Sand, gravel and other materials used for surfaces are a science in themselves. Chapter five gives an introduction and overview and points out characteristics that are valuable to be aware of when building or renovating a riding arena.
THE CHAPTER IN BRIEF
We often use the word sand for the material, which is the major ingredient in a riding arena, but this is a rather non-specific term. It is preferable for the material to be defined based on the following characteristics:
1. Is it natural sand or manufactured material from crushed rock?
2. What is the geological origin of the material?
3. Sand and gravel are granular materials resulting from the natural disintegration of rock or from rock crushed in a aggregates crushing plant.
4. The fraction of fine materials such as silt and clay.
5. Additional additive materials such as fibre, wax or wood.
These characteristics are important to understand since they affect the properties of your arena. One example is the size of the grains of sand, which are described according to their diameter in millimetres, mm. However, this does not describe the distribution of these sizes, which affects the properties of an arena. Availability of different sand qualities varies with locality and may influence your transportation costs. Research has found that you can get very similar properties in arenas with different materials. Therefore, you have a choice of solutions to obtain the “right” arena by, for example, adding other materials such as wood, wax, rubber or fibre to sand. The functional properties of sand are significantly affected by water (moisture) content. To find the best solution it is important to understand the properties of the materials and how you can affect them with maintenance.
It can be a challenge to try and understand something as different as sand and gravel when your real interest is horses and equestrian sport. But for someone who wants to build a riding arena it can be essential. As the consumer, you need to become aware of what factors influence the properties of the sand and the maintenance required to retain the properties of the material. The specifications of materials sold from quarries are normally insufficient. Hopefully, within a near future,we will have access to better analysis, which will provide more relevant information about the material before buying. Unless a ready-made surface is purchased form a commercial producer, in general it is best to get help from specialists with experience with the local materials and climate. In this chapter you can learn the basics for being an informed customer when planning a riding arena.
The first point to remember is that what we call sand in fact consists of varying proportions of:
- sand (particles in the rock material <63μm)
The first question when selecting materials for a riding arena is:
WHAT TYPE OF ROCK (GEOLOGY)
Sand and gravel are granular materials resulting from the natural disintegration of rock or from rock crushed in an aggregates crushing plant. Rock can vary in hardness,depending on the mineral content, which produces sand with different properties. The type of sand that is available is therefore determined by the geology in a region, and will vary between, and within, even small regions.Therefore your location will determine some of the properties of the sand available if you source the sand locally. The hardness and shape of the sand you choose will influence the lifespan of an arena.
Commercial producers of riding arenas and consultants normally prefer a high percentage of quartz or silica. The access to silica(quartz) sands varies between regions. The selection of especially silica sand requires some vigilance regarding dust, as fine particles can damage the airways. This is primarily an issue in indoor environments. It is possible for a rock to be both hard and fragile, such as porphyry, and the resulting sand will have a shorter life span. If you buy a ready-made arena surface it is important to find out about the composition of the sand beforehand and make sure that it does not contain excessive fine materials and is sufficiently durable.
The selection of natural sand will make a great difference for arena properties. Natural sand, was once rock, but has disintegrated to small particles and is deposited in nature. In continental Europe one important source of sands are glacial deposits from the Ice Age. The grains have been“ground” by water, typically giving a more rounded shape. One type of natural sand is sea sand, which some producers regard a shaving grains that are too rounded and too similar in grain size, resulting in an unwanted ball bearing effect. Sand grains with a more angulated shape will hold together better but with greater wear on the hoof/shoe. The use of additives such as fibre allows this more rounded material to be used.
Advantages: Natural sand has properties which are beneficial in riding surfaces, with rounded and slightly angular grains.
Disadvantages: Natural sand is a finite resource. Glacial deposits of natural sand can be an important source of groundwater. Some countries, including Sweden, have an official policy for a long-term reduction of the use of natural sand and aim for a changeover to crushed rock. Common problems with natural sand surfaces are that the sand is installed too deep,or with a suboptimal distribution of grain sizes.
The alternative to natural sand is crushed rock. This is produced when a quarry blasts bedrock into different sized rocks and particles and then crushes the resulting material. The bigger fractions are called road base because that is one of the main uses(example 2-6, 8-16, 16-32 and so on; road base are sometimes also known as aggregates).
The relatively smaller fractions that is called well graded road base is used in many arena constructions as foundation layer. Sand is the smaller particles with silt and clay sized particles as the finest fractions. The silt and clay are sometimes called fines, with clean sand having very little of the finer material.
While the grains in natural sand are to various degrees rounded, having been polished by water over time, crushed materials usually have a much more angular shape. Just holding some grains in your hand you can feel they are “sharper”. These more angular materials “hold” together better than natural sand because the sharp edges of the sand grains will contact the adjoining sand particles. You can utilise this effect to get the surface to “hold” together, but the result can also be that such a surface gets hard due to fine particles generated from fracturing of the corners of the sand, that in turn cause high degree of compaction. These broken corners effectively change the particle size distribution which in turn increases the surfaces ability to compact.
