Precast concrete delivers a much-needed water tank – fast
By Daina Manning
Building a water tank is one thing. Finishing all the precast concrete production in eight days is quite another. But that was the challenge Dutchland Inc. faced with the Reserves at Eagle project in Chester Springs, Pa.
“We bid the job, they imposed on us mid-stream and we just geared up,” recalls David Beiler, sales engineer for Dutchland Inc., Gap, Pa. “We realized the client we were serving here was one of the largest -residential builders in the United States, and in order to meet the demands of a very large and good client, we just ratcheted up and got the job done.”
The proposed structure was essentially a water recycling center associated with an upscale housing development: the station and tank receive clean-treated wastewater, which is then dispersed into open land through drip fields for irrigation.
“The homes were selling fast,” says Beiler. “This facility needed to be in operation for them to continue to sell homes – couldn’t get occupancy permits to move into their homes until this project was done.”
The project was originally specified for cast-in-place concrete, says Matt Boggs, project manager for Yerkes Associates, Westchester, Pa. Additionally, the original designs had the cast-in-place tank outside the building, says Sandra L. Morgan, P.E., assistant project manager for URS Corp., Wilmington, Del.
But this approach posed several problems. “ waited to start construction until fairly late, and under weather conditions that would not be the most advantageous for cast-in-place concrete,” says Morgan.
“There’s no way they could have cast-in-place,” says Beiler. “You’re fighting the weather, temperatures and all the challenges associated with outside, on-site construction.”
Water tank construction efforts are eased when the panels that comprise it can be poured horizontally – a process somewhat analogous to painting a door when it’s hung as opposed to a door lying flat on the ground.
For precast concrete, the sections that comprise the tank can be poured in steel forms laying flat on the floor, so that it has great width and length but the height equals the thickness of the finished piece. For cast-in-place, it’s necessary to pour a section that has great height and length but a narrow width. It becomes a much more challenging and hazardous project to assemble, requiring scaffolding, bracing and a longer time frame.
When the Reserves at Eagle project needed a faster, easier way to accomplish the job, “the contractor came up with the idea for going precast to speed things up,” says Boggs.
Dutchland geared up for the construction equivalent of finishing a term paper by pulling an all-nighter, which was made possible with the controlled environment of a precast plant. “If you’re doing it on site, you can form up so much in a day, and you can only cast so much in a day,” says Beiler. “But when you’re in a large precast production facility, you’re able to do multiple pours in a day’s time. That’s what allows us to shorten the time frame.”
Then “it was a matter of basically allocating additional manufacturing resources toward it, more floor space pouring more panels per day,” says Beiler. A higher-strength concrete formulation was also used to attain faster curing. “We could ship the panels to the site in a time frame that was less than normal,” he says. Each section of the tank was a custom piece, although stock forms could be utilized by adjusting for thickness and width.
Once complete, the sections were transported to the site. “We have large hydraulic cranes that are able to sit next to the excavation, pick the sections right off the truck and right into the final resting place,” Beiler says.
For the final phase of the project, the precaster applied post-tensioning technology. “The foundation of the tank is cast on the site, flat, and then the precast sections are put into place,” Beiler says. “Then they’re all pulled together and compressed by cables that we insert into the wall, pull it all together and lock and seal it off.”
Beiler explains that in post-tensioning, high-strength steel compresses the structure’s joints and makes them watertight. “You’ve got a structure with a lot of energy pulling it together.”
Precasting also led to a major design change for the project: Instead of a separate unit, the tank was now located beneath the building. Following the subterranean excavation, the top of the precast tank – at ground level – served as the foundation of the building. Precast proved advantageous for this space-saving configuration, which was not possible with alternatives such as a cast-in-place or steel tank or an open pond. The latter entails the safety risks of people falling into the pond, and it also becomes a mosquito attraction,” Beiler says.
Despite less than ideal circumstances, the project came together smoothly. Boggs says that using precast concrete makes life easier for engineers due to quality control standards. “When you use cast-in-place, you need to supply the structural calculation,” he says. “ precast, the liability is on the to come up with the correct sizing and certifications for the structure.”
For this project, the precaster was also subject to detailed inspection. “The township had an inspector there,” Boggs says. Morgan points out that the wastewater facilities were constructed by the developers, but will be dedicated to the township. “So the township is actively reviewing the design and construction to make sure it conforms to township standards.”
Apparently, everyone liked what they saw: Boggs recounts that the precasters later received an amiable letter stating how well things went and how quickly the project was put into place. “It was very complimentary.” Precast concrete proved itself as the go-to material of choice when the job was both challenging and deadline driven.
Project: Reserves at Eagle, Chester Springs, Pa.
Owner: Toll Bros./Upper Uwchlan Township, Chester County, Pa.
Engineers: Yerkes Associates, West Chester, Pa.
Contractor: MGK Industries Inc., Pottstown, Pa.
Precast Manufacturer: Dutchland Inc., Gap, Pa.
* Dutchland Inc. is a certified plant under NPCA’s Quality Assurance/Plant Certification Program.