Chapter nine
Introduction to Part Two
The first step in creating an outdoor sports facility is to provide an acceptable playing surface, isolated, if necessary, by peripheral, interception drainage. The next step is to make certain that the soil on the playing area can infilter the water required to sustain grass growth and then discharge any excess in order to maintain an adequately aerated root environment and a tractively efficient surface. Finally, when an appropriate grass sward has been established, it must be used and maintained in a manner that will retain the virtues of the construction for the purpose intended.
9.1
Review of basic principles of construction
9.1.1
Surface configuration
Excess rainfall will tend to accumulate on the surface or within the soil and, given the chance, will run off downhill, more or less rapidly, according as the movement takes place over the surface or through the soil.
We may take advantage of surface flow as one means of rapidly coping with the sort of temporary excess that can arise at any time when rainfall intensity exceeds the infiltration capacity of the soil. However, the mere fact of surface flow carries with it the possibility of the moving water accumulating in local depressions unless the surface as a whole is at a sufficient gradient to allow it to keep water moving downhill. This applies also to hard porous surfaces (Chapter 13) and synthetic surfaces. In fact, any sports surface will benefit from being constructed with either a uniform, cross slope, or some form of crowned profile to minimize the risk of ponding, at least within the playing area.
9.1.2
Peripheral interception
Encroachments of surface and/or ground water, originating from sources outwith the site, may have to be located, intercepted and diverted away before work can begin on the proposed playing area. Whether the problem originates from water discharging downhill, off land further up slope, or from the periodic backing up of a ground watercourse, interception is likely to be best achieved initially by peripheral ditching. Though this may be converted eventually to some form of catchwater drain (Chapter 2, page 23) the initial ditch system should be left open for as long as possible to confirm its effectiveness.
9.1.3
Soil structure and drainage
In British agriculture, the standard excess rainfall for which we make pipe underdrainage provision is of the order of 1 mm/hr; in sports turf we usually double this figure. In practice, discharge rates of this order can be achieved, through soil to pipes at 5–10 m intervals, only if the soil itself is a loamy sand (more than 75% sand), or has an adequate, water-stable, granular structure such as might occur naturally under worm-worked pasture.
The pore sizes required for rapid infiltration are those large enough to be seen by the naked eye. Pores of this size are formed by particles or aggregates similar in size to the fine sand in sand dunes, but most soils have sufficient silt and clay to fill the pore space within a close-packed arrangement of their coarser particle content. To avoid this a soil would have to be over 70% one grade of sand. Therefore, the rapid passage of excess water through most soils is dependent on avoiding a close-packed arrangement of the primary particles.
A clay topsoil that has been ploughed and allowed to crack down to a fine blocky tilth, or a loam, stably aggregated by worm casting, may transmit water at the rate of 250 mm per hour—a rate approaching that of dune sand (compare SN15 and SN21, Table 7.1). Such a soil, providing its structure holds up, and providing it can discharge excess water to an adequate subsoil soakaway, should never be waterlogged.
However, open structure, developed in loams by a combination of root activity, cracking and worm aggregation, is neither strong nor long lasting. It can disperse during storage and will fail to regenerate if the soil becomes acid and earthworms are thereby excluded. It can easily be damaged by wheel traffic and machinery used in construction. It can be poached out of existence by players treading the surface when wet. It cannot be preserved if the soil is abused at the very time of the year when most agriculturalists would advise staying off. It is surprising, in fact, that any of our pitches escape being waterlogged if they are used when wet in the winter.
In most situations, therefore, the only hope for intensively used grass surfaces is to link the surface directly through to the permeable fill above pipe underdrains by an adequate and reliable system of drainage routes. The aim must be to by-pass any impediment to drainage within the indigenous soil, utilizing an interconnecting system of vertical channels, stabilized by infilling with appropriate, freely permeable sands and gravels.
9.1.4
Precise specification of sands and gravels used in sports-field construction
Because existing systems of classification used in Britain for specifying sands and gravels are intended primarily to serve the needs of the building industry, it has been necessary to develop alternative systems for sports turf. One such is the Stewart zone system, summarized in Table 1.4 and, in greater detail, utilizing summation graphs, in section 7.1.6. It is to these materials that reference will be made in the constructions described in the chapters that follow.
In the Stewart zone system a topsoil sand is a material suitable for topdressing and topsoil construction. A blinding sand is suitable for bringing permeable backfill through to the surface over gravel, or for preventing a fine-textured topsoil from infiltering an underlying gravel bed. A trench gravel is suitable for use as the gravel element in a two-part, sand/gravel slit or pipe trench, or for use as the gravel bed in a rafted construction. These are new terms introduced to improve the precision of specifications and the efficiency of the end product.
9.2
Construction options for sports turf
The main alternative approaches now used for the construction and maintenance of sports-turf facilities are summarized in Table 6.2. Note the logical consequences that follow for construction and maintenance according as the eco-system established involves the presence or absence of burrowing earthworms. An import-ant consequence of this relationship is that maintenance has to be tailored to the special needs of the type of construction chosen. All concerned should appreciate what this implies for equipment, manpower and long-term financial commitment, before construction begins.
