Beneath the surface

When we play our favourite sport on one of the huge varieties of modern synthetic surface, we seldom consider what may lie below our feet. As in every type of construction, only the best foundations will provide the long-term performance demanded by players and facility operators. Let’s look at some of the major elements in the construction of a suitable base.

The first, and most obvious consideration is the selection of the site. In many cases the selection process boils down to “Hobson’s choice” because there is only one location available. If, however, the luxury of selection exists, careful consideration should be given to the factors that influence the construction processes employed.

Flat surfaces
Sports surfaces are laid to very flat gradients, meaning that the slope on the finished surface is minimised to ensure that the effect on the sport is imperceptible. In sports where the ball rolls on the surface, such as football and hockey, the path taken by the ball will be influenced by the slope, as will be the pace, depending on whether the slope is helping or hindering the roll. In sports where the ball bounces, such as tennis, the effect of the slope is more likely to be felt by the players than any material effect on the bouncing ball.

In an ideal world the surface would be laid totally flat, but consideration must be given to dealing with the surface water drainage, thus all surfaces tend to be laid with some degree of fall towards the drainage outlet. The various governing bodies of each sport have recommendations on slopes and falls on playing surfaces for their sport and due cognisance should be taken of these requirements when planning a new facility.

It is likely that some degree of earthworks will be required to achieve the single plane required for the playing surface but, ideally, this should be kept to a minimum. Disposal of surplus excavated material off-site is a very expensive business in these days of landfill tax so, if possible, retaining the materials on site should be an objective when designing the finished profile.

One of the most common misconceptions is the area of land required for an individual sports facility. The client looking for a tennis court in their back garden will invariably consider the dimension taken on the play lines, finding (when taking side-runs, runbacks, fencing, drainage and excavated banks into account), that the actual area required can be more than three times the playing area.

To a lesser degree, this can also be the case when constructing athletics tracks and artificial grass pitches. It follows that an accurate site survey is always required before a site is chosen. This site survey will also define the proposed orientation of the facility. In an ideal world, the preferred orientation is north-south in the UK to avoid playing into the setting sun, from one end, in the evening. The survey should also identify site boundaries, trees, and existing services and plot the position of roads and access to the site.

A comprehensive sub-soil survey is also a basic requirement prior to final site selection and certainly prior to construction. It is essential to know the load bearing potential of the sub-soil as well as its drainage characteristics and depth of topsoil.

Construction phase
This information will be used in designing the overall thickness of the baseworks construction to carry the superimposed loads during the construction phase. In most cases, the maximum load imposed on a synthetic surface construction is from the contractor’s plant at the time of installing the facility.

When the pitch, track or court is finished, the predominant load is pedestrian (players) with light vehicular equipment for maintenance purposes.

In certain cases it may be necessary to make provision for heavier long-term loading from vehicles at specific access points. A typical example would be a crossing point on an athletics track to allow heavy vehicles to pass from the external road on to the central grass pitch area. This provision can be made by installing an extra depth of structural base at the defined access points.

Where the sub-soil has a tendency to be unstable, such as certain clays or frost susceptible soils, a greater depth of base construction is installed to ensure that the surface layers are stable and will be unaffected by the passage of construction traffic, or winter frosts, in future years. The excavated formation should be treated with an appropriate residual herbicide to ensure that any vegetative growth is neutralised.

Structural stone base layer
It is usual to install a geotextile membrane on the formation to prevent contamination of the structural stone base layer by sub-soil punching up from below. This membrane is semi-permeable and allows the passage of water through the textile while preventing silt passing up into the structural layers.

The dry stone structural base comprises a layer of clean crushed rock that is graded to ensure a “lock” of the individual pieces of stone, and is compacted to a finished thickness of between 150 and 250mm, depending on the sub-soil conditions. This layer must be porous on completion to allow the passage of surface water through open-textured systems. It will normally be necessary to install the base stone in at least two layers with the final layer laid to a fine tolerance. Bound base layers for artificial sports surface construction normally consist of bituminous macadam. This macadam will be open-textured in the case of porous systems, and densely graded for impervious systems.

The construction layers of a pitch, track or court are normally retained around the perimeter by a pre-cast concrete kerb. This kerb will be laid on a mass concrete foundation and will be haunched with concrete to provide an accurate line and level for the edge of the facility.

Consider drainage
It is essential that the drainage of the proposed facility should be given proper consideration. It is all too common to find a tennis court that has been installed without any thought as to where and how quickly the surface water will discharge.

In sites with poor sub-soil drainage characteristics, it will be necessary to install a pattern of drains below the surface of a pitch. This seldom needs to be done in the case of tracks or tennis courts because the horizontal distance the water will have to travel over, or under, the surface is much less than the case of a soccer or hockey pitch.

Provision must be made to accommodate water that may be inherited from outside the area, such as from adjacent higher ground or cut banks. This is normally done by providing a cut-off drain on the outside edge of the facility. It is equally important to cope with the excess water that will run off the surface of the facility during periods of torrential rain.

This resultant surface water must be conveyed away from the facility to an adjacent watercourse. In some cases the only option will be a soakaway, but design and construction of soakaways is a specialist area and should only be undertaken as a last resort and then with great care.

The new “all-weather” facility will not, in itself, create any more surface water from the area than was created in the past. The difference is the intensity at which the surface water will reach the outlets. The period of time between the rain falling and the resultant volume of water reaching the outlets will be much less than when the topsoil, turf and vegetation are retained in the run-off allowing a much slower percolation to the drains. This “flash run-off” could result in existing drains surcharging, briefly, until the storm subsides.

The playing surface is laid on the baseworks described above. There are numerous types of synthetic sports surface available, but the list may be broken down into three major generic types of artificial grass, polymerics and acrylics. The factor they all have in common is that construction considerations are essential requirements if the system is to provide a top- class sports facility that will perform to the highest standards throughout its life.

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