Water management key to field success
Adequate drainage is a key factor in sports field performance. What that consists of and how it is achieved will vary with each project to match site specifics, field use expectations, the construction or reconstruction time frame and the budget.
My company, Millennium Sports Technologies, Inc., works exclusively in the sports field market. We provide professional design and consultation services for both synthetic and natural grass systems. During the design process, we work with the architects and engineers, as well as the facility owners, testing laboratories, material providers and the playing field contractors. We develop the designs and specifications in-house so we can control all the components to ensure the integrated properties that result in a properly functioning field.
The first step on any project is researching the existing soils information through study of the laboratory analysis of numerous borings and the geotechnical reports. We need to know what kind of soil we’re going to be working with underneath the field to determine drainage and stability requirements. Sites that are naturally gravelly and sandy won’t need as much internal drainage as a nonporous, heavy clay with little or no vertical drainage. We also need to determine how effectively the water will move through the rootzone matrix and down into the gravel layer.
Next comes the identification and analysis of the storm sewer systems that are available to the field to determine how to effectively move the water off the playing field. The next step is discussion of the performance expectations to determine how quickly the client wants the field to drain. The demands are high for fast drainage for baseball and softball fields, as the clay infield areas are so susceptible to rainouts. A rained-out game translates to lost revenue and unhappy fans, so rapid drainage at all levels, be it a professional or collegiate venue, is important.
All of these factors must be taken into consideration when designing the field, as the required infiltration and percolation rates for the gravel, sand rootzones and permeable aggregate materials’ selection all factor in to the sizing and placement of the piping and, ultimately, the success of the field’s drainage.
We prefer the more traditional drainage design of creating a series of trenches for the perforated piping system. Some of the newer technologies promote placing the piping directly on the subgrade. That does save labor, but we believe the additional cost of trenching is justified because if the piping becomes damaged or plugged, the water will still find its way into the trench and then follow the slope of the trench to the point of discharge in the storm sewer system.
The primary factors determining the design of the subsurface piping system are the type of field and where the point of discharge is located for the main outlet pipe. A baseball field drainage system looks more like a herringbone. For a rectangular field, the piping design may be more linear. We also want the termination points of the drainage system to be placed where they can be accessible. Each pipe should have a cleanout so the system can be flushed. Also, should any drainage issues develop over time, a camera line can be run through the pipe to search for a break or other problems in the pipe.
With baseball fields, we tie the drainage system into surface quick drains to facilitate rapid movement of the rainwater from the tarp. These quick drains are generally located adjacent to the warning track and are monitored by the turf manager to prevent contamination by the warning track material or grass clippings.
The drainage pipe typically is placed in the trench and surrounded by gravel on the top, bottom and sides. We specify lining the trenches with filter fabric so the gravel is kept separate from the soil of the walls and the bottom of the trench. In fields with high water tables, the filter fabric serves as a barrier to prevent contamination of the gravel as water pressure tries to move the soil upward.
The standard field profile design is to provide for at least a 4-inch gravel blanket placed over the entire subgrade of the field, with the sand rootzone placed over that. We find some interesting situations with the depth of the gravel in some of our fields because of the jurisdictional requirements put on the project concerning the amount of water runoff that can be released off the site during a significant rainfall event.
In these cases, we can specify a greater gravel depth, as many gravels have a 40 to 50 percent void space, which allows the field’s subsurface to become the storage site to meet those requirements.
The field design for TD Ameritrade Park, in Omaha, Neb., is such a project. The field will be the new home of the College World Series and Creighton University’s baseball program, and will host additional events such as concerts and football play. During our initial design meetings with the project architect, civil engineers and geotechnical engineers, we learned they were considering putting huge underground storage tanks beneath the parking lots to retain the hundreds of thousands of gallons of anticipated water runoff mandated by the sites’ retention requirements and the close proximity to the Missouri River. Through a combination of engineering, laboratory testing of materials and a lot of math, we designed the subsurface of the field with up to 30 inches of precisely specified gravel over the entrenched pipe, enabling it to capture and hold water during a rain event of 7.5 inches, thus satisfying the water retention requirements for the site.