9.2.1
Preliminaries
Preparatory site work
The initial circumstances of any site may well require some modification before the construction of the actual sports-turf facility can begin. The examples presented in subsequent chapters should progress only after the following conditions have been satisfied.
1. The land does not require any further major grading, and existing slopes are acceptable.
2. The soil, if required to form the topsoil or to accommodate slit trenching, is free of toxins and other noxious materials liable to endanger players or the healthy growth of grass. In addition, the soil is free of buried obstructions that might impede excavations by normal methods of trenching.
3. Full perimeter drainage has been installed to intercept extraneous water likely to reach the site from surrounding land, making the area to be used for playing-field construction an isolated catchment.
Records
As no two situations are alike, details of specifications will vary and may even have to be modified during the period of construction. Though during such negotiations verbal agreement may appear to have been reached it is important to keep written records, confirmed by both parties. The mere act of committing thoughts to paper can do wonders to clarify the mind and, in case of dispute, such records are often the only tangible evidence of what was intended. For personal reference it is also a good idea to keep a note of all calculations until the work is completed and handed over.
9.2.2
Options
Apart from sites where a very low limit is placed on financial expenditure, sports-turf constructions nowadays tend to take one of two main alternative routes: a slit drainage system capped by a shallow or deep layer of sand, or a sand soil rafted over a drained, gravel bed. The former is more generally adopted for the coarser turf surfaces used for the more rigorous, winter games; the latter for the fine-turf surfaces on which the impact and roll of a small ball is a crucial part of the game.
With coarse turf, earthworm activity should be encouraged for its beneficial effect on the incorporation of organic residues, drainage, deep rooting and nutrition. With fine turf, surface casting of earthworms can be a problem, particularly on the specially groomed surfaces used for golf and bowls, but also, though to a lesser extent, cricket and hockey.
If the soil-conditioning activities of earthworms are excluded their beneficial effects must be achieved some other way. Much can be done by constructing the soil of sand and by very careful maintenance aimed at combating the inevitability of mat development which, in turn, will increase the risks of shallow rooting, desiccation, disease and insect pests. It is important for players as well as greenkeepers always to keep in mind that on a matted surface there may be nothing left from which natural regeneration can occur if a dislodged divot is not immediately heeled back into place.
Low-budget constructions
An expensive construction may not be necessary if a site is selected which has a naturally free-draining soil and the users are prepared both to adapt their requirements to what is available and make arrangements to avoid casual use. Thus a worm-worked pasture with a modest drainage scheme may be developed for use by juveniles, or by adults when dry enough not to cut up, and not be very demanding on maintenance. On the no-earthworm side, providing the area is not large and can be adequately sloped to clear excess water by surface run-off, a cheap but adequate playing surface can be created for crown bowls, or for putting greens on a low-budget golf course with modest ambitions. However, the standard of maintenance will still have to be high to counteract the inevitable tendency for the surface to become organically matted.
Good quality coarse-turf surfaces for winter activities
A soil surface, drained vertically through sand/ gravel slits, linked for discharge to pipe underdrains, may acquire a progressively deeper sand cap by successive topdressings applied annually. Alternatively it may be carpeted initially by a deep (e.g. 100-mm (4-in)) layer of ameliorated sand. Crucial to the long-term success of a standard slit system is the maintenance of the permeable link right through to the surface. With the sand-carpeted alternative the permeable link is assured, but there is then a further problem of keeping the sward rooted through the sand carpet to benefit from the water and nutrient reserves in the underlying soil.
The sand carpet option provides such a reserve of readily available water storage capacity that the risk of surface ponding is virtually eliminated. However, the extra sand required greatly increases the cost and does not reduce the need for further increments of sand to be applied during maintenance (section 6.3.1). Where finance is limited, therefore, thel00-mm (4-in) depth of sand in the carpet might better be regarded as a 20-year reserve of sand to be used for progressively building a sand-dominated surface by sand topdressings applied annually.
Good quality, fine-turf surfaces for flat rink bowling greens, golf greens and croquet lawns
The sand soil rafted over a gravel drainage bed represents by far the most expensive approach to construction, and though it might benefit from earthworm activity when used for a large area such as a soccer pitch, it will normally have to manage without.
The required maintenance is bound to be very demanding. All steps will have to be taken to avoid surface accumulation of organic debris and the attendant problems of surface-rooting fungal disease and insect pests. Therefore, due priority will have to be given to continuous removal of organic debris and regular sand topdressing (Appendix F).
Any water reserve against drought is strictly limited to what can be stored within the fixed depth of the soil layer above the drained gravel bed, and even this can only be exploited to the effective rooting depth. In these circumstances there is no possibility of any replenishment from below by capillarity from a ground water-table.
