Natural grass surfaces Advantages

Natural grass remains the most user-friendly of sur-faces, and the only permissible choice for some sports. The advantages of natural grass are:

• It is aesthetically attractive.

• It gives a speed of rebound and a degree of roll-ing resistance that is just about right for most ball sports.

• It provides reasonable (though variable) purchase for players feet when dry or wet.

• It gives a surface that is neither excessively hard nor excessively soft for comfortable running.

• It is less injurious to players who fall than most alternative finishes.

• If irrigated it is a relatively cool surface in hot climates.

• It will continually self-repair and regenerate.

Disadvantages

The major limitation to the use of grass surfaces is that they cannot be used in roofed stadia, and are difficult to keep healthy even under partial cover.

The reason is that grass needs ample light for really healthy growth, and air movement, humidity and temperature levels need to be kept within fairly strict parameters. To date it has proved impossible to arrange all these matters satisfactorily in a totally enclosed stadium, even using the most transparent roofing materials. In theory supplemental artificial lighting does help but there are no real-life appli-cations to demonstrate the effectiveness of this solution, and in any case the energy costs of such lighting may be prohibitive.

Even in a partly-roofed stadium the size of the roof aperture, the shadow-effects of the surrounding structure and other such factors may lead to dis-appointing results. One example of failure is the San Siro Stadium in Milan which was redeveloped to accommodate 80 000 spectators for the soccer World Cup in 1990. Only the spectator seating areas are roofed, with a central opening over the playing area; but even though this aperture is approximately

Table 7.1 Playing characteristics of tennis court surfaces

Surfaces Ball–surface Spin Player–surface

interaction interaction

Speed Height Trueness Topspin Slice Sliding/ Traction Resilience of court of bounce of bounce firm footing (slip or (hardness)

non-slip)

Grass Fast Low Variable Little Yes Firm footing Slip Soft

with partial

slide

Synthetic Fast Medium Variable Little Yes Firm footing Mainly Medium to soft

turf to low but partial non-slip

slide on sand

filled

Impervious Medium Medium Uniform Yes Yes Firm footing Non-slip Hard to medium acrylic

Porous Slow High Almost Yes Little Firm footing Non-slip Hard

macadam uniform

Shale Medium Medium Variable Yes Yes Sliding Slip Medium to soft Continental Slow Medium Almost Yes Yes Sliding Non-slip Medium to soft

clay uniform

Source: Tennis Courts, published by the LTA Court Advisory Service. Reproduced by kind permission of Christopher Trickey.

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81 the size of the football pitch below, the grass is

struggling to survive. The lesson to designers must be one of caution.

A second limitation is that grass cannot survive the same intensity and frequency of punishment as most artificial surfaces. This relative fragility will inevitably conflict with the stadium’s need to maximize the number of event days per annum for profitability.

The ‘pitch replacement’ concept

One response to the problems outlined above is the concept of ‘pitch replacement’ on an organized and systematic basis. The principle is to remove the grass when not needed, to allow other events to take place on an artificial surface underneath.

There are many removal techniques: (i) a Canadian method of growing the turf in large boxes which can then be moved out of the stadium on rails; (ii) a German method of growing the turf on pallets 4 m square which are then moved on the hovercraft prin-ciple; and (iii) a Dutch concept of leaving the nat-ural grass in place and constructing above it a new platform supported on remote-controlled hydraulic legs. In the UK, Odsal Stadium in Bradford has used a simple system of restoring the corners of a football pitch which had been cut off by a speedway track around the pitch: grass was grown on wooden pallets with a reinforced plastic mesh sub-base, and these were moved away to storage by forklift truck before speedway events. Further notes on this topic are provided in Section 5.4.2.

Installation

Planting and maintaining a grass pitch is a task for specialists. All the advice given below is for general background understanding only: a specialist con-sultant should be retained from the outset to give advice, draw up a detailed specification, invite ten-ders and supervise the work.

Figure 7.1 shows the elements of a typical grass-turfed surface, and should be studied in conjunction with the following notes.

For bowling greens and croquet the upper grass surface must be smooth, true and absolutely level, necessitating very good subsoil drainage arrange-ments. For other sports the grass surface can be less exacting but should be smooth and free from surface unevenness, and possibly laid to a slight fall for water disposal. Maximum permissible gradients must be checked before design with the relevant governing bodies, because such rules are constantly being upgraded, and the main fall should ideally be from the centre to both sides of the pitch and not in the direction of play.

