Bringing Home the Bacon

Grocery Store Satisfies Regulations Through Retention/Detention System

Moving water may be an ancient practice, but that did not
keep bright engineering minds from developing a more efficient way to do it.
Balancing an aggressive, stormwater pipeline design concept with a conservative
approach to prove its performance, the result was a new engineered stormwater
system.

It is a familiar scene outside of shopping centers and
office plazas nationwide during construction. Still unpaved, the parking lot at
a Chicago-area grocery store chain was covered with crushed stone while
72,000-pound ready-mix concrete trucks and earth-moving equipment finished
constructing the curbs and parking lot. What went unnoticed by most at this
Jewel-Osco in Elmhurst, Ill., was Jim Goddard, the chief engineer for Advanced
Drainage Systems, Inc. (ADS), crawling through the 42?-diameter N-12 HDPE
pipes that make up the stormwater drainage system.

He was inspecting the new design to make sure it did not
move. This detention/retention system replaces a traditional corrugated steel
underground system. It also features a lower cost and a smaller footprint.

The Motivation

According to the Environmental Protection Agency (EPA), the
number one cause of water pollution in the United States is paving over earth.
As new or renovated commercial and residential developments are built in cities
and suburbs, officials must have a plan to control stormwater quality and
quantity.

The expanding city of Elmhurst is no exception. When retail
centers and other commercial property owners construct or redevelop a site,
Elmhurst officials require strict compliance with current storm water codes.
Most commercial developments in Elmhurst must comply with an ordinance that
states the release of stormwater cannot exceed 0.1 cubic feet per second, per
acre of disturbed area.

Similar requirements are being mandated in communities
across the country in response to Phase II of EPA's National Pollution
Discharge Elimination System (NPDES). NPDES is designed to improve water
quality and quantity by regulating point sources that discharge pollutants into
surrounding waters. This includes pollutants such as oil and other fluids found
on shopping center parking lots.

At the Jewel-Osco store, a customary, corrugated steel
retention system originally was specified to adhere to Elmhurst's codes, but it
never made it into the ground. The contractor and the engineer agreed that the
new system design by ADS was better suited for the site and more economical. It
justified discarding the old plans and redrawing them with HDPE pipe.

Developing a New Process

When compared to traditional underground steel retention
systems, Goddard's design narrowed the width of the trench by placing pipes in
contact with each other. This feature reduces the volume of earth needed to be
excavated while achieving the large storage volume requirement.

These reductions were reached through a novel concept. The
large diameter lateral pipe runs are placed next to each other, eliminating the
spacings and supporting backfill required between laterals. Instead, smaller diameter
HDPE pipes are placed in the upper and lower haunches of the larger pipes. The
smaller pipe produces a bridging effect between the soil and the laterals. For
retention systems, the haunch pipe also can provide additional storage
capacity.

The bundles, composed of 3 to 4 lateral runs, then are
wrapped with a geogrid to hold the system together tightly during installation,
to minimize tensile stress, to distribute overburden loads and to bridge void
areas. A geotextile is added to act as a filter for soil and backfill fines.

Contractor Switches

After talking to ADS, project contractor Tom Merryman of
Merryman Excavating realized that by installing the Storm Compressor system,
the total combined cost of the project would be less than using the corrugated
steel. This was even though the steel itself had a lower material cost per foot
than HDPE.

Mike Swedick, ADS zone manager, said that the original plans
in Elmhurst called for a four-foot-diameter corrugated steel pipe system in a
six-foot-deep hole. The larger diameter steel pipe would have reduced the area
of the original footprint, but the depth required to install pipes that size
was not appropriate for the Jewel-Osco site.

"With this (HDPE) design, we used 42-inch pipe, saved
three feet in depth, and were still able to limit the size of the
footprint," Merryman said. "The cost savings are significant because
you have so much less volume to excavate and spoils to dispose."

Installing the new system at Jewel-Osco meant 45 percent
less excavation volume, 78 percent less backfill used and a 45-percent smaller
footprint than a traditional retention/detention design such as the corrugated
steel system first proposed.

In addition, when the system was installed at the Jewel-Osco
site, it took two-and-a-half days rather than the four or five days a
traditional retention/detention system requires to install. The job's quicker
completion is the result of not having to add backfill between the lateral runs
of pipe or compact extra soil in the haunches.

Designer Convinced

The next step was to convince project design engineer Ben
Bussman to rewrite the corrugated steel specification to include HDPE pipe.

Bussman admitted that at first glance he did not have a
comfortable feeling about the design. It was so different from what he was used
to. However, after seeing studies and other projects where the system is
working, Bussman became confident with the new system.

"It is very unusual for us to change the pipe specs at
the last minute like that," Bussman said. "It takes a lot, including
making sure the city engineer finds it totally acceptable. However, there were
so many compelling factors it was the right thing to do."

While the most compelling advantage of the Storm Compressor
system from an owner's perspective is cost saving, a close second is the
minimum inconvenience to the store's customers. The drainage system at
Jewel-Osco was part of a redevelopment project, and business had to carry on
while workers were digging, installing pipe and repaving a portion of the
parking lot. Given the fast turnaround time and the reduction in the project
footprint, the impact on the store's patrons was negligible.

Bussman believes that the system eventually will catch on
with other retailers and municipalities because of these advantages.

"I feel comfortable with the durability of the plastic
pipe," Bussman said. "It's going to hold up better in some conditions
than concrete or steel. There's a lot of salt and chemicals in parking lots,
especially here in the Midwest."

Final Approval

The basic criterion Elmhurst city engineer Mike Dever uses
to approve or reject a subsurface stormwater detention system is it has to work
in the real world. "I don't have an opinion about one type of material or
one system over another," Dever said, "as long as the system
works."

In Elmhurst, this means complying with the county ordinance
that states the release of stormwater cannot exceed 0.1 cfs per acre of
disturbed area.

If designed for detention only, the system can hold
stormwater for controlled release until surface streams or city storm sewer
systems can handle the flow. The Jewel-Osco system has a capacity of 40,000
cubic feet and is under the county ordinance limit. However, instead of just
collecting stormwater for the city to process, the engineers added a
groundwater recharge element to the design. Some of the collected storm water
is detained, filtered and then slowly released to the subsoil through
perforations in the pipe and geotextile wrap.

The Future

Municipalities all over the country are accelerating their
efforts to manage stormwater as the EPA's Phase II regulation compliance
approaches. There are 6,100 cities and towns across the United States that must
recognize the EPA's deadline to develop a comprehensive plan that addresses
both water quality and quantity for protected watersheds.

"Unfortunately, there still are municipalities that
don't allow the use of HDPE pipe," Goddard said. "They're just not
fully aware of the benefits plastic pipe brings to the equation. But once these
city engineers and specifiers see this system working, and the overall
performance and economic benefits, they'll realize this is a better
solution."

Information for this article was provided by Advanced Drainage Systems, Inc., Hilliard, Ohio.

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