Gravel Drainage Specifications
Numerous studies have been conducted to evaluate the safety and playability of traditional (non-infill) synthetic turf surfaces. Three methodologies are used to compare the safety and performance of various surfaces. These include 1) material tests where mechanical devices simulate human movement and measure the associated forces; 2) human performance tests where researchers measure the forces associated with the interaction of a human subject and a surface; and 3) epidemiological studies in which the number and type of injuries sustained by athletes during actual sporting events are counted.
Material tests have been completed that measure the shoe-surface traction and surface hardness of synthetic turf surfaces (Bowers and Martin, 1975; McNitt and Petrunak, 2001; Valiant, 1990). Human subject tests have shown improved athlete performance on traditional synthetic turf when compared to natural turfgrass (Krahenbuhl, 1974; Morehouse and Morrison, 1975) and epidemiological studies have counted the number of knee and ankle injuries on synthetic versus natural turfgrass (Meyers and Barnhill, 2004; Powell and Schootman, 1992; Powell and Schootman, 1993).
No large-scale epidemiological studies have been published comparing the number of surface-related injures sustained by athletes playing on infill synthetic turf systems to the number of injures sustained on either traditional synthetic turf or natural turfgrass surfaces. One study (Meyers and Barnhill, 2004) compared injury incidence of eight high school (American) football teams in Texas USA playing on infilled synthetic surfaces (FieldTurf) and natural turfgrass surfaces. Although similarities in injury occurrence existed between FieldTurf and natural grass fields over a five-year period of competitive play, there were significant differences in injury time loss, injury mechanism, anatomical location of injury, and type of tissue injured between playing surfaces. The researchers reported higher incidences of 0-day time loss injuries, noncontact injuries, surface/epidermal injuries, muscle-related trauma, and injuries during higher temperatures on FieldTurf compared to natural turfgrass surfaces. Higher incidences of 1- to 2-day time loss injuries, 22+ day time loss injuries, head and neural trauma, and ligament injuries were recorded on natural turfgrass fields compared to FieldTurf. The researchers state a number of limitations to their study including the random variation in injury typically observed in high-collision team sports and the percentage of influence from risk factors, other than simply surface type. Field conditions at the time of injury were not measured although the researchers noted that the majority of injuries (84.4%) occurred on natural turfgrass surfaces under conditions of no precipitation (dry surface).
The United States National Collegiate Athletic Association (NCAA) is collecting injury data from numerous men's and women's sporting events across the United States using a computerized system called " NCAA Injury Surveillance System" (National Collegiate Athletic Association, 2004) but presently does not have sufficient data from which to draw conclusions (R. Dick, 2004, personal communication).
Stefanyshyn et al. (2002) used human performance comparisons to evaluate 20 configurations of infill synthetic turf systems. Human subjects performed various maneuvers on the surfaces and the forces associated with the cleated foot interacting with the surface were recorded in the laboratory using a force plate installed beneath the turf surface. Stefanyshyn et al. (2002) reported a significant range of traction and surface hardness differences among the infill synthetic surfaces (Table 1) and grouped the 20 infill surfaces into categories of highly recommended, recommended, and not recommended based on surface hardness and both the rotational and translational (linear) traction recorded on these surfaces.
Shorten et al. (2003) performed material tests in which weighted shoes were dragged across varying infill synthetic turf systems and traditional synthetic turf. The translational and rotational traction of the various shoe-surface combinations were measured. The researchers concluded that both shoes and surfaces significantly affect traction. On all surfaces tested, shoes with lower profile cleats or studs had better overall traction performance compared to shoes with longer cleats and infill systems had better traction performance than traditional synthetic turf. Traction performance was calculated using an index where rotational traction values were subtracted from translational traction values. To eliminate scaling and range differences between the translational and rotational resistance measures, calculations were done using "standard scores" rather than raw data. The standard score is a measure of where a particular result lies relative to the average and distribution of all the results recorded: ex. Standard Score = (Actual Score − Average Score) / (Standard Deviation of All Scores). The researchers stated that further research is required to determine the effects of moisture, temperature and aging on surface traction performance. Both the study by Stefanyshyn et al. (2002) and the study by Shorten et al. (2003) were performed on newly constructed infill systems in a laboratory setting.
Selection and Placement of Materials When the Intermediate Layer Is Used
The tables above describe the particle size requirements of the gravel and the intermediate layer material.
The intermediate layer shall be spread to a uniform thickness of two to four inches (50 to 100 mm) over the gravel drainage blanket (e.g., if a 3-inch depth is selected, the material shall be kept at that depth across the entire area), and the surface shall conform to the contours of the proposed finished grade.
Selection of Gravel
Selection of this gravel is based on the particle size distribution of the intermediate layer material. The construction superintendent must work closely with the soil testing laboratory in selecting the appropriate gravel. Either of the following two methods may be used:
Send samples of different gravel materials to the lab when submitting samples of components for the intermediate layer material. As a general guideline, look for gravel in the 2 mm to 9.5 mm range. The lab first will determine the best intermediate layer material, and then will test the gravel samples to determine if any meet the guidelines outlined below.
Submit samples of the components for intermediate layer material, and ask the laboratory to provide a description, based on the intermediate layer material tests, of the particle size distribution required of the gravel. Use the description to locate one or more appropriate gravel materials, and submit them to the laboratory for confirmation.
It is not necessary to understand the details of these recommendations; the key is to work closely with the soil testing laboratory in selecting the gravel. Strict adherence to these criteria is imperative; failure to follow these guidelines could result in drainage failure.
The criteria are based on engineering principles which rely on the largest 15% of the root zone particles “bridging” with the smallest 15% of the gravel particles. Smaller voids are produced, and they prevent migration of root zone particles into the gravel yet maintain adequate permeability. The D85 (root zone) is defined as the particle diameter below which 85% of the soil particles (by weight) are smaller. The D15 (gravel) is defined as the particle diameter below which 15% of the gravel particles (by weight) are smaller.
- For bridging to occur, the D15 (gravel) must be less than or equal to eight times the D85 (root zone).
- To maintain adequate permeability across the root zone/gravel interface, the D15 (gravel) shall be greater than or equal to five times the D15 (root zone).
- The gravel shall have a uniformity coefficient (Gravel D90/Gravel D15) of less than or equal to 3.0.
Furthermore, any gravel selected shall have 100% passing a ½" (12 mm) sieve and not more than 10% passing a No. 10 (2 mm) sieve, including not more than 5% passing a No. 18 (1 mm) sieve.