A sound foundation is an essential component of any structure. This statement also holds true for artificial turf athletic field surfaces. To have a pristine field surface, the foundation must be designed and constructed with the utmost precision. A structurally sound and consistent surface is essential for playability and uninterrupted use. Natural turf fields require extensive maintenance, particularly during and after severe weather events. This maintenance is drastically reduced, if not completely removed, with the use of an artificial turf surface accompanied by a sound foundation. One of the greatest benefits of an artificial field surface is the ability to have it utilized in all weather conditions, most specifically during or after severe rain events. Most people don’t realize that the surface they are playing on consists of multiple layers of material. Most artificial turf fields are constructed upon a three-tiered base as its foundation. These three tiers consist of a virgin soil base and two separate layers of crushed stone — also called engineered fill — with each layer being a different size of stone. In some instances, an additional tier, called a “shock pad," is added to soften the playing surface of the field. Typically, a geotextile filter fabric is laid upon the soil base to prevent migration of the soil into the stone layers. Each of these layers is fine-graded with laser precision and then compacted with mechanical equipment to provide a strong, consistent surface. In addition to the fill, there is an extensive underdrain system designed to handle the water runoff draining through the playing surface.
The performance of an artificial turf surface relies on the performance of the base and underdrain system, which exists between the soil layer and stone fill. The underdrain system acts as the “veins” of the field, and are made up of interconnected perforated pipes. These pipes are connected to a header, which acts to collect the flow of water from the under drain system, and discharges in one of two ways: through either an infiltration or positive drainage system. An infiltration drainage system is utilized when clean, sandy soils exist beneath the field and groundwater conditions allow for its use. This system works in conjunction with the surrounding sandy soil, acting as a filter that serves to remove pollutants from the water. A positive drainage system is typically utilized when an infiltration drainage system is prohibited due to existing soil or groundwater conditions.
This system directs the runoff from the underdrain system to a series of pipes, which ultimately discharges into a body of water. However, unlike with infiltration drainage, no means of naturally filtering the water exists, so another structure would be required for filtration prior to discharge into a body of water. The utilization of an artificial field surface during rain events also depends on its porosity, allowing stormwater to filter through the surface into the underlying soil or under drain system. This allows games to be played on the surface regardless of weather conditions. These combined attributes allow for more frequent use of fields with little to no effect on the games played, as compared to that of a natural field, which mostly relies on positive overland flow for stormwater. Natural fields are also more susceptible to flooding and ponding, as the soils tend to become saturated. This can affect the playability of the surface, as well as future games and events. As far as artificial turf surfaces go, it is easy to see why a properly constructed base and underdrain system foundation is such an integral part of its design. And despite it being the part of the field you don’t tend to see, it creates a far superior surface to that of its natural counterpart. Mike Bowden, P.E. is a Senior Project Engineer in H2M's Civil Engineering group. You can reach Mike at email@example.com