By Debbie Sniderman
Using traditional concrete products for innovative projects excites specifiers and precast manufacturers alike, challenging them to think outside the norm. In many cases, such projects benefit local communities and the Earth as a whole. Two recently completed underground precast projects did just that, addressing the ever-present need for effective stormwater solutions.
Unique stormwater treatment systems
The Mississippi State Port Authority is investing more than $57 million in construction projects to extend and deepen the Port of Gulfport. When MSPA completes the restoration program’s environmental plan, the Port of Gulfport could become the second coastal port in the U.S. to successfully achieve a green designation. Precast concrete is playing an important role in the unique stormwater treatment systems that are a major part of this plan.
Travis Dickey, director of business development, marketing and sales at Design Precast & Pipe of Gulfport, Miss., said his company provided a wide variety of underground precast products for the Port’s 60-acre expansion. In addition to massive stormwater treatment systems, Design Precast & Pipe manufactured precast for the 140 electrical transport systems and more than 15,500 linear feet of reinforced concrete pipe and reinforced concrete arch pipe.
With the help of CONTECH Engineered Solutions, Design Precast & Pipe produced 14 large stormwater structures: 12-foot-tall boxes with 15-foot-by-22-foot and 14-foot-by-20-foot inner diameters. They contain integrated vortex swirl chambers that thoroughly clean the stormwater before it is released into the Mississippi Sound.
Each box weighed more than 100,000 pounds and the entire structure – including its large aluminum vortex chambers – weighed 250,000 pounds. When water enters the box through an entry hole, it spins as it goes through the vortex chamber. Waste, oil and sediments are captured in filters on the inside before the water is released.
Although Design Precast & Pipe is experienced making large precast structures, this is the company’s first job that includes swirl chambers. Teaming with another company and learning how to add an element to a structure for the first time was a challenge. Instead of simply adding wire and rebar, this project involved adding a metal structure that was engineered before the precaster was involved. Constructed from bent and shaped aluminum sheets, it was fixed inside the box and bolted into the wall.
Initially, the precaster didn’t know how to assemble the large aluminum swirl chambers, but designers came to the precast plant and showed them how to build the first one. Design Precast & Pipe built the rest without assistance.
Saving time, protecting workers
Other than having high strength and an extended service life, the main reason the underground precast stormwater treatment structures were specified was time constraint. Due to the project’s location on the water and the complexity of the port expansion, excavation and installation needed to be completed in a matter of days. As such, casting in place wasn’t possible.
Safety factors also came into play. The sheer size of the structures was very concerning to the installation contractors. They didn’t want workers to remain in such large structures in such large holes for a long time. Precast helped minimize these concerns.
Dickey said compared to casting in place, digging a hole and placing a precast product makes the project much easier. Using a precast structure is also more convenient for on-site contractors that have multiple projects going on – when they are ready, the structures are ready.
Four boxes were installed at a time, each set close to one another: two large 15-foot-by-22-foot pieces with 8-foot-by-10-foot structures in between them. All boxes were tied together with pipe to make the stormwater loop. Dickey said the precast structures saved time and money during installation.
“They were built to grade and there were no hiccups,” he said. “The contractors did a good job digging to the depth they needed, and the surface was prepared well. Unique jobs like this show that there’s no limitation to what precast can do. If engineered and built properly, precast technologies can be used to solve almost any problem.”
Watertight, done right
Located near Nashville, Green Hills, Tenn., is a hot area for development. But with so many people moving in, infrastructure changes are necessary. One necessary upgrade was the town’s local fire station. Inspired by the nearby town of Brentwood, the Green Hills fire chief sought to save money on the city’s water bill by installing a cistern that holds rainwater on site, making it available for re-use.
The Green Hills chief wanted a rechargeable system that would hold rainwater to irrigate the grounds and supply hoses for training exercises. This type of system allows the fire department to use less city water, providing both environmental advantages and cost savings. Directly and indirectly, taxpayers benefit over time.
After experimenting with non-concrete solutions that did not perform to expectations, the Green Hills fire chief became frustrated. He then mandated the system be absolutely watertight, with a simple design and easy installation. A precast concrete cistern design met all of the criteria, bringing speed, efficiency and quality control to the project.
Sherman-Dixie Concrete Industries of Nashville bid on and won the project. Along with a consulting engineer and an installation contractor, Sherman-Dixie put together several design options. According to Michael Kusch, Sherman-Dixie’s director of technical marketing, the one that was ultimately selected was a series of large vault catch basins.
Measuring 9 feet on all sides, the selected model allowed for a total monolithic pour with no horizontal seams or joints. Sherman-Dixie created a series of boxes suitable for sanitary or water-lined applications where nothing can get in or out, other than through a hatch on top used for servicing and a series of pipes that feed the system.
The system consists of five identical boxes linked together and installed underground that holds 27,300 gallons of rainwater harvested from roof drains. Gaskets in the walls ensure the cisterns are absolutely watertight.
There are many ways to hold water, but precast concrete offers the most efficient solution. Kusch said one reason precast concrete was used for this project was because of the speed of installation. All five vaults could be set in one day. With cast-in-place, it would have taken two weeks.
While there was no design criteria for product lifetime specified for the project, precast cisterns are built to last. Where flexible products have 25-year life spans, precast concrete products can last 70 to 100 years. According to Kusch, over the life of the concrete, this can save owners the headache of having to consistently re-install tanks made from plastic or fiberglass.
Precast concrete products offer endless advantages and can often be used in unexpected situations. As the stormwater systems above highlight, precast provides an adaptable, long-term solution with the ability to lead any project to success.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.