The Dirt on ESCS Lightweight Aggregate for Stormwater Management
Stormwater management is a large and ever-increasing priority in environmental and civil engineering, particularly as the United States continues to experience record-breaking precipitation events across the country. According to the Center for Climate and Energy Solutions, the most-immediate impact of excessive precipitation is flooding, which can push the stormwater systems we have in place to their limit and degrade water quality. Even more concerning, the Environmental Protection Agency reports that approximately 772 U.S. cities have combined sewer systems, where stormwater and wastewater are mixed, treated and released. Heavy rainfall can overwhelm such systems, sending excess stormwater and untreated wastewater directly into the environment—harming public health and ecosystems.
To protect water quality for current and future generations, more environmental and civil engineers are looking to flexible stormwater-management programs. One way to design an effective and sustainable stormwater system is to use an expanded shale, clay or slate (ESCS) lightweight aggregate. An effective alternative to conventional aggregates such as sand or gravel, engineers can use ESCS in low-impact development (LID) techniques to help mitigate pressing health and environmental concerns.
What Is ESCS?
ESCS lightweight aggregate is made by expanding raw shale, clay or slate in a rotary kiln at temperatures reaching 2,200 degrees Fahrenheit. As the material cools down, the bubbles form an unconnected network of pores surrounded by a hardened, high-quality ceramic material. This “clinker” material is then crushed or screened into standard aggregate gradations. The result is a lightweight, low density and structurally strong material that’s non-toxic and won’t degrade through time.
ESCS as a Filtering Media
ESCS’ premium performance as a filtration media stems from its unique physical properties created during the vitrification process. In application, ESCS is very durable, and, coupled with its porous structure and large surface area, it’s an exceptional filtering material in a variety of applications for removing harmful elements such as nutrients, metalloids, pathogens and suspended solids.
The surface area of filtration media has a significant impact on performance. The term “specific surface area” refers to the surface area per unit mass of a filtration media and is expressed as m²/g. The specific surface area is closely related to—and often the dominant factor in—a media’s performance in the physical adsorption of suspended particles and chemical compounds. The specific surface area of ESCS fine media (< 4.75 mm) can be up to 50 m²/g, which is much greater than traditional media. For comparison, the specific surface area of ordinary sands can be as low as 0.0011 m²/g. This increase in surface area is the key for a media with a larger effective size and a higher-than-usual uniformity coefficient to achieve the desired filtration results with improved water flow and less clogging.
In addition to having a large surface area, ESCS lightweight aggregates also contain a uniformly distributed network of pores that range in size from approximately 5-300μm. The interstitial voids act like tiny reservoirs for the absorption and storage of moisture and water-soluble nutrients. The cellular structure within the particles is developed by heating the raw materials to incipient fusion. At this temperature, gasses evolve within the pyroclastic mass, causing expansion that’s retained upon cooling. Pores close to the surface are readily permeable and fill with water within the first few hours to a few days of exposure to moisture. Interior pores, however, fill extremely slowly, with many months of submersion required to approach saturation.
Different ESCS Uses as Filtering Media
Environmental and civil engineers can optimize the unique physical properties of ESCS lightweight aggregates in a variety of filter media applications, making it a multitasking stormwater management tool.
ESCS in filter strips—areas of vegetation over which sheet or dispersed runoff flows at a shallow depth—promote faster water percolation through the filtering system and will reduce the amount of time standing water remains exposed at the surface level. ESCS porosity provides deeper aeration for vigorous root systems that contribute to pollutant removal. The hardworking material also facilitates cationic exchange and microbial action, which break down organic substances and reduce pollutants in the water supply.
Unlike a swale or filter strip, rain gardens are designed to collect stormwater runoff for onsite infiltration. In this setting, ESCS provides the same filtering function as it does in filter strips. The material is an excellent medium for amending heavy clay or compacted soils to promote plant root growth, improve drainage and retain moisture during dry periods.
Bioretention rain gardens in commercial settings typically include underdrains that collect, pretreat and then discharge the exfiltrate. Many regions that include nutrient-sensitive watersheds have treatment guidelines that ESCS achieves. These stormwater utilities become landscape features as the vegetation thrives in a media balanced with air and water.
ESCS can be blended with topsoil to drastically increase the infiltration rate. A 50/50 blend has an infiltration rate of approximately 20 inches per hour that, in some cases, allows for smaller pipe sizing due to the reduced runoff rate. Bioswales are easily incorporated into parks, residential communities, university campuses and city properties. They require less maintenance than other LID systems.
Structural Soil for Urban Trees
The U.S. Department of Agriculture estimates that a typical medium-sized tree intercepts 2,380 gallons of rainfall per year. Healthy urban trees reduce the amount of water diverted to engineered systems and are a favored stormwater practice. Urban trees need the proper volume of soil with adequate organics, voids for air and water storage, and strength to support concrete or permeable paving. Structural soils with ESCS typically are blended with 80 percent lightweight aggregates and 20 percent sandy clay. This media holds water longer than sand-based systems while reducing the fluctuations in soil temperature that can be detrimental to newly planted trees.