A depth of 150 mm (6 in) of root-exploited soil will provide of the order of 25 mm (1 in) of available water reserve, starting from the drained state following saturation (field capacity). Water loss per day from a fully grassed surface in summer will range mostly between 21/2 and 5 mm (1/10–1/5 in) according to weather conditions. A week of hot, dry weather in mid-summer, therefore, is a significant drought and, in a sand soil, when desiccation threatens, any adverse effect is likely to be sudden. With a sand soil perched over gravel, it is not only necessary to make provision for irrigation, it is necessary also to plan ahead how best to use the water supplied. For more specific advice on this subject see Chapter 10, page 162.
This type of construction is forced upon those requiring a quick-draining, worm-free, close-mown and smooth surface but, outside the extravagances of professional sport, it can only be justified for the relatively small, specialized greens required for golf, bowls and croquet.
Special difficulties involved in providing grass surfaces for hockey, cricket, tennis and horse racing
Hockey, cricket, tennis and horse racing are particularly challenging sports for which to provide turf surfaces. They fall between the one extreme of soccer and rugby where the pitches can benefit greatly and suffer little from a soil continuously processed by earthworms, and the other extreme of the fine-turf surfaces required for golf and bowls where earthworms are unacceptable.
Hockey, cricket and tennis
The extensive, smooth surface required for hockey clashes with the fact that the game is played at a season of the year when a natural soil needs all the drainage benefit it can get from earthworm activity. No wonder that our hockey players turn increasingly to hard porous or synthetic surfaces, where these are available.
Similar conditions are required for cricket outfields as for hockey but, because cricket is a summer game, most County grounds try to get away without the assistance of earthworms for drainage. However, they are liable to become a sorry sight when the heavens open during a thunderstorm unless slit drained to bypass surface water through the compacted soil.
The pros and cons of earthworms on cricket squares are discussed at length in Chapter 12, page 188. The choice is by no means as clear cut as the prejudices of those on either side of the argument might suggest.
On cricket squares and tennis courts the conditions required are similar, particularly at top level. Both games need a surface that is uniform, smooth, non-slippery and strong enough to provide the special qualities of true bounce and pace. To achieve all this on a grassed, soil surface not only must the grass tolerate frequent close mowing and remain anchored effectively within a compact soil, the soil itself must have sufficient clay to strengthen the compact surface against deformation. Thus drainage through surfaces of this type can virtually be discounted when prepared for play.
Though the soil requirements for cricket and tennis are very demanding they do not preclude the retention of an earthworm population. Some groundsmen prefer on balance to keep them because of their beneficial effects on root distribution and in counteracting any tendency towards surface layering. They appreciate that surface casting is a potential problem, especially if the soil brought on to the surface is texturally unsuitable for the development of pace. However, both cricket and tennis require dry surfaces for play and these are the conditions when casting is minimal and any residual casts can be dispersed easily by brushing.
Because of the extreme hardness of the ball used for cricket the soil-binding strength required makes a clay content over 25–30% a major objective. If this is a feature of the local soil then the topdressing involved in surface renovation may well be left to the earthworms to provide in their peak phase of casting during the warm, moist conditions of autumn. But if the local soil is low in clay then a suitably clay-rich topdressing may have to be imported and problems may well arise over poor surface integration when incompatible materials are allowed to form layers, liable to crack apart on drying (Chapter 12).
These problems also occur on the special courts used for major lawn tennis tournaments but such courts provide an inappropriate model for others to try and emulate as they tend to be used for only two or three weeks each year. Elsewhere a compromise should be accepted between the occasional hazard of a bad bounce off an undispersed cast and the work done by earthworms to keep the soil in good condition below ground. As so much is involved in the preparation and maintenance of the special turf surfaces required for hockey, cricket and tennis, consideration should be given to the desirability of providing alternative, synthetic surfaces to protect the grass surfaces from excessive use.
Horse racing
A galloping horse puts tremendous strain on its own legs and on the shear strength of the turf. A soft, giving surface will cushion impact but may leave a hoof-print depression six inches or more deep. A firm surface will benefit forward movement but, if too hard, will jar joints severely on impact.
Judging from the extreme contrast seen in 1967 between the worm-worked soil profile at Newmarket and the worm deficient soil profile at The Curragh, there would appear to have been no consensus of view at that time in the horse racing world on which type of soil is best for horse racing. If the thick turf at Newmarket gave way it would expose a very well granulated soil underneath, excellent for drainage but possibly a little too soft to be ideal for traction. At The Curragh, the root-bound mat of unincorporated organic debris would have been like a cushioning layer of foam rubber over the compact mineral soil beneath. This may have become soggy after heavy rain descended but would have provided the house with a shock-free landing and a firm, though turf-shearing, take-off.
We still need to know which of these two very contrasting surfaces the horses prefer and how well they do if brought up and trained predominantly on one type of soil rather than the other. Only when we know this can we logically advise on the best way to construct and maintain a race course.