From site selection to subgrade preparation to seed and sod implementation, there's more to constructing a natural-grass athletic field than meets the eye.
The varsity soccer field at Boston College is tucked within a grove of towering shade trees. On a New England fall game day, the pristine green stage of natural turf is surrounded by a curtain of orange and yellow. "It's a tremendous venue - as nice a place to go see a soccer game as you can imagine," says Patrick Maguire, president of Geller Sport, the Boston-based landscape architecture firm that handled the project.
But given the choice, Maguire would rather have placed the playing surface somewhere else. Unfortunately, the urban setting of a Boston campus wouldn't allow it. So this wooded lot on BC's law school campus in Newton became the next best thing, even though it would require the extra site preparation and expense involved in removing dozens of mature trees. "They had no choice but to build this field where we ended up building it," Maguire says.
Identifying a workable location is just the first step in building a high-performance competition athletic field. Designers must also take into account native soil characteristics and potential soil amendments, drainage and irrigation requirements, and the type of turf installation that best suits the timeline of the field's first expected use. It's a process that is part science, part common sense. And if carried out correctly, the end result is a field that will look and function like new for years to come.
Here, then, is an overview of the most common considerations when constructing an athletic field from the ground up.
Site Selection Like Boston College, many colleges, school districts and parks departments are hamstrung by their own real estate realities, but when options are available, it is critical to choose a site with the least number of constraints. These constraints may include a severe slope that dictates significant cut-and-fill efforts, an existing shelf of bedrock adversely affecting the grade to the point that costly blasting or importation of material is required, the existence of trees that will need removal, or the field's proximity to protected wetlands. The latter scenario also will likely prove costly, both financially and politically, as destruction of wetlands typically involves the mandated replication of those wetlands at twice the area (or more) than was destroyed. "Although regulations might allow it, it isn't something that conservation commissions and environmental groups will necessarily look upon as a good thing," Maguire says.
Key to the construction of any outdoor field is adequate space - typically between two-and-a-half and four acres, depending on the inclusion of such amenities as parking, rest rooms and concessions areas. While baseball diamonds edge out regulation-size soccer pitches as the largest of the outdoor athletic fields, extra space is sometimes allocated around soccer fields so they can be shifted forward and backward or side to side, or even rotated a full 90 degrees, to alleviate sideline and goal-mouth wear.
In short, the ideal location for an athletic field is large, flat, well drained and mostly devoid of trees. Some peripheral trees can be desirable, since they reduce wind and thus cut irrigation demand. But while some shade is good for spectators, too much on the field (or portions of the field) can lead to irrigation-management challenges, ongoing grounds maintenance issues regarding fallen leaves, or, worse, outbreaks of turf disease.
Soil Testing Once a suitable site has been identified, the native soil should be tested to determine its composition and chemical makeup. Samples should be gathered by a competent soil scientist - preferably a Ph.D. in agronomy who specializes in sports fields - and lab-tested to ensure that a pH level suitable to sustain quality turf exists, or if amendments are called for.
"On any field, you have a minimal area to deal with, and before you start planting grass, you have to make sure that profile is as high-quality as it can be," says Scott Pierce, a partner at the Atlanta-based Turf Solutions Group who handles the firm's projects in the Southwest. Improving the profile may involve removing contaminants (everything from rocks to petroleum spills), importing soils that will help mitigate compaction, or introducing preplant fertilizers or preemergent herbicides.
In terms of physical composition, individual soil particles come in many shapes and sizes. Soils heavy in fine particulates such as silt and clay will likely compact easily and inhibit free drainage, while coarser material such as sand won't. Infiltration rates should be tested under normal compaction conditions, giving a sense of how the field will need to be designed for optimal performance. Low infiltration rates will dictate that the bulk of drainage is handled at the surface by crowning or sloping the field and allowing water to move horizontally toward the sidelines. High infiltration rates reveal that water can easily move vertically through the soil profile, and may signal the need for a subsurface drainage system.
