Airports are under constant change and development. This can be challenging with the mass of buried infrastructure and the need to excavate, expand, and make upgrades. And with more impermeable surfaces comes the need for improved stormwater control. The “Collapse Soil Site” at DFW Airport proved why lightweight aggregates were the perfect choice for a design with limited solutions.
The project site is located on the far southwest corner of the property between an active runway (13R-31L) and airport road ARFF. It has poor drainage with minimal surface grade and is best described as a narrow swale approximately 900 feet in length with a spaghetti network of sensitive and aging utilities including gas, electric, water, a petroleum pipeline, FAA communication cables, and electrical vaults. Of particular concern was the possibility of standing water and the attraction of birds and wildlife not desirable near an active runway.
The project consisted of drainage improvements, sidewalk repair, and PAPI (precision approach path indicator) conduit and electrical cable upgrades. There was a drainage inlet at the southern end of the site with a 33” RCP outflow. RS&H Engineers of Dallas, Texas added a “Type A” inlet at the north end with three 6” perforated pipes set at a base grade of approximately .50%. These pipes connected to the existing drainage inlet some 800’ away. The engineers studied subsurface drainage vaults to collect stormwater but this idea failed due to the underground network of utilities, making installation impossible. It was also difficult to use larger pipe based on the elevations of different utilities.
The perforated pipe system bedded in lightweight aggregate proved to be the easiest solution for a few reasons. The 3/8 x 10 gradation has a total pore space of 62% (41% air filled porosity, 21% moisture retention) meaning it could store a lot of stormwater. The depth of fill ranged from 6” to 36” and being half the weight of normal pea gravel it was easy to install and compact around existing utilities. It was also economical to transport at approximately 37 cubic yards per load. The overall drainage system was designed for a 25-year storm and guaranteed to have no ponding water.
Placed above the lightweight aggregate was a 4” layer of infiltration media made by blending select proportions of expanded shale and sandy clay that drains approximately 20 inches/hour. RS&H specified a sand-grown Bermuda sod placed over this to provide the best infiltration and eliminate the compaction issues resulting from sod grown in clay. Project specifications called for fully charging the infiltration media with water before sodding and to follow up during grow-in with the use of a water truck. In all, some 1300 cy of lightweight and 350 cy of infiltration media were installed. The infiltration media sets up firm and has good water retention and pore space needed for growing quality turfgrass. It can be mowed without the worry of rutting as with saturated clay soils.
Lightweight aggregates may be the solution for other projects with similar constraints of utilities and the need for improved drainage. Engineers may also find that they are able to downsize their pipes due to a lower coefficient of runoff. Lightweight aggregate manufacturers are seeing more use of their materials for stormwater control in bioswales, bioretention systems and infiltration strips.