Technical
Stone sub base
banding, or altering soil structure, for example by verti-draining, is the equivalent to altering the types and arrangements of materials in the layers beneath a carpet.
Natural soils have void space in the same way that a stone carpet has void space, although the extent of this is very variable and far less easy to predict. These factors affect the capacity of the topsoil to retain water before it is able to pass into whatever drainage or soakaway capacity may lie beneath it.
It is dependent on factors such as the
soil texture, the degree of soil compaction and also the time of year as seasonal variations in soil moisture content also occur. Soil structure and texture also affects the speed with which water transfer to an underlying drainage or soakaway system takes place. For artificial surfaces, that transfer is assumed to be instantaneous. For natural turf, it certainly is not, and it is here that the complications really arise. A brief mention of infiltration rate measurement is appropriate here. This is a key component for example of the Sport England ‘playing quality assessment’ method for natural turf surfaces.
That measurement refers to the rate at which surface water is able to penetrate the topsoil. It could be related to the rate at which soil water enters the subsoil and/or drainage system, but this is less easy to be sure of. What it is not is a measure of the rate of subsoil infiltration as referred to above.
Sand carpet for natural turf surface in preparation
To measure this, very deep and substantial holes have to be excavated and filled with water, and the measurement taken according to a very strict and clearly defined method (BRE 365).
Having got through the topsoil, or pitch construction, water will behave beneath a natural turf surface in exactly the same way it will beneath an artificial surface. So, the criteria relating to soakaway design or drainage discussed above may be applied from the topsoil/subsoil interface in exactly the same way.
It is in the nature of the topsoil or natural turf pitch construction that the most variability exists, and this upper section of the facility, as we have seen, greatly influences the design of the associated drainage or soakaway system. I believe that, by thinking about natural and artificial sports surfaces in this way, we are getting closer to a ‘Grand Unifying Theory’ of sports surface optimisation and flood prevention. It would appear that such a theory is necessary in order to ensure that projects are able safely to go ahead in a sustainable manner, thereby bringing more sport to more people in a cost effective way, which is surely our ultimate objective.
Dr Tim Lodge PhD, Agrostis Sports Surface Consulting, RIPTA
www.agrostis.co.uk
Sand band cross section
“By thinking about natural and artificial sports surfaces in this way, we are getting
closer to a Grand Unifying Theory”
JUNE/JULY 2013 PC 117
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