Soil tests often are conducted with augers or splitspoon samplers, which provide a cross-section of the soil strata. "You have to make sure that what you're looking at is consistent," Pierce says. "You don't want a third of your field to be contaminated or not comparable to the rest. You have to start with a clean slate, and that means testing not just a couple samples, but taking and testing 20 or 30 different samples on a grid system."
Ideally, the process should also involve digging holes on the site at least five feet deep with a backhoe in locations covering all four corners of the proposed field, allowing those conducting the test to climb into the hole and get a close-up view of how the various soil layers have been interacting naturally. There may be half a dozen layers of material to inspect, according to Maguire. "Someone who knows what to look for can actually get down in there and see what's going on, feel how compacted those profiles might be," he says. "Water can be poured in the hole, to see how quickly water is going to filter down."
Native soil that is deemed deficient can be amended by tilling into the seed/sod bed a percentage of sand or calcine clay to address compaction and drainage concerns, or compost to improve the nutrient makeup of the would-be root zone, which may measure a full 12 inches deep. Successful root-zone mixes will vary by region, with the aggressive nature of Bermuda grass providing enough of a root structure and thatching capability on fields in the South that are at least 80 percent sand to remain sufficiently stable, given adequate irrigation and fertilization. Meanwhile, successful sand-based fields in the North, where ryegrass, bluegrass and fescue rooting isn't as aggressive, may consist of 70 percent sand, 15 percent native soil and 15 percent compost. "A root zone that is magnificent down in the southern part of the country may be a miserable failure in the North, and vice versa," Maguire says.
Drainage For the field to function at its best, water must be easily moved through the soil profile a full two to three feet below the surface. If a backhoe test finds the water table a foot from the surface, a subsurface drainage system will need to be installed. In fact, subsurface drainage systems are more effective at mitigating high groundwater concerns below the surface than they are at handling rainwater introduced on top of it.
Drainage pipes are typically laid at least 18 inches below the playing surface, in foot-wide trenches filled with stone or within full drainage blankets composed of six to 12 inches of stone. The stone allows water to filter through to the perforations in the pipes while preventing silt from clogging them. How well water moves laterally through the soil profile, as determined during early soil testing, will dictate the spacing of the drainage pipes - typically on 15- to 20-foot centers. In the absence of a drainage blanket, a subgrade that exhibits poor lateral water movement will require pipes to be placed in closer proximity to each other, perhaps a mere two or three feet apart, for a sufficient amount of water to reach them. However, such a scenario often proves cost-prohibitive, at which point surface drainage becomes the only alternative. "You have to be very careful what you do," says Mike Flowers, president of Championship Turf Services in Harwinton, Conn., "because you could just be burying money."
In addition to the parallel placement of pipes, newer - and more expensive - drainage system designs incorporate pipes laid one on top of the other in a perpendicular grid pattern, or pipes laid on a slant from the center of the field in a herringbone pattern. One increasingly popular system doesn't incorporate pipes or gravel at all, but rather creates a one-inch layer of open air 11 inches below the surface using a plastic support grid sandwiched between permeable (top) and impervious (bottom) layers of PVC.
Should a traditional drainage blanket be specified, which is often the case in sand-based fields, care must be taken so that the size of the stone in the blanket layer works in concert with the particle size of the root-zone mix above it. Drainage blanket stones should be sufficiently larger than root-zone particles to allow for the vertical movement of water through the profile, yet not so large as to allow the root-zone particles to migrate into the drainage blanket. "Anytime you have a finer material that's over a coarser material, the pull of gravity needs to be greater than the pull of each one of those soil particles on the water," Maguire says. "So if you have a very fine soil over a very coarse soil, it actually won't drain at all, because those soil particles will hold the water by adhesion and it won't flush through freely."
