fines and sports ground drainage

The Impact of Fines on Sports Ground Drainage

We thought we would put an article together on what seems too  be a current and future issue. The impact of fines on sport ground drainage, and their impact on the infiltration rate of a soil.

The smallest of soil particles, can have a big impact on sports ground drainage, and how sports grounds perform. This is especially the case when you play on them in wet weather.

As a qualified Soil Scientist I have always taken an interest in the impact that these tiny particles have on the in filtration rate of a soil.

Sand Root Zones.

• Modern sports grounds often use sand to build their rootzone. The main reason for these sand based systems is the benefit of improved soil drainage that allows play even after torrential rainfall.
• Often sand based systems comprise a sand root zone, a gravel layer, and an underlying drainage system.
• The type of sand you use is very important as sands can vary a lot in their particle size and shape.
• The sand is placed over a gravel layer to create a perched water table above the gravel. This keeps the root zone moist in dry conditions.
• A drainage system is then installed under the gravel to remove any excess water.
• To maintain this freely draining medium it is important to maintain the drainage characteristics.
• Any fines contamination (silt and/or clay) will negatively affect the sports ground drainage, and the infiltration rate of the soil.
• If contamination occurs it tends to lead to issues with compaction and poor water infiltration into the profile.
• Sand root zones have a maximum allowable limit for these ‘fines’ in sports grounds. ¹ACT Government’s Design Standards for Urban Infrastructure  ²Recommendations for a Method of Putting Green Construction. 2004  ³Baker SW. Provision of turf surfaces for major football stadiums in Europe :environmental issues and design criteria. 1995. Multifunctional Grasslands in a Changing World Volume 2, pp 283

What is The Impact of Fines on Sports Ground Drainage?

•  ≥20% by volume of fines to a USGA sand, increases the soil moisture content to a point where it is unacceptable for trafficked turf sites⁴Brown, P.J., McCarty, LB., Quisenberry, VL., Hubbard, LR., & Addy, MB. (2019). Influence of Increasing Fines on Soil Physical Properties of U.S. Golf Association Sand. HortScience, 54(11), 2063-2066. https://www.doi.org/10.21273/HORTSCI14110-19.
• You should not add even a small amount of fine material without prior evaluation to a freely draining rootzone.
• In an 80:20 sand to silt + clay profile, a 2 cm layer formed under the turfgrass⁵Kesgin E, Gezici K, Tan Kesgin Rİ, Ağaçcıoğlu H, Karaagaçli M, Sağlam MT. Assessing Drainage Performance of Turfgrass Rootzone in Sports Fields. JARNAS. December 2024;10(4):803-818. doi:10.28979/jarnas.1530185. This slowed water infiltration of the soil. In this case, the impact of fines made this unsuitable for sports fields.
• A study in 1998 and 1999⁶Henderson, JJ., Crum JR, Wolf TF, and Rogers, JN. 2005. Effects of Particle Size Distribution and Water Content at Compaction on Saturated Hydraulic Conductivity and Strength of High Sand Content Root Zone Materials. Soil Science. 170: 315–324. looked at the amount of silt and clay you can add to a sand in order to increase its strength without any loss in in the infiltration rate of a soil. Only those with 10% silt + clay or less had an acceptable, in terms of drainage (150-200 mm/hr).

Silt and Clay vs Sands.

• Silt particles are smaller than sand, but bigger than clay.
• Soils that contain a high proportion of fines are more easy to compact.
• When compaction occurs this reduces the soil pore space, and limits the movement of air and water through the soil.
• The air spaces and water channels in high silt content soils are more restricted than in sandy soils.
• Silt holds more moisture than a sand. This is due to a combination of surface area and pore size.
• Fine soil particles have a larger surface area than sand particles. As the surface area increases the amount of water that these fine particles can absorb increases.

 What is Silt?

• Silt is a type of soil particle that has a floury feel when dry, and feels soapy or slippery when wet. Silt is often found in rivers, where it is transported by flowing water and deposited as sediment over the ground. 

What is Clay?