Underground Filter Fill (Permeable Surfaces)
Underground filter fill is the layered aggregate fill material beneath permeable paved surfaces, which store water underground and allow water to percolate into the subgrade. In addition to new construction, these permeable surfaces with underground storage are being increasingly used in retrofits where a best management practice is required to treat stormwater, but perhaps the available land can’t accommodate a pond or basin. In these cases, a permeable surface allows the engineer to treat the stormwater underground.
Unlike the typical compacted stone base usually found under conventional pavements, permeable technologies use three layers of open-graded aggregates under the bearing surface. With a 40 percent void space in the underlying 12-inch aggregate layers, a 4.8-inch rain event can be stored and slowly released back into the subgrade. An alternative to conventional aggregates because of its large surface area, greater porosity and total pore space (60 percent or more), ESCS is a better medium for storing water and capturing suspended solids, hydrocarbons, metals, nitrogen and phosphorus.
ESCS in Application
Many engineers will agree that the design elements you don’t see often contribute most to a project’s success. That can certainly be said of the following two projects that use ESCS lightweight aggregate as an integral component of their long-term stormwater reclamation solution.
ESCS Solves Design Challenges at Collapse Soil Site Near Airport Runway
Airports are under constant change and development, but renovations on any scale can be challenging with a mass of buried infrastructure. So when the Dallas Fort Worth International Airport needed to replace underground “precision approach path indicator” conduit and electrical cables in a location with poor drainage and sensitive utilities, the project team needed a creative design solution.
Located on the southwest corner of the airport, the construction site had poor drainage with minimal surface grade. Beneath the narrow swale approximately 900 feet in length was a “spaghetti” network of sensitive and aging utilities, including gas, electric, water, a petroleum pipeline, communication cables and electrical vaults. To implement drainage improvements, project engineers explored the idea of installing subsurface drainage vaults to collect stormwater. However, the underground network of utilities and pipelines (at varying depths) made this idea futile.
A perforated pipe system bedded in lightweight aggregate proved to be the best stormwater solution. Expanded shale was chosen with a gradation of 3/8 inches by 10 inches. This media has a saturated hydraulic conductivity of 1,200 inches per hour and a total pore space of 62 percent (41 percent air-filled porosity and 21 percent moisture retention). This provides great drainage and storage of stormwater beyond the pipe discharge system. From a construction standpoint, the lightweight aggregate (52 lbs./ft3) was easy to install and compact around the sensitive pipelines and utilities. It also was economical to transport to the airport at approximately 37 cubic yards per load. Leveraging the long-term performance benefits of ESCS, the new drainage system was designed for a 25-year storm and guaranteed to drain any standing water.
Placed above the lightweight aggregate was a 4-inch layer of vegetative filter media. The soil was a blend of expanded shale and sandy clay that drained approximately 20 inches per hour. A sand-backed Bermuda sod finished off the construction process.
Use of ESCS in Stabilized Horticultural Subsoils at Largest Urban Park in Austin
The largest urban park in Austin, Texas, Waterloo Park showcases the disciplines of civil and structural engineering, landscape architecture, hydrology, soil science, ecology, and horticulture. The 11-acre park has everything: walking and biking trails, a concert pavilion, an elevated skywalk, and lush landscaping reminiscent of Central Texas. The finished project—phase one in a series of green spaces and amenities tied to the city’s stormwater improvement plan—didn’t come without a robust set of challenges.
To solve the park’s flooding problem, the project team designed an underground stormwater channel measuring 10.5 feet in diameter. Rain basins and underground pipes were installed to collect rainwater and store in a cistern, where it’s then pumped into a series of rain gardens for filtering and reuse.
The soil was an integral part of the rainwater capture and filtering system at Waterloo Park. Beneath the topsoil was a layer of “stabilized horticultural subsoil” that ranged in depth from 8 inches to 30 inches, depending on the application. They were blended to meet the agronomic conditions of the performance lawn, street trees and plantings on-structure. They were valued for drainage (performance lawn), strength (street trees placed beneath pavement) and weight reduction (planting soils on-structure). The horticultural subsoils consisted of two-parts sandy loam, three-parts expanded shale lightweight aggregate and one-part compost. These materials drain well and provide added pore space for optimal growing conditions.
A Sustainable Stormwater Management Solution
With its unique composition and premium performance qualities, ESCS lightweight aggregate as a filter media can be an effective tool in an engineer’s arsenal of stormwater management solutions. By leveraging the benefits of this multitasking material, the environmental and civil engineering communities can continue to stand at the forefront of LID techniques and green infrastructure principles, helping U.S. cities to reduce runoff, delay input into the drainage system and improve water quality. In turn, we can take steps toward a cleaner, more-sustainable future.
About Eric Nelson
Eric Nelson is a sales representative with Arcosa Lightweight and a member of the Expanded Shale, Clay and Slate Institute (ESCSI); email: [email protected] Debbie Stringer is a sales representative with Carolina Stalite Company and also a member of the ESCSI; email: [email protected]