Precast Manholes in Stormwater Application

Precast concrete manholes become part of the stormwater solution

By Fernando Pages Ruiz

When American Fork, Utah, decided to approve construction of a 175-acre commercial center, the city engineers had to consider the environmental effect it would have on a 45-acre wetland right next door.

Under the EPA’s Phase II Storm Water Rules, adapted in late 1999, almost any city large enough to require a storm sewer system must balance growth with the environmental impacts of urbanization. When small-town cornfields and prairie grasses become asphalt and concrete, rainfall can wash big-city pollutants – oils, greases, salts and trash – into delicate wetlands, streams and, sometimes, even drinking water.

PEPG Engineering project engineers designed a sophisticated stormwater runoff treatment system for its new commercial center, comprised of big box stores like The Home Depot, Wal-Mart, Kohl’s and a 12-screen Cinemark Theater. Since the commercial center’s runoff would wash straight into the protected Mitchell Hollow Wetland Conservation area, the challenge meant more than a regulatory annoyance; it was a legitimate environmental concern.

Oil and Wetlands Don’t Mix

While wetlands have an excellent capacity for capturing and removing pollutants, overloading a wetland with pollutants can reduce their capacity to filter and remove pollutants such as oils and greases, metals, nitrogen, phosphorous, and other chemicals. The best solution, according to the EPA, is to prevent pollutants from entering the wetland in the first place.

“But traditional systems designed to separate water, sediment and oil have their limitations,” says Ralph Kuntz of Amcor Precast, Ogden, Utah.

“ cast-in-place oil and water separators consist of an in-line vault with two baffles,” says Kuntz. “As water comes into the vault…sediment gets trapped in the first chamber and then oil floats to the top and gets trapped in the second.” The system relies on gravity and time to do the work of separating pollutants and water; manholes provide access for cleaning. Under most circumstances, these systems work adequately – that is, until a big storm hits and the system cannot handle the increased flow rate.

A major storm stirs up any sediment settled in the tank, while the grease and oil churn, emulsify and recombine with the storm water. Suddenly all the pollutants isolated and stored over several months of treatment froth into a sludgy mire that can enter the water stream like a poison cocktail. Fortunately, Amcor Precast could provide city engineers with a state-of-the-art solution.

Coexisting With Conservation

For Andy Spencer of PEPG Engineering, Draper, Utah, who designed the stormwater system for the American Fork Commercial Center, the project represents a hallmark – not only for economic development, but also for “cooperation between development and conservation,” he says. Engineers couldn’t afford to specify anything but the best treatment system, one that wouldn’t fail in a storm.

To avoid the problem of purging during high flows, Spencer specified a two-stage oil and sediment separator manufactured by BaySaver Inc. of Mount Airy, Md. The two-stage unit includes a bypass for large storm events so that when flows exceed capacity, the unit allows water to run through it without disturbing accumulated sediments and oil, keeping pollutants out of downstream waters.

The system has another feature Spencer likes: a large storage capacity that utilizes two 60-inch precast concrete manholes instead of a smaller cast-in-place vault. Since the effectiveness of most water-scrubbing devices relies on a strict schedule of cleanup, the separation unit provides a big advantage to a small-town public works department. On average, the system requires cleaning only once a year.

How it Works

In principle, a separation system works the same as any other oil and sediment separator, using gravity, flow and density differences to remove free oils, suspended sediments and floating debris from stormwater. But instead of relying on a footprint of elaborate, cast-in-place chambers, the BaySaver system requires only two off-the-shelf precast manhole structures with a lightweight, high-density polyethylene flow-regulator to connect them.

The system includes three components: A primary precast concrete manhole set in line with the storm sewer pipe, the BaySaver separator unit and a second manhole for storage.

One or more inlets spill into the primary manhole, which collects water and serves as a catch basin to reduce turbulence and provide a first-stage settlement tank for coarse sediment. Water then spills from this primary manhole into the separator unit. The BaySaver is one unit in the overall system. The separator unit controls the water flow, then channels it into the storage manhole, or sometimes allows it to bypass.

During light to moderate storms, water flows through the separator unit into the storage manhole. A weir at the separator inlet skims oil and floating debris from the main manhole, channeling pollutants into the storage inlet pipe.

Once water and pollutants travel into the storage manhole, gravity draws fine sediment to the bottom of the tank; differences in viscosity and buoyancy float oil and debris to the top. An outlet pipe in the storage manhole displaces clean water from the storage basin back into the outflow. A manhole cover allows access for maintenance.

During a heavy rain, however, rising water levels force the incoming surge over a separation plate in the unit, bypassing the storage manhole inlet so that turbulence cannot force pollutants back into the system. Other components, such as “T” pipes, also help reduce turbulence and prevent backflow. Since the system is entirely passive, relying on gravity, water flow and variations in viscosity, there are no mechanical parts to break down or wear out.

Installation is Simple

The basic installation is simple: Dig, set two manholes, slip the separation unit in place and backfill. After the excavator digs a hole, the utility contractor places one manhole structure in line with the storm sewer pipe to act as a catch basin and then to the right or left to act as a silo. Since the BaySaver unit comes in both right-handed and left-handed configurations, it offers some flexibility around other utilities and obstacles.

The primary manhole has an outlet large enough for the separation unit to slip into place, with two pipes dropping into the catch basin and two outlets. Polyethylene pipes connect the unit to the storage manhole, and a reducer connects the unit to the storm sewer pipe. Gravel backfill supports all the components. After manhole frames bring the structures to finished grade, dirt backfill and paving complete the installation.

Precast Advantages

According to Jim Schaefer, project engineer for Ames Construction, which built most of the utilities for the American Fork Commercial Center, it took less than a day to install each system. It’s a good thing, given that the project required about 30 manholes.

This contrasts sharply with the traditional cast-in-place system, which can take five to seven days for each installation when allowing for curing before backfill.

“It would have been impractical to use anything but precast concrete, given that high groundwater required constant dewatering,” Schaefer says of the American Fork project. “Trying to keep the holes dry while concrete cured would have required pumps running 24 hours a day and a full-time attendant to make sure motors didn’t break down or run out of gas.” Reduced construction time also decreased the liability exposure associated with an open excavation, he says.

Off-the-shelf precast concrete components coupled with a pre-engineered product also make for quick delivery times and reliable system performance guarantees. Because precast concrete manholes do not require the elaborate structural engineering needed for a cast-in-place vault, the separation system can be installed virtually anywhere – under a parking lot or in a grassy field. The system rests entirely underground, requiring very little real estate except for two manhole covers, which must remain accessible for maintenance.

Next time you’re in a parking lot, see if you can spot two manhole covers in relative proximity. If you do, there’s a good chance you’re looking at the discrete clues of one of the most advanced environmental stormwater treatment systems available through your local precast concrete producer.

Project Profile

Project Name: American Fork Commercial Center

Owner: AFCC Limited, Salt Lake City, Utah

Engineer: PEPG Engineering, Draper, Utah

Contractors: HE Davis & Sons, Spanish Fork, Utah Ames Construction, Salt Lake City, Utah Geneva Rock Products, North Orem, Utah

Precast Manufacturer: Amcor Precast (a division of Oldcastle Precast Inc.), Ogden, Utah*

* Amcor Precast is a certified plant under NPCA’s Quality Assurance/Plant Certification program.

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