Proper drainage will also be a function of the depth of each layer, a calculation that also must take into account initial soil-testing results. While insufficient drainage can render a field unplayable, too much drainage can leave it susceptible to drought. "I've seen mistakes made where people had a soil scientist figure these things out based on a 10-inch root-zone mix and a six-inch gravel blanket, and then they decided to cut costs by reducing their root-zone mix to six inches," Flowers says. "They changed the dynamic, and now the field's not functioning the way it should."
To function properly, the drainage blanket (like any subgrade material) also should be crowned to the exact same slope specifications as is planned for the finished grade.
If the construction budget doesn't allow for a subsurface drainage system or drainage blanket (both of which can add thousands of dollars to the project's final price tag), a far less expensive means of improving subsurface drainage includes employing a paraplow to fracture the hardpan subgrade to a depth up to 36 inches. The resulting fracture channels will allow for improved drainage, so long as care is taken during the rest of the construction process not to recompact the subgrade. This means using low-pressure equipment such as bulldozers on tracks and graders on rubber tires.
Irrigation While water removal capability is an integral part of field design, an equally important consideration is the ability to add moisture to the finished product. Proper care of any competition field will require irrigation beyond natural rainfall. Says Flowers, "If you want to keep a field in good condition, it needs water, and nature just doesn't provide enough of it."
That is particularly true in the Southwest, where proper spacing of irrigation equipment is essential. "We have a saying - if you have a square foot of ground that isn't being hit by three sprinklers, you've got a brown spot," says Pierce. "Out east or in the northern parts of the country where you're getting an average amount of rainfall, some of those inconsistencies may never show."
In-ground irrigation systems are not only convenient (eliminating the labor involved with moving hoses and other equipment to and from the field), they also allow for more uniform precipitation coverage and better adjustment of precipitation rates. And in-ground irrigation systems facilitate not only the watering of grass plants, but the conditioning of the soil in which those plants grow. Just as a baseball diamond's clay infield needs moisture to prevent compaction and hardening, so does the soil in a grass field. Says Maguire, "If you're in a place that has heavy soil and you let it dry out, it becomes incredibly hard, incredibly compact, and can even become cracked."
Like subsurface drainage systems, the network of irrigation pipes should be placed at least 18 inches below field level, so as to avoid potential damage to the system by deep-tine aeration or freezing. There are typically a minimum of four rows of irrigation heads on a football field - one outside each sideline and two running down the middle of the field. The number of heads will depend on how much natural rainfall a region of the country receives. For example, fields in the Northeast may feature fewer heads with a longer throw, while fields in the Southwest will have more, lower-profile heads to help combat high winds and evaporation rates. "Just like anything else, irrigation is site-specific," Pierce says. "If you're in Texas, you might get 24 inches of rain, but you may go through a dry June, July and August. You have to design that system based on your worst-case scenario."
Moreover, various irrigation heads are available for different applications. Rotors that throw water the greatest distance are recommended for baseball infields, for example, while short-throw rotors are suitable for football sidelines.
Grading Laser leveling has become the standard to ensure that the finished grade is properly crowned or sloped for adequate surface drainage. It should also be used to grade the subsurface - including a drainage blanket, if one is being installed. Fields that drain extremely well may be specified with a crown ranging from a half to three-quarters of a degree of slope per foot. Fields with average drainage capabilities may need a crown of one degree per foot, while poorly draining fields are likely to need at least a two-degree crown. Flowers has even seen fields with three-degree crowns. The latter scenario is least conducive to the hosting of athletic events, particularly ball-roll sports such as soccer, since such a severe crown may translate to a rise in elevation from sideline to midfield of a couple of feet. "I remember playing on a field that had a crown that was so severe that if you were sitting on the bench, you could not see the ball on the other side of the field," says Maguire, who played collegiate soccer at the University of Rhode Island.
However, field construction experts agree that under no circumstances should a soccer field be built completely flat. No matter how conducive a native soil is to vertical drainage, a flat native-soil field will not drain adequately, and foot traffic over time will form areas of standing water. Even a sand-based field with its excellent drainage capabilities should exhibit some degree of slope. An alternative to crowning the field is to tilt the entire playing surface from one corner of the field to the opposite corner. Though almost imperceptible, a half a degree of slope will represent roughly a four- to five-foot difference in elevation from one end of the field to the other. "Over 360 feet, it's not that easy to see it, but it's there," says Flowers. "A good coach will make sure he's got that downhill run in the second half."