• Clay is a fine grained material that has a particle size of 0.002 mm.
• Small clay particles are able to fit together more tightly than large grains of sand.
• Soils with high levels of clay tend to have more micropores.
• Clays can absorb a relatively large amount of water, and have a larger water holding capacity than sands.
• More of the water is unavailable in clay soils to plants than in sands.
• Clays retain soil water and do not drain as well as sands.

How Do Fines in Sand Rootzones Affect The Drainage?

From a sports ground perspective, sand constructions with high silt and clay contents tend to have a lower infiltration rate of the soil in comparison to sand root zones. High fines content negatively affect the infiltration rate of a soil⁷Sor, K., and W. D. Kemper. 1959. Estimation of hydrateable surface area of soils and clays from the amount of adsorption and retention of ethylene glycol. Soil Sci. Soc. Am. Proc. 23:105-110.. D. Foth Henry, “Fundamentals of Soil Science,” 8th Edition, Michigan State University, John Wiley & Sons, New York, 1990.⁸Foth HD, Fundamentals of Soil Science, 8th Edition, Michigan State University, John Wiley & Sons, New York, 1990.

These fine particles like silt, reduce the pore space in sands. This makes it harder for water to infiltrate and move through the soil. The end result is surface pooling of water, reduced playability, and an increased risk of compaction and waterlogging, especially in wet weather.

Less Pore Space.

Silt particles have a much smaller particle size than sand particles. This means that there is much less space between the particles for any water to flow. So high levels of silt in sand rootzones will limit how quickly the profile, and in some cases the surface drains. This lowers the infiltration rate of a soil.

Lower Infiltration.

When there is a high level of silt or clay present in the soil, water takes a much longer time to move through the soil in comparison to a sand-based rootzone. This is because the finer silt particles form a physical barrier to water flow.

Layering of Particle Sizes.

Surface Pooling of Water.

Because the drainage is impeded, water tends to build up at the turf surface. When it rains the result is wet, muddy conditions.

When soils with a high silt content become wet, the fine silt particles tend to float in the water. As a result, when these soils come under any degree of pressure it’s easy for these silt particles to be pushed into soil voids.

In areas that are compacted the result is shallow rooted grass where this compaction occurs.

Increased Compaction.

High levels of silt in sand rootzones are more prone to become compact. This in turn further reduces sports ground drainage capacity.

When you play on soils with a high silt content in wet weather, there can be large increases in the soil bulk density. In some cases this means that the infiltration rate can drop to < 2 mm per hour.

Playability.

Soils becomes less able to cope with wear when they drain poorly. This is due to the shallow root zone and water remaining at the surface.

How To Limit The Impact of Fines on Sports Ground Drainage?

To limit the impact of fines in sand rootzones, consider the following:

Ensure that you use Turf that is Free of Silt and Clay.

This may seem like common sense, but we have often seen unwashed turf put down over sand root zones. This is despite the fact the turf is clearly contaminated with finer material.

If the site is a new construction or if you are re-turfing you must make sure to limit the impact of fines on the sand rootzone. The easiest way to do this is to ensure the turf is clean of fines like silt and clay.

We were recently involved with the specification for a major Sydney venue. The project involved resurfacing the ground, and due to time and quality constraints, the turf had to be silt free and also not from an area prone to flooding.

There were two reasons for this:

• There was a very small window to carry out the required work. This meant if a turf farm flooded the window for the work would be compromised due to limited road access.
• For all the reasons above the turf needed to be silt and clay free. Washing would remove the majority of the fines, but there was always the concern that there was potential for root zone contamination to occur.

Flooding.

• If turf grass is buried under sediment for any period of time, it damages the grass due to a lack of light and smothers the vegetation.
• Any Silt and clay that is deposited over an existing freely draining root zone will tend to cause long term infiltration and sports ground drainage issues.
• This blog covers helpful tips on how to limit turfgrass damage in floods.

This makes it very important to remove this sediment from sand root zones as it stops a surface cap of silt and clay over the profile.

sediment deposits on turfgrass after flooding

Optimized Sand-Silt/Clay Ratio.

