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Stone me!


Dr Tim Lodge believes that the sporting bodies should develop absolute standards for their individual sports with regard to stone tolerance in new surfaces


Throughout the country, the amount and the nature of stones in topsoil varies widely. The Cotswold Brash, for example, is notorious not only for the amount of stones within it but also for the nature of those stones. They tend to be flat so many pass vertically through the bars of a stone burier and avoid removal from the surface. But the oolitic limestone of the Cotswolds tends to break up very easily and can simply disintegrate while lying on the surface. Elsewhere, for example over the chalks of the south east, flints of varying sizes often arise and these certainly do not disintegrate and can be very prolific. They are also sharp and consequently dangerous. Stones are, therefore, a major preoccupation for those involved in the creation of new sports turf surfaces. Recently I was made aware of an unfortunate situation in which a sports turf contractor was being denied a very substantial sum owed to him. This was because the client considered that the turf surface that had been prepared contained too many stones. It transpired that the problem had arisen due to the inadequate definition of the quality of surface with regard to stones in the original


specification. The contractor had offered to remove ‘all significant stone’. This was a mistake because one


person’s ‘significant’ differs from another’s. In the light


of this, I


revisited my own specifications


and these stated that ‘all surface stones having a diameter larger than 25 mm’ should be removed. I now see that in many situations, for example on a Cotswold Brash, this much tighter statement is inappropriate and may actually be unachievable, particularly over a very large area. What is required is an understanding of the practical realities of achieving a particular finish in a given situation while satisfying the client’s demands for a good playing surface. This must be achieved through a more precise definition in the specification of the extent of surface stoniness that must be brought about.


My solution, which I now incorporate into all specifications, takes the form of a statement of ‘tolerance’ with regard to stones. The tolerance limits may be altered with regard to factors such as the budget available for the project, the soil type and the area being treated. However, it is vital that the client is made aware from the outset of the limitations on the final quality of the surface with regard to stones that such variation will impose.


The statement of stoniness considers the density of stones of varying sizes. Density can be expressed as the average distance one would need to go before finding another stone of the same size. The clearest way of stating this is as a graph of maximum tolerable stone density against stone diameter. So, in this example (right), maximum density for stones greater than 150mm would be one every 150 metres, for stones of 100 mm diameter one every 20 metres and for 50 mm stones one every 2 metres. In practice, it may be easier to define classes of stones representing a range of diameters, under 25mm, 25 to 40mm, 40 to 60mm and soon. The actual numbers need to be reviewed for each individual circumstance. This example was derived for polo grounds


Dr Tim Lodge, formerly with the STRI, now runs Agrostis Turf Consultancy


on a 29 acre site on Cotswold Brash. Another pass with the stone burier would have brought about relatively little improvement beyond this but would have added several thousand pounds to the cost of the project. However, this stoniness element of the specification would very probably prove too lenient for a single, league soccer pitch in chalky (flinty) Sussex. This would require a much tighter set of stone tolerances. Alternatively, there may be merit in the sporting bodies developing absolute standards for their individual sports with regard to stone tolerance in new surfaces. These would ideally be expressed in the manner described above and would require some relatively inexpensive research in order to be developed. Such standards would remove the flexibility to vary specifications between sites (which would be costly to people in the Cotswolds) but could be useful in litigation, for example concerning injury involving stones. The ability to state that a particular standard had or had not been achieved would be a powerful statement in any legal dispute. Despite the importance of this issue in the sports turf industry, surprisingly little has been written about it. Agrostis continues to refine its approach to stones in specifications and consultancy in general and observes with interest developments on the subject.


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