Runoff from a crowned or sloped field must be dealt with, as well. Swales built beyond the boundaries of a field or in between fields in a sports complex naturally direct runoff away from the area. Collection drains can also be specified to move water from the field through a system of underground pipes. Fields in regions of the country that are susceptible to flash flooding must be designed to handle, say, three inches of rainfall in 30 minutes, says Pierce. "Utilize deposit points - places where you can move the water via a vertical pipe to get it out of your area. At that deposit point, you might create a large, deep pit filled with heavy gravel where a three-inch rainfall eventually percolates into the soil."
Turf implementation The final stage in the construction process is the actual implementation of the natural-grass playing surface. Given the choice, Northern-based field construction professionals would prefer seeding the field as opposed to installing sod. Seed established in the soil bed prepared for it on site will ultimately produce the most stable turf. However, grass that has germinated on site takes at least two growing seasons before it can stand up to the punishment of athletic play. That means that the field will be out of commission a minimum of nine months to a full year. In the South, where quality turfgrass doesn't come in seed form, the spreading of sprigs (grass plant segments consisting of a stem or rhizome and at least one crown) or the laying of sod is preferred.
Regardless of region, if the field must be played on in the short term, sod is the only solution. Sod cut to a thickness of two inches, as opposed to one inch, will support immediate use simply because of its deeper root structure and heavier weight. Although roots cut during the harvesting process are instantly killed, these dead roots will continue to stabilize the grass plant without actually nourishing it. Entirely new roots must grow from the plant crown at the surface. Ideally, sod designated for collision sports such as football should be given six to 10 weeks during the spring or fall growing seasons to establish roots at least three inches into the ground (baseball can be played on sod two to three weeks after installation). Sod composed of aggressive hybrid Bermuda grass, meanwhile, may be football-ready in a mere 45 days, according to Pierce.
Sod's convenience comes with a price. Not only is it much more expensive than seed, it introduces potential problems. "With sod, you're lifting soil out of one location and rolling it back out on top of another soil. It's not going to be the same soil," says Flowers. "You could have a fairly well-draining soil, but if you bring in a sod soil that's a little bit heavier, it can seal the top."
Also, the thicker the sod is cut, the harder it can be to establish - with new roots having to reach farther to infiltrate the native soil. For this reason, sod installation often amounts to a stopgap measure. Flowers points to NFL teams that can afford to install thick-cut sod for a playoff game or two, then remove it before the start of the next season. Finally, one means of mitigating the soil-compatibility risks associated with sod installation is to specify washed sod. In this scenario, all the soil used to grow the turf is rinsed from the root zone prior to installation over the prepared root-zone soil mix.
The finished product may be a picturesque venue on a bucolic college campus, or simply a nondescript but otherwise functional field within a municipal park. In any case, there's more to constructing a natural-grass athletic field than meets the eye.
In fact, the most common construction mistake often involves underestimating the importance of intelligence gathered underground - intelligence that will help designers plan for the long-term viability of the playing field via suitable soil composition, drainage and irrigation. "A lot of times fields are designed by people who design the grade on top without really looking at the physical properties of the soils below," Flowers says. "It's not as uncomplicated as people think."
"The absolute most important thing you need to do is prepare the subsurface soil - no matter what turf you put down, no matter where you are in the country," says Pierce, adding that it's a process that can't be rushed. "Laypeople see all the action in the beginning and say, `Well, when are you going to make it green?' I tell them that that's the last thing I do. I'm going to bake this cake before I put icing on it, and I want to make sure it has the right ingredients. Growing and maintaining grass is simple if you have a quality growing medium. Once you have that, you're going to have a quality field for a long time."