By specification, multiple laboratory tests will be conducted to make sure that the gravel needed will be compatible with the rootzone materials. TD Ameritrade Park will have a SubAir system that can be used to provide ideal oxygen levels, and to speed water movement from the field. The system will be complete with gas injectors, which will allow warm air to be forced up into the field to keep the rootzone warm earlier and later in the season.
Before the introduction of advanced laser grading technology, it was difficult and time consuming to hit specified tolerances. Now grading can be more precise and should meet design specifications of plus or minus no more than .50 inch in 25 feet. Sometimes, due to the nature of the project or the materials, such as a low uniformity coefficient in the sand rootzone, it can be difficult to achieve the specified grading tolerances. Simply walking across a sand-based field will imprint it, but usually the grading tolerances can be met.
We specify laser grading the subgrade, and then certifying that subgrade with a survey. We also require laser grading and certification of the final finished grading prior to the installation of the natural or synthetic turf. Most of the professional field builders we work with provide laser grading as a rule. They know the importance of proper grading, drainage and subsurface installation that must meet exacting standards for the field system to function properly.
As preferred by most of our clients, especially professional baseball and soccer, we design most of our sand-based natural grass fields and baseball outfields without surface drainage, in other words “flat.” We rely on the vertical drainage to channel water through the turf, rootzone and into the gravel layer. We generally design the infield segment of a baseball field with a slope of .75 to 1 percent from the mound out 360 degrees on most all major league and collegiate-level fields. They need that slope to assist in moving the water off the tarp, especially at the collegiate and minor league levels, where manpower to remove the tarp may not be so readily available.
We usually design native soil and modified native soil fields with 1 to 1.5 percent slope for surface drainage. If the budget will allow it, we’ll include one of the vertical bypass systems, such as the QwikDrain System, that incorporate a matrix of sand slits and perforated pipe to channel water from the field surface into the main subsurface collectors that tie into the storm drain system. When these systems are included in the design, surface drainage often can be reduced to .5 percent.
All of our synthetic turf field designs incorporate subsurface drainage, often incorporating trenching and piping similar to a natural turf sand-based field. If the gravel to meet our preferred specifications is readily available, we may be able to work with a 6 to 8-inch gravel layer, laser graded and rolled to meet our requirements, which can directly support the synthetic turf system. Sometimes availability requires us to specify a 4 to 6-inch layer of coarse stone or gravel, topped with a 2 or 3-inch layer of finer material to provide the necessary leveling and stability. Laboratory testing is critical in this situation. If the two materials are not uniformly compatible, some of the finer material may filter into the voids in the larger material, blocking, rather than facilitating, water movement. Even with compatible materials, laser grading and survey certification are always necessary.
The synthetic turf permeable backing and fibers, infill material and any underlying padding must also be tested to ensure proper drainage. Some of the synthetic turf system installations are designed with a very light crown for surface drainage, others are completely flat.
Where additional, or more rapid, drainage is required on synthetic fields, or where subsurface piping will be at a minimum, we’ve incorporated the drainage mat system from Brock USA. We’ve found it efficient and structurally sound, and it generally eliminates the need for a stone base.
We’ve also designed sand-based natural grass fields to include the AirField System, which incorporates a series of connected rings with a filter fabric to facilitate drainage. It provided a workable drainage solution for two sand-based football fields built over an abandoned landfill at Baylor University. The project was heavily scrutinized to ensure there was no penetration into the landfill. The Brock USA or the AirField System is also viable when fields are to be built over a rock ledge or a stone subsurface where trenching is either impossible or extremely expensive. There are definite benefits to incorporating these systems in those types of applications.
With drainage there are multiple avenues to move water from point A to point B. Once you’ve researched the variables, it becomes quite clear from a design standpoint what you’re dealing with; the challenge is solving the problem. Facility owners have high expectations for the performance of their fields. We all have an obligation to keep up to date on the dynamic changes in the technologies and products continually becoming available to the sports field market, and do the research necessary to produce the best possible results in terms of safety and playability for the athletes and aesthetics for the fans.
Daniel R. Almond is owner of Millennium Sports Technologies, Inc., based in Littleton, Colo., and has over 30 years of experience working with sports fields.