Make sure that the root zone material that you use in a grounds construction has a high percentage of sand to facilitate water flow. See the specification above.

Subsurface Drainage.

The installation of an efficient drainage system helps remove excess water from the root zone. That’s obviously under the proviso that water can actually bypass any surface issues and reach the underlying drains.

Reduce The Impact of Fines on Sports Ground Drainage By Topdressing

Regularly topdress with sand. This helps with the drainage and aeration. This is even more important if the soil is compact or the surface is contaminated with silt or clay. 

Aeration:

Aerating the turf can help break up compaction and improve water infiltration. However, if silt or clay contamination is the issue this won’t fix the problem in the long term.

In fact there is an argument that if the silt or clay is present at the surface if you punch holes to depth it may physically move this fine material further down into the root zone.

How to Determine if a Silt or Clay Problem Exists.

• Determining if you have a silt or clay problem can go a major way towards identifying if this is the cause of poor water infiltration.
• The maximum rate at which water enters a soil is called the infiltration capacity. Different soils have different infiltration capacities.
• A freely draining Sportsground with a sand rootzone can have an initial infiltration rate of over 1000 mm/Hr.
• In contrast, a soil with a high silt/clay content can have infiltration rates below 10 mm/Hr.

How to Test if Fines in Sports Ground Drainage are a Problem?

If you suspect you have an issue with fines contamination there are a few easy tests you can carry out before you send samples away to a laboratory for testing.

The following are in increasing difficulty.

Pot Test For Fines in Sports Ground Drainage.

The simplest test is to carry out a “pot test”. Thanks to McIntyre and Jacobsen for this.

• Take a small pot and fill it with the rootzone material.
• Then drop it for 4 x from about 100 mm.
• Add water to the top of the pot.
• If a significant amount of dirty water runs out of the pot then it probably has a high level of silt.
• In contrast, if the soil has low silt levels, the water will not be dirty when it comes out of the bottom of the pot.

Check the Infiltration Rate.

Infiltration Rings.

The final test to carry out before you send samples away is to use what is called a double ring infiltrometer. This is a relatively simple and accurate way to measure the infiltration rate “in-situ”, and the potential impact of fines on a sand rootzone.

Some factors that affect the infiltration capacity at the soil surface are:

• Soil compaction caused by machinery, concerts or play.
• Fine particles such as silt and clay washing into surface pores, and
• Cracks and fissures (macro pores).

The double ring method has a number of advantages over a single ring infiltrometer. The main one is that it measures the water infiltration without the loss of water by sideways movement. This often occurs if you use a single ring infiltrometer.

The metal ‘infiltration rings’ are partially pushed into the soil and you then fill them with water. You then measure the speed of water infiltration. The double ring limits the lateral spread of water after infiltration.

Double ring infiltrometers come in various sizes and the small units are really simple to use. However, they do have limitations.

Limitations of the Double Ring Infiltrometer.

• A double ring infiltrometer is very sensitive to fissures, cracks or aeration holes. Any of these will result in a much faster flow of water than you would otherwise see. As many of these cracks are often not visible at the surface it is not always possible to avoid them when you choose a site.
• The results from small rings do not always agree with those from larger units. It’s common for people to often use small infiltrometers as they are much easier to than the larger sets of rings. Small units are easy to carry and use less water.
• However, the affects of lateral and horizontal flow are much greater with small double ring infiltrometers. In fact small rings tend to overestimate the infiltration rate.
• If you use larger rings, the proportion of water lost to sideways movement is a lot less. This means that they are more accurate.
• In some situations it is not practical to measure the infiltration rate of a sand rootzone.
• On some sites where hybrids turf or replacement systems are present, the presence of a backing or a reinforcement material gives a false reading and overestimates the infiltration rate.

Hopefully you have found this article of interest. We have discussed the impact of fines on sports ground drainage and how these small particles can cause big issues with the infiltration rates of soils.

These small soil particles can have a major impact on water movement in modern day sports ground constructions. Bearing this in mind, every effort should be made to reduce any potential for contamination of these freely draining soil profiles.