalination in California.
Simon Wong, P.E., S.E., M.ASCE is vice president of corporate business development at Kleinfelder. Located in Kleinfelder’s San Diego, California, headquarters, he can be reached at 619-831-4553 or [email protected].
Zeenat Chandrasekhar, S.E., is a principal professional for Kleinfelder. She can be reached at 619-831-4551 or [email protected]
Kleinfelder is a 51-year-old full-service engineering, architecture and science consulting firm headquartered in San Diego.