Infill Depth Perfection
A key ingredient in maximizing synthetic-turf field playability and safety
By Thomas P. Shay
Photos: Woodard & Curran
After nearly six decades on the market, synthetic-turf fields for recreation and athletic facilities are an integral part of being able to schedule and complete games. While the technology has evolved since the Houston Astros played its first season on synthetic turf in 1966, the application remains resource-intensive to install and replace, along with regular maintenance. These concerns are valid, but there’s a path to improving the lifespan of a synthetic-turf field, and it is all about paying attention to the depth of infill.
Infill is often a neglected consideration even though it accounts for up to 90 percent of material within a synthetic-turf system (not including the shock pad underlayment). Infill is literally the basis of the surface on which athletes compete and children play. So, let’s focus on the significance of infill during planning, design, construction, and, most importantly—maintenance and operation. Acknowledging how important infill depth is and taking action will help parks and recreation departments get more life out of synthetic fields, save money, and reduce the likelihood of player injuries.
Perfecting infill depth begins with planning, design, and construction. However, if there is no follow-through on operations and maintenance, all that upfront effort will be for nothing. We know by heat-mapping fields infill depth varies significantly across the surface in both aged and newly installed facilities. Measuring infill depth as part of routine operations and maintenance tasks is necessary to ensure the performance and longevity of the field and is simply not being done.
Laying A Good Foundation
Infill depth is critically important because it affects an athlete’s movement, traction, and impact-absorption as well as dictating how the game ball rolls, rebounds, and reacts with the surface. Infill is also crucial in supporting the synthetic fibers, keeping the fiber upright, and protecting them from wear and tear. Whether there was too little infill installed to begin with or the infill migrated over time, shallow infill depth means fibers start to lay flat, causing more damage to the turf, exposing the fibers to ultraviolet light, and degrading the material more quickly. The fibers will split, tear, and break, requiring repairs or replacement and potential environmental contamination as microplastics when the fibers are lost.
It is worth noting that a properly designed turf system with a shock pad will improve field surface consistency, safety, and performance, while also reducing the amount of infill needed during installation, repairs, and subsequent replacements. Introducing a shock pad into a synthetic-turf system is recommended for improved athletic performance and safety, especially when there is a high probability that infill depths are improperly managed.
Considerations For Planning And Design
Deciding what type of infill to use largely dictates what type of synthetic turf and shock pad is used in the system based on their respective properties, which is a critical decision and coordination item in the planning and design phase. Parks and recreation staff will also want to consider what type of sports will be played on the field to further inform infill-depth standards and tolerances, resilience, performance characteristics, warranties, maintenance, replacement cost, disposal of materials, climate, and overall impact on the environment.
Some sports-governing bodies have identified infill-depth standards. For example, the European Committee for Standardization issued guidance EN 1969 (Surfaces for Sports Areas - Determination of Thickness of Synthetic Sports Surfaces), which is referenced by the Federation Internationale de Football Association Test Method 21, World Rugby, and Synthetic Turf Council Performance Guidelines. However, this standard is limited in its recommendation for infill-depth measurement locations (17 to 19). On fields larger than 80,000 square feet, this is an insufficient number of measurement locations to ensure equally distributed and maintained infill depth. Based on our observations, a minimum of 100 infill depth measurements are required.
The Moment Of Truth
Synthetic-turf systems are performance-tested in a controlled laboratory environment. Decisions are made during planning and design based on these controlled results, which include a prescribed and consistent amount of infill. Unfortunately, during construction, measuring infill depth is often overlooked, even though it is written into installation manuals. Without measuring during installation, fields then have incomplete and inconsistent infill depths from the start, with low points frequently seen at the edges of fields and sidelines, and sometimes even in the body of the field in high-wear areas. Therefore, it is important that contractors measure the infill depth as it is applied to create a sufficient and consistent depth across the surface, setting the field up for success from the start. It is also important to check the infill quantity delivered against the amount estimated during design and reconcile it if the amount delivered is insufficient. Furthermore, if a contractor utilizes a broadcast spreader, inconsistent infill depths are almost certain because the user is trying to avoid spreading infill outside of the field limits. When we consider how important infill is—for both athletes and the game ball—contractors should receive an adequate amount of infill, use appropriate tools for application, monitor production rate, and measure infill depths; this is critical to field performance in providing what owners expect and pay for.
Evolving turf systems, innovative infill materials, and fast-tracked projects also pose a challenge to contractors. Contractors should be made aware of the system and infill specifications. Meanwhile, with infill application occurring toward the end of construction, project owners—whether school, university, or parks and recreation department—should not rush a project to meet a hard stop, if possible. Rather, there is value in adding schedule flexibility to allow time for contractors to conduct infill-depth measurements during application. This may mean rethinking the project schedule entirely, especially if an owner is trying to shoehorn the construction in during the summer months when there is less athletic programming and other facilities are often under the same schedule and stress.
Leveraging Technology To Measure Infill Depth
Originally, project teams relied on sports-surface testing laboratories to complete infill-depth measurements during the final stages of surface-performance testing. This typically included measurements taken at 10 to 19 locations across the field. However, the limited number of test points has shortcomings. Later, a grid of infill-depth measurements taken across the surface and tabulated in a list was commonly used to improve accuracy. The grid approach revealed increasing variations of infill depths across a field, but also proved challenging when conveying to contractors where infill depths needed modifications, were insufficient, or exceeded the recommended amount.
We now use a GPS-enabled device to geolocate hundreds of measurement locations across a field. Data are used to generate color-coded, infill-depth heat maps. Increasing the number of measurements taken and using technology to map out the findings has proven effective in achieving the target average depth across a surface area. It also helps maintain an infill depth within a percentage of a specified depth target. The map becomes a quantitative and visual tool for the contractor to identify areas of concern and level out the infill in certain spots rather than rework the whole surface. This approach not only provides specific areas of concern but also reduces the amount of labor required, resulting in a higher quality surface.
Field Maintenance Is Key
While synthetic turf does not require the same level of skill and effort to maintain as does quality natural grass, it still needs regular maintenance to realize the best return on investment, especially for high-use fields or portions thereof. Synthetic-turf field systems come with a manufacturer's guide for operation and maintenance. We know, however, that very often recommendations for regular infill-depth measurements and adjustments are not followed. Yet, based on what we have learned, it is likely the single-most important task (and the simplest) in maintaining a field’s performance and lifespan.
Parks and recreation staff should consider the following when tasked with maintaining synthetic-turf fields:
Understanding that the type of infill material strongly influences behavior and performance, as well as necessary operation and maintenance tasks.
Keeping infill material in stock and in an easy-to-handle and transport form (such as 50-pound bags).
Being proactive about maintenance, especially infill depths, ensuring longevity.
Keeping the synthetic-turf manufacturer and infill supplier manuals on hand to easily reference operations and maintenance guidelines.
Considering how infill migration and fiber loss impacts the environment and incorporating containment or recovery elements in regular operations and maintenance.
Level Out The Playing Field
While using an infill gauge may seem tedious, field owners should see it as a value. Recording infill-depth measurements during installation before a contractor leaves and routinely once a playing field is in use is the best way to keep a field level. Rely on the synthetic-turf field system and infill manufacturers’ standard operating procedures, and leverage available technology to manage maintenance. Making use of simple tools and a little effort to maintain adequate infill depths will significantly improve the performance and longevity of a synthetic-turf field.
Thomas P. Shay, PE is a principal and technical manager at Woodard & Curran, an integrated engineering, science, and operations firm. He specializes in site civil engineering and sports and recreation facility design. Reach him at tshay@woodardcurran.com.