The species of turf grass must be carefully chosen for the correct playing characteristics, resistance to wear and disease, and suitability for its particular climatic and physical environment and the season of play. An appropriate cultivar or mix of cultivars will be specified by the consultant and supplied by spe-cialist growers. ‘Fescues’ and ‘bents’ are commonly

Figure 7.1 Elements of a typical natural grass playing surface.

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chosen species. As an instance, the famous grass surface of Wimbledon Centre Court is resown every year with 66 per cent Troubadour perennial rye grass, 17 per cent Bingo chewings fescue, and 17 per cent Regent creeping red fescue. This is the best mix for the specific soil, drainage and other conditions found at Wimbledon, but other situations will demand other specifications.

Immediately beneath the grass surface is a layer of topsoil, often consisting largely of sand, with a depth of not less than 100 mm and usually averaging about 150 mm. This layer must contain no stones or injurious material, must be permeable enough for water drainage, and must be uncontaminated and well-fertilized for healthy growth. Using some local suitable-seeming soil will not be good enough:

the mater ial will almost certainly be obtained from specialist suppliers to the precise specification of the consultant, and will probably contain a large quantity of graded sand.

Beneath the topsoil is a blinding layer of fine mater-ial (ash, crushed stone or the like) to fill the voids in the surface below and provide a smooth base for the topsoil.

Beneath the blinding layer is a zone of graded stone to ensure that all excess water can drain away freely to pipes laid in trenches below. There may be sheets of tough water-permeable membrane laid between the foundation layer and the formation surface to prevent soil from being forced up into the founda-tion layer and obstructing the free flow of water.

This decision, the depth of the graded stone layer, and the layout and fall of the drains will depend on subsoil conditions and will all be decided by a specialist.

Drainage

Adequate drainage is a necessity, and the above methods may need to be supplemented to avoid standing pools of surface water after heavy rain-fall and to minimize expensive ‘down-time’ in wet weather. There are basically two methods of drain-age – passive and active.

The passive approach relies on gravity to drain away the water, and one method of enhancing the basic system described above would be the cutting of deep

‘slit drains’ into the subsoil by specialist machine, and filling these with sand or fine gravel to help sur-face water flow down quickly into the land drains.

This is quite expensive and needs to be carefully costed before a decision is taken.

The active approach uses pumps, usually activated by water-sensing electronic devices in the field, to literally suck the water off the pitch and into under-ground storage chambers, thus clearing the surface very quickly and maximizing the availability of the pitch for revenue-generating activity. Special drainage pipes may be laid for this purpose, or alternatively cellular technology may use the same underground network of pipes both for irrigation and drainage sim-ply by reversing the direction of flow by computer control.

Irrigation

Traditionally grass pitches have been watered by sprinklers, usually of the pop-up kind, but these are being challenged by underground water delivery systems. Using special porous low-pressure water supply pipes (or possibly the underground drainage system with the direction of flow reversed by com-puter control as suggested above) which allow a uni-form ‘weep rate’ along the whole length of the pipe, a steady supply of water – possibly mixed with fer-tilizer and weed-control additives – seeps directly to the grass root zone. The advantages that are claimed for sub-surface irrigation include:

• A lusher, tougher growth of turf grass than over-head watering, which tends to create a stressful cycle of drought and flood conditions.

• A deep root system rather than a shallow one.

• Less tendency for the soil to compact (this being a major problem with intensively-used pitches).

• Probably less water-loss by evaporation than would be the case with surface irrigation.

• Similarly, a conservation of fertilizers, insecticides and herbicides.

Underground irrigation pipes are normally laid between 150 mm and 350 mm below the surface, spaced from 450 mm to 900 mm apart; but specialist advice must be sought.

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83 Heating

Many major stadia in cold climates use some form of under-pitch heating, the most common type being based on a system of hot water pipes operated by gas boilers and thermostatic sensor controls.

Electrical heating on the principle of an electric blan-ket is an alternative commonly used method.

The most important aspect of this type of installation is the laying of the pipes or cables, which must be high enough to heat the pitch but low enough not be damaged by pitch aeration and other surface works.

A free-draining pitch is essential if heating is to be considered.

Maintenance

Day-to-day maintenance operations are discussed in Section 22.2.1.

7.1.4 Artificial grass (synthetic turf) surfaces