Precast tanks offer superior wastewater/water storage solutions.
By Leslie Lichtenberg
The Eagle Hunt Effluent Storage Tank in Chester County, Pa., is a circular precast post-tensioned concrete tank that serves as a large detention basin for treated water. Located in the upscale residential community of Eagle Hunt, a neighborhood of about 130 homes in Chester Springs, the 2.2-million-gallon tank provided a cost-effective, maintenance-free water storage tank with extreme durability and structural integrity. Because of its high visibility in a residential development, the 108-by-36-foot structure also met the aesthetic standards of the project owner, Pulte Homes.
“The original designs called for the tank to be installed in a lagoon at the entrance to the development,” explained Fred Ebert of RF Preston Engineering, Bluebell, Pa. “By using a concrete tank, we were able to move it to a top corner of the site closer to the spray field, which not only saved us about 10,000 feet of force main, but also enabled the builder to actually add building lots to the project,” he added.
Precast concrete was chosen for the tank’s wall panels over other materials for many reasons, including a considerable reduction in cost. Because concrete does not require many of the labor-intensive, high-maintenance applications typically associated with steel tanks, such as sandblasting and exterior coating, precast was considered a cost-effective, virtually maintenance-free alternative. In addition, the engineer and owner preferred the AWWA D-115 construction methods over less stringent AWWA D-110 concrete or cast-in-place concrete for reasons associated with cost and longevity.
“For this project, we were representing the interests of the largest residential home builder in the country,” said Dave Beiler, sales manager for Dutchland Inc. in Gap, Pa, manufacturers and installers of the Eagle Hunt Effluent Storage Tank. Not only was pricing an issue, but also cost-effectiveness. “The high costs of inspection and maintenance made precast the most sensible choice for our customer,” explained Beiler, adding that the concrete in this type of structure does not require interior or exterior coatings, as is often the case with steel tanks, which can deteriorate and discolor when exposed to the outdoors.
Another benefit of precast tanks is that they are not limited to aboveground installation, as are steel tanks. Two-thirds of the Eagle Hunt tank is underground and therefore out of sight of adjacent homes, the closest of which is 50 feet.
“Partial in-ground installation was another aesthetic advantage associated with this tank that enabled the builder to sell premium homes in the community without a problem,” said Ebert.
Working on a tight schedule, Dutchland was able to design, manufacture and install the Eagle Hunt Storage Tank in short order. The use of precast concrete also expedited the on-site erection time for the $600,000 project, which began in March 2003 and was completed approximately six months later.
An extremely high measure of quality control is yet another advantage of using precast concrete walls for a project of this size and scope. Unlike cast-in-place walls, which are cast vertically, the wall panels for the Eagle Hunt tank were cast in molds horizontally, thereby ensuring precision in concrete consolidation and reinforcement positioning. At the site, bearing pads and shims were placed in the keyways, and the wall panels were placed on the base slab. Post-tensioning provided not only active reinforcement of the concrete, but also a dense and watertight exterior. The post-tensioning was achieved through the use of tension duct extensions, which were placed and sealed between the wall panels. Once the tendons were threaded into the ducts, vertical joints were formed and certified PTI (Post-Tensioning Institute) personnel administered circumferential post-tensioning. Using post-tensioned concrete ensures that the tank is held in constant compression and allows for minimal deflection and cracking.
The wall panels are constructed as one continuous thickness, rather than applying multiple layers of shot crete and cables (as done for AWWA D-110 concrete tanks), providing a more consistent and durable finish.
A similar project designed, manufactured and installed by Dutchland is the Long Run Pump Station in Irwin, Pa. In North Huntington Township, a community of about 30,000 residents, two precast concrete post-tensioned tanks were built to supplement the operation of an existing 600,000-gallon tank and to help provide equalization of water flow in the event of storm surges. A new circular tank measuring 40 feet in diameter is used as a wet well to hold submersible pumps, while a rectangular structure holds 3 million gallons of water. Precast concrete post-tensioned rectangular structures are typically built for a variety of wastewater treatment applications, as was the case with the Long Run Pump Station.
“Because this is an older community with a long history of problems with stormwater inflow, these tanks were built to supplement our sanitary sewage system,” explained Kate Petrosky of the North Huntington Township Municipal Authority, owners of the project. “The purpose of the tank is to retain excess water until the pump station can catch up with it and send it to the treatment plant.”
Cost was the primary factor in choosing precast over cast-in-place concrete for this $5 million project. Flexibility was also critical in allowing the tank to be installed during the winter months. Overall, the project, which took approximately a year and a half to complete, proved to be a longer and more complex job than the Eagle Hunt project.
The rectangular tank, measuring 65 feet wide and approximately 250 feet long, was constructed of precast concrete walls, corners and tees, which were set on a cast-in-place slab. “Although weather was not a factor in the pouring of the walls and top of the tank, it was factor in pouring the floor and sealing the tank,” said Rich Hinkle of Glenn Engineering & Associates, engineers on the Long Run Pump Station.
The panels were set in place and alignment was checked with laser equipment. Like the Eagle Hunt Storage Tank, all walls were cast horizontally in steel forms and exterior coatings were not required. The rectangular retention tank was installed 30 feet deep in the ground. Again, post-tensioning provided active reinforcement through internal ducts cast within the tank walls.
The advantages of precast concrete in both the Eagle Hunt Effluent Storage Tank and the Long Run Pump Station centered on a high level of quality control throughout the fabrication process; cost effectiveness owing to savings on time, labor and maintenance; and an overall aesthetically pleasing product.
As environmental solutions for water storage and wastewater treatment, these precast post-tensioned tanks offer superior functionality and durability that cannot be matched when using steel, cast-in-place concrete or other alternative materials.
Project Name: Eagle Hunt Effluent Storage Tank
Owner: Pulte Homes, Trevose, Pa.
Engineer: R.F. Preston Engineering, Bluebell, Pa.
Contractor/Precaster: Dutchland Inc., Gap, Pa.*
Project Name: Long Run Pump Station
Owner: North Huntington Township Municipal Authority, Westmoreland County, Pa.
Engineer: Glenn Engineering & Associates Ltd., North Huntington, Pa. Contractor/Precaster: Dutchland Inc., Gap, Pa.*
*Dutchland Inc. is a certified plant under NPCA’s Quality Assurance/Plant Certification program.
Luke Smith says
Thanks for sharing this with us, Leslie! I’ve been trying to learn about different types of concrete. I think it’s interesting how more and more companies are using post tension concrete. I think you hit on a valuable point: post tension concrete is very durable and structurally strong. Next time I build anything, I’ll make sure to build it with this concrete!