A Soil Study: Engineered soil is crucial to state-of-the-art storm water management system at the Georgia Institute of Technology

By Laura Drotleff and Don Eberly, Big River Industries, Inc.

With the completion of the Christopher W. Klaus Advanced Computing Building site in 2006, the Georgia Institute of Technology (Georgia Tech), and those involved in the site’s development, set a competitive environmental benchmark in storm water management.

Georgia Tech’s primary goal was to develop a retention system that would capture the first flush, or the first 1.2 inches, of each rain event and hold it on site as reclaimed irrigation water. The 414,000-square-foot building sits on a 6.2-acre site with a 30-foot grade change from the rear of the building down to the front. With a relatively small site and a large building, including a three-story, below-grade parking structure, the remaining open space was heavily impacted by construction activities. The soil quality was ideal for building support, but lacked the necessary infiltration for adequate storm water management.

Landscape architecture and urban planning firm Ecos Environmental Designs saw an opportunity to use new solutions, embracing the university’s challenge to recreate pre-development hydrology, preserve the site’s native ecology, and emphasize open green space. Through a collaborate effort with the Environmental Protection Agency Region 4 and the Georgia Department of Community Affairs (to fund a grant studying the “Use of Engineered Soils and Landscape Systems to Meet Storm Water Runoff Quality and Quantity Management Requirements),” Ecos partnered with ERTH Products to engineer a soil mix for a bioretention and landscape area. The goal was to capture storm water while minimizing run off on the site’s dramatic grade change.

“It turned out to be a perfect fit,” says Stephen Brooks, Ecos vice president. “We were able to achieve the needed infiltration rates while maintaining a certain amount of moisture, combined with good organic content to support proper soil biology for ample plant life.”

Soil Considerations

The bioretention area of the site used 350 cubic yards of engineered soil, containing 40 percent clay topsoil, 20 percent sand, 20 percent ERTH food compost, and 20 percent HydRocks. Manufactured by Big River Industries, HydRocks is an expanded clay lightweight aggregate product, manufactured through a rotary kiln process in which selectively mined clay is fired at 2000 degrees Fahrenheit. “This process produces a consistent and predictable, high-quality ceramic aggregate that is structurally strong, physically stable, durable, environmentally inert, lightweight, and highly insulative,” says Jeff Speck, Big River Industries’ vice president of sales and marketing. “As a filter medium, HydRocks increases surface area and allows fast, free drainage, helps remove or reduce toxins, and absorbs nutrients for long-term, sustainable water treatment.” For site developers and storm water management professionals, it improves soil’s functionality and service life, saving material, labor and transportation costs.

HydRocks’ features support the physical requirements considered in developing engineered soil mix, according to Scott King of ERTH Products. “This design allows for good surface infiltration of storm water, along with high groundwater holding capacity, while not creating a continuously saturated soil, which would be detrimental to plant life,” King says. “HydRocks accomplishes this and provides long-term soil structure with pore space for air, water and nutrient exchange in the soil profile.”

Chemical and biological considerations included creating a living soil containing organic macro- and micro-nutrients and a diverse population of beneficial microbes. A living soil was essential given it requires fewer chemical inputs, breaks down contaminants, and provides movement within the soil, which increases infiltration, water holding capacity and the overall air and water exchange.

Many Parts Create A Whole

The storm water retention design aimed to accept the building’s roof runoff and first flush, absorbing storm water into the landscape and depositing the surplus into two underground concrete cisterns with a combined 174,149-gallon volume. After the cisterns were installed, Ecos constructed a series of retaining walls, 25 feet long by 5 feet wide and 30 inches tall. Built from local, natural granite, the walls were set perpendicular to the flow of the bioretention area, providing grade retention and serving as the delivery vehicle to infiltrate roof runoff through the bioretention area to the cisterns below.

“The roof’s downspouts were connected to the end of the walls to an interior channel, which has a series of openings on the downstream side,” explains Brooks. “Roof runoff passes from the downspout to the walls’ interior, turns 90 degrees, and exits to the landscape. The reinforced channel of the walls withstands the four stories of velocity from the roof, preventing soil erosion.”

Ecos excavated four-foot deep cells between the retention walls, where it laid under-drain pipe that connects to the underground cisterns and wrapped the area with gravel and filter fabric. The engineered soil mix was then installed in a series of lifts, each watered down to ensure soil settlement until design elevation was reached. The channels were lined with native river rock, broken up by large boulders salvaged during excavation of the building site. Cranes placed the boulders on graded aggregate to ensure that they did not move. The boulders were slightly elevated to absorb grade and encourage pooling behind them, maximizing infiltration time.

The bioretention area was then planted with a mix of native plant species to mimic a perennial stream condition in Georgia’s Piedmont region, providing a drought-tolerant landscape. Any storm water that does not absorb is captured by the under-drain and sent to the cisterns, where irrigation pumps recycle it through the grounds.

At the rear of the Klaus building is a large lawn space used for student gatherings and social functions. Under the sod, Ecos used the same bioretention soil mix formulated with Big River Industries’ HydRocks to capture storm water sheeting off hardscapes. The water is routed through a series of under-drains emptying into the cisterns.

Mission Accomplished

Today, Georgia Tech’s innovative storm water management system provides a constant three-week independent supply of reclaimed irrigation water. The facility earned a coveted LEED Gold Certification from the U.S. Green Building Council for sustainable site development using environmental materials from local sources. The experience and its ultimate result not only achieved the university’s goals for storm water retention, but set a new standard for future development projects.

To learn more:

Don Eberly is president / CEO of Eberly & Collard Public Relations, Laura Drotleff is a writer for the firm. The company specializes in landscape, agribusiness, design, construction industry public / media relations and writing. www.eberlycollardpr.com.

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