by Eric Nelson, Trinity Lightweight
Location: Dallas/Fort Worth Airport (DFW), Texas
Owner: Perot Development Company
Engineer: Halff Associates
Developer: Hillwood Investment Properties
Earthwork Contractor: RE Cupp Construction
The Story: Expanded shale was specified to enhance the drainage properties of vegetated swales
Two newly constructed developments outside DFW Airport utilized expanded shale in their vegetated swale to control storm runoff. Both developments were logistic centers (office and warehouse space) that included vast pavement areas facing the Grapevine Creek Corridor. Soils in this region are defined as “Houston Black,” also known as “black gumbo” for their high clay content and low permeability.
Filter strips and grassy swales are garnering more attention because they protect our environment. The grass and soil media help filter unwanted contaminants like nitrogen, phosphorus and total suspended solids. Placement of these “low impact development” techniques are frequently used around large pavement areas at stadiums, shopping malls, big box stores, transportation hubs, airports and the like. They are also specified along agricultural plots, animal feedlots, parks and golf courses to protect nearby watersheds.
The engineering firm of Halff Associates specified a vegetated swale mix of 3/8” expanded shale (50%) and sandy clay (50%). This was placed in an 8” lift over a sub-base of pea gravel, following the contours on the grading plan. With temporary irrigation in place, the swale was charged with water before being sodded with Bermuda 419. The longitudinal slope and infiltration characteristics of the swale media control storm runoff, thereby protecting the Grapevine Creek Corridor. Developments of this type may benefit from a reduction or elimination of drainage pipe.
The properties of expanded shale are great for low impact developments of this type. The 3/8” gradation has a saturated hydraulic conductivity of over 1,000 inches per hour with the ability to absorb up to 35% of its weight in water. It is a durable product used in everything from road construction to lightweight concrete and geotechnical fills. It increases the porosity and oxygen level of clay soils thereby promoting better growing conditions for plants and turf grass. Expanded shale is also specified in green roof media, structural soils for urban trees, bio-retention and permeable grass parking. Many grassy swales and filter strips would reap benefits from 50% expanded shale blended into existing clay soil.