Comment
“
Any discussion about artificial surfaces tends to become very technical very quickly. This may be because it is in the non-technical areas that the case for artificial sports surfaces is at its weakest
Performance testing on 3G pitch
great deal of understanding of a wide range of factors. Compared to the solid, yet technically quite simple, foundations upon which the design and construction of artificial turf sub-base and drainage and attenuation systems are built, the equivalent understanding pertaining to natural turf is woefully inadequate. Circumstances cannot readily be explained to planners and funders, and so mistakes and misdirected investment occur whilst client expectations are continually
mismanaged. This has led to a diminution of the overall standing of natural turf in the current sport and recreation industry, a tragedy to say the least.
Infiltration rate
Now for some detail. It is a given that an artificial surface will be suitably free draining to meet the needs of the sport. Infiltration rates pertaining for example to the carpet are usually known with some accuracy. Some variability exists in relation to the infiltration rate of the overall installation (the combined influence of the carpet, macadam and stone layers for example), but these rates are almost always extremely high relative to natural turf and most rainfall events rarely affect the playing quality to any great extent or for a long period. This is one of the main reasons for installing an artificial surface in the first place, of course. In the case of natural turf, Sport England
Double ring infiltrometer for measurement of infiltration rate
Soil structure sampling equipment 16 I PC AUGUST/SEPTEMBER 2016
have indicated tolerance limits of infiltration rate for some sports, including football and rugby. This is assumed to be measured using double ring infiltrometers. Straightaway, we run into an oversimplification of the circumstances, however. Firstly, it will be found that infiltration rates, especially on clay soils, can be very high indeed during the summer months, but fall to near zero on the same soil during the winter. This is because clay soils shrink as they dry and this creates fissures through which water may flow very freely. With the coming of winter, persistent wetting of the soil causes the clay to expand once more, which closes the fissures and reduces the infiltration rate. The time of year at which measurement of infiltration rate takes place is, therefore, highly significant in the interpretation of such data. Sometimes, a very low infiltration rate may not be correlated with poor drainage performance. This is because no account is made of the influence of surface runoff. Sports pitches with a marked slope can often shed water to the side more rapidly than
they can absorb it, so the effect on overall performance is the same as if the pitch had a relatively high infiltration rate. Sport England’s maximum tolerance of pitch cross- fall is 1 in 60 and this is certainly sufficient to achieve a significant runoff effect of this kind. In fact, improving the infiltration of pitches that behave like this can sometimes actually diminish their performance. The soil is made capable of becoming saturated more rapidly as it absorbs water that would previously have run off the side. As you see, the situation is more complicated than it first appears.
Void space and total porosity
The materials that make up the sub-base of an artificial sports pitch have a quality known as the ‘void space’, which is the percentage of the overall volume of the layer that is air and therefore capable of temporarily storing water. What used to be known as MOT Type 3 aggregate, for example, is often selected for its high void space qualities, in addition to providing the structural support for the surface itself. This might have a void space of up to 40% which, for a sub base depth of 250mm, would be sufficient to contain 100mm of water, a heck of a flood. Natural soils or sports turf rootzones have an equivalent property to void space which is known as total porosity. Near the surface of a sports pitch, this property may vary between about 30 and 40% and tends to decline with depth (in natural soils). In recent years, we have consistently been measuring total porosity on natural turf sports field sites with the aim of achieving greater understanding of how this quality of soil is related to performance as a sports surface. Ultimately, we hope to identify actual tolerance limits for total porosity which may be related directly to the overall quality of a particular surface, existing or yet to be made. Such tolerance limits might ultimately find their way into a practical set of performance quality standards.
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