30 SportsTurf | May 2010 www.sportsturfonline.com

SO YOU’VE MADE UP YOUR MIND thatyou are going synthetic. You were a bit pan-icked when the idea was first proposed, butnow you have studied your current field useprogramming, maintenance levels and avail-

able space options. You have explored all of the knownheat, recycling and sustainability issues. You have workedwith a qualified design professional to determine thehighest and best use of the land and budget available.You are fully versed in the latest materials, methods andmanufacturers all the while paying attention to everydetail imaginable. From ASTM test to insured warran-tees, you have considered it all. Well, almost all. Whatabout the field’s base? You might ask, “Doesn’t thatcome as part of the synthetic turf?”

Of course you do need a base under any synthetic turffield but no, fields do not typically come with a base orcomposite base materials as part of the system although afew offer it as an option. The base under any syntheticturf field serves two distinct functions, neither of whichare typically considered in natural turf fields: a structurallysound base for field construction and a media for drainageof the field.

The structural component of any synthetic turf base isdesigned to ensure that once the field is in place, it nevermoves. Differential settlement, expansive soils, saturatedsubgrades or an inability to drain water are all disastrousto a new synthetic turf field. Unique to this type of proj-ect, it includes a 360’+ straight edge by which the qualityof construction will be judged for life. While a ½-inchsettlement on a natural turf field may go unnoticed, itwill stick out like a sore thumb on synthetic turf. Withthat in mind, any synthetic turf field installation shouldbegin with a geotechnical report.

A good geotechnical report will contain informationessential to providing a firm and unyielding base for thefield. Existing soil conditions should be examined andrecommendations for mitigation of existing sub gradeconditions included. By requesting an expansion index aspart of the report, you get an immediate idea of thepotential for movement if the subgrade is exposed tomoisture or freeze-thaw cycles.

Recommendations for treatment of the subgrade con-ditions typically range from removal of topsoil and com-paction to lime or cement treatment and can greatlyimpact the overall cost. Achieving an initial value of

Synthetic field bases 101

Facility&Operations | By Tony Wood

92%+ compaction at the sub gradelevel will typically ensure stabilitythroughout the life of the field.

Based on the existing subgradeconditions, consideration for a geot-echnical fabric should be included aspart of the initial soil investigation. Inhighly expansive soils a woven geot-echnical fabric can prevent penetra-tion of water into the compacted subgrade to avoid expansion. In lesssevere conditions, a non-woven geot-echnical fabric may help insure thatthe specified stone base does not

migrate while still allowing somemeasure of water in to the sub grade.In the most severe conditions, a PVCor other impermeable material may beused as a liner to ensure that no sur-face water ever reaches the subgrade.These fabric options should beexplored with the geotechnical engi-neer before design ever begins.

BUILDING THE FIELDWe are now ready to begin build-

ing the field section. Drainage is thesecond critical component. Based on

In the most severe conditions,a PVC or other impermeablematerial may be used as a linerto ensure that no surface waterever reaches the subgrade.

32 SportsTurf | May 2010 www.sportsturfonline.com

the findings of the geotechnical report wemay be able to allow some water to percolatein to the subgrade, but that is the exceptionand not the rule. If you have an existing subgrade that can be compacted, with a highpercolation rate and a low expansion index,you are extremely fortunate. Typically, mostif not all of the surface water that penetratesthe synthetic turf will need to use the base asa transportation medium to the storm waterdrainage system.

Generally, there are at least two baseoptions for consideration when building asynthetic turf field section: a stone base ordrainage mats. Both are used today with apreference that is often based on regionalavailability of materials, native soil condi-tions and experience of the constructionteam. The first synthetic turf fields used acombination of aggregates to achieve thestructural stability required while maintain-ing the drainage characteristics to keep thefield playable in the worst conditions. Thismethod is still often referred to as a “tradi-tional” stone base.

The stone must be angular enough toallow pore space for drainage, the sieve sizevaried enough that it will interlock to form astable surface without settlement, and thematerial strong enough that it will not turnto dust when compacted. A final layer ofstone is often required with the same charac-teristics in a size that can be fine graded tomeet strict tolerances for planarity.

Engineering of a good stone base isjust the start. A perfect mix of aggregates canbe ruined by a bad installation. Best prac-tices require testing of materials at the sourceand upon delivery to the site. Continual test-ing for compaction and porosity should bescheduled regularly as the aggregate is beinginstalled. An experienced contractor or sub-contractor is key. A low-bid environmentmay not achieve the best results.

Drainage mat systems underlay the syn-thetic turf and come in a variety of materials.Cost of raw materials and petroleum, con-cerns for safety, and the occasional failedflood test have resulted in the design of mul-tiple systems. The intent is for easier con-struction, predictable drainage and in somecases a tertiary benefit of increased or consis-tent shock attenuation.

It is worth taking a moment to talk aboutshock attenuation, i.e. Gmax. It is the mostoften used term when discussing the safetyof synthetic turf fields. Drainage mat systemshave been shown to influence the Gmax of asynthetic turf field, but few are sold as partof a synthetic turf system. The potential forfuture claims or disputes and the associatedfinger pointing, should the Gmax exceed safelimits during the warranty period, are real.For this reason, we strongly recommend thatthe Gmax performance of the synthetic turfalone be guaranteed to meet the specifica-tions regardless of the base it is installed on.Any anticipated impact that a drainage mat

may have on the Gmax of the field shouldbe considered a bonus and closely coordinat-ed between the drainage mat manufacturerand the synthetic turf manufacturer to avoidwarranty conflicts.

The most significant difference between atraditional stone base and a drainage matsystem may be in how they handle water.Concerns about over compaction and poros-ity of the stone are not a factor in mostdrainage mat systems. If a stone layer isinstalled, its function can be structural only.This makes the task of providing a firm andunyielding base much more comfortable tomost grading contractors. An aggregate basecan be installed and compacted to fulfill thecore requirements of structural stability andplanarity.

Drainage mats may require an additionallayer of geotechnical fabric to ensure that thehorizontal surface flow under the syntheticturf does not erode the aggregate base. Theunit flow rate of some drainage mat systemshas tested at 26 gal/min/ft2. Translated,that’s more rain than any 100-year event inU.S. history and far exceeds the limits of thesynthetic turf above it. A layer of woven geo-textile fabric can help distribute the waterevenly and guarantee that the aggregateand/or subgrade below it are protectedagainst erosion.

In instances where an existing sand-basedfield is being replaced, some of the existingsystem may be able to remain. Sand-basedfields typically have a minimal potential forexpansion. The geotechnical engineer willgive recommendations about the stability ofthe subgrade and if any treatment would berequired to modify the remaining sand base.Modifications will still be necessary. Mostsand based fields are designed with the root-zone as the structural component to stabilizethe playing surface. In the absence of livingturf, most sand based fields feel more likebeaches and less like athletic fields. If thesand base material is deemed stable, thepotential for removal of the synthetic turfand reuse of the field base for a natural turffield remains.

Regardless of the base option chosen, astorm drain outlet is required. Pipe sizesand materials will vary based on regionaland engineer’s preferences, but all serve

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Engineering of a good stone base is justthe start. A perfect mix of aggregates canbe ruined by a bad installation.

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similar purposes and share similar problems. Often times the opengraded stone used as drain rock around pipes is rounded and strug-gles to compact to specified levels. If lime or cement treatment wasused to stabilize the subgrade, care should be taken in areas wherethe depth of the pipe exceeds the depth of the treatment. Lining ofthe trench with the woven geotextile fabric used below thedrainage mat is a common practice. Locating the trenches thatcontain the drainage piping beyond the field boundaries helpsensure that differential settlement over the life of the field will notimpact the use.

SUSTAINABLE OPTIONSConsider sustainable or “green” options. Many of the composite

drainage products contain recycled materials or can be recycledthemselves. The benefit of keeping the field section shallower with adrainage mat can minimize earthwork, off haul and equipment costsas well as the carbon footprint of the project. Most drainage mat sys-tems can be delivered in a single load.

The potential for rainwater harvesting exists with any syntheticturf base option. In a traditional stone base, the storage capacity ofthe stone and the filtration of the water through the base and syn-thetic turf make an excellent start towards re-use of the rainwater forirrigation to cool the field or irrigate surrounding landscape areas.The same benefit is realized with any drainage mat option with theonly limiting factors being storage capacity and required filtering ofthe water collected. Either system generates a water supply that iscleaner than most run off from natural turf fields. Compared to thepotential contamination of silt, herbicides and pesticides commonlyused to maintain natural turf fields at their peak the harvested rain-water is often cleaner than municipal recycled water sources.

Base systems are gaining their own importance in the life of afield. They now offer a warranty that meets or exceeds that of thesynthetic turf manufacturers. One composite base provider hasoffered a 20-year warranty on their materials as standard. While noguarantee is iron-clad, that level of confidence in a system to per-form provides a level of comfort that traditional stone base construc-tion cannot match with a standard construction warranty of oneyear.

Once you have made the decision that synthetic turf is in yourfuture, be sure that your selection of base materials provide the struc-tural stability and drainage performance you need. Local landscapearchitects, geotechnical engineers and contractors with specific expe-rience in this specialized field can help you with material selection,budgets, timelines, constructability and sustainability options. ■

Tony Wood, a landscape architect with Beals Alliance, Sacramento,CA has completed hundreds of facilities in the past 21 years with abroad range of scope, budget, and program needs.

The potential for rainwaterharvesting exists with any syntheticturf base option.