An even distribution of different particle sizes compacts much more than a material that consists of the same sized particles. Crushed rock or manufactured sand is more angular producing good grip but it is likely to cause more wear on the horse’s shoes (or hooves if barefoot) due to the abrasiveness of the angular grains.
Advantages: Unlimited resource. New methods for crushing are being developed may make it possible to produce functionally designed gravel, sand and fines.
Disadvantages: Less durable than natural sand. Sharper grain surfaces that increase wear on the hoof/shoe. Requires know-how to produce a surface that does not compact too much.
Three important parameters used to assess sand and its properties are sand shape, size and sorting.
THE GRAIN SHAPE/ROUNDING
The shape or rounding of the grains particles is one of three critical factors in determining how the sand will function as a riding surface. This is somewhat an issue for the difference between natural sand versus manufactured. Natural sand will have rounded or subrounded grains, as over time the sharp angles have been subject to abrasion and impact with other particles, but the grain shape can vary significantly even in natural sand.
The tendency of more rounded sand grains to create a loose surface can, to some extent, be controlled by using a wider distribution of grain sizes and fractions. Some specialists suggest that “suitably” subangular or subrounded grains in natural sand produce a combination of dampening, cushioning and stability because they will not compact entirely.
In contrast, crushed rock materials have sharply angular grains when new.
HOW BIG ARE THE GRAINS?
The manner in which the size of the sand grains affects the performance properties of the arena is well accepted. Most people who have been involved in conversations about riding arenas have heard definitions such as “0-2 (4, 6 etc)” or “100 sieve sand”used when describing the sand used in an arena. The separation of sand by size is performed by putting the sand through screens.
0-6 describes the higher and lower end of the grain diameter in millimetres. This description does not, however, mean that the sizes are evenly distributed between those two limits. In some cases there may even be a small proportion of grains that are larger than the specified maximum size. In general the most important characteristic of arena sand is actually the size of the pores between the sand. Smaller pores are created either by smaller sand particles or by larger particles mixed with smaller material that can fill the gap. The problem with the latter case however is that the material can sort out overtime and become very hard, where smaller grains with a very consistent size will retain the energy absorbing characteristics which are so important in an arena surface.
The language associated with the sand can be confusing depending on the goal of the supplier and the type of material. A stable base is required when constructing roads or foundations for buildings. The material which is used for these applications is sometimes called well graded. This is a good material for a foundation for an arena but would not make a good top surface since it would compact and get hard, fine for house building but not good for horses. What these same suppliers would call a “poorly graded” soil both has better drainage and will not get hard as easily. The goals for sports fields or agricultural applications are very different and have developed separate descriptive wording as well, where “poorly graded” materials can become “well sorted”.
The critical characteristic of an arena surface is that the material in a basic sand arena cannot have all of the grains of sand all of the same size or the effect can be compared to that of peas “rolling” in a bag. This will provide the horse with poor grip. When fibre or wax and fibre are added to this same material however, the opportunity exists to have a well sorted material that in combination with the synthetic additive will support the hoof of the horse. This type of surface with open pores and rounded but durable sand will tend to have higher initial cost,but will require less maintenance and will have reduced sensitivity to moisture content because of the ability to drain water through the surface.
If a grain of sand had the same diameter as the wheel of a bike,a grain of silt would have the same diameter as a bottle cap and a grain of clay would be smaller than the head of a pin.The diameter of a fairly large grain of sand is roughly equivalent to the head of a matchstick, while clay particles are so small they can not be seen with the naked eye.
As we have said, the size distribution of sand is very important for function, and is, therefore, essential information when ordering sand for an arena. To get a description, you order a sieve analysis from the quarry (or discuss with the contractor/producer). For the analysis the sand is separated through screens with progressively finer mesh, rather like wheat flour when divided into fine flour or wholemeal. A quarry can also mix sands into a predetermined profile for the customer. This test, like many with natural materials, is deceptively simple. Caution should be exercised when looking results with a larger percentage of fine material, silt or clay. The silt and clay can in some cases be very difficult to separate and make sand grains appear larger, either by covering their surface or by forming agglomerates of clay particles. Other methods are needed for profiling these materials, which involve soaking and use of chemicals to dissociate the clay particles. The test methods used are critical and incorrect results can result in the construction of an arena which may only be acceptable for a short period after installation.
MORE ON SAND AND FACTORS THAT INFLUENCE THE MATERIAL
How big is the natural fraction of fine materials such as silt and clay?
The smallest grain size which will be specified in a sieve analysis of natural sand is normally at 0.063 mm in diameter. It is important to also know how much of the material is smaller than that. The smallest grains are not sand but particles of clay, silt and various biological sediments from nature (humus). This can be classified as “filler” or “fines”. That is also the name for the finest particles in crushed materials.
In fact there are other critical characteristics of the smaller particles that are more important than the size. In general is it best to refer to the small particles identified in standard size testing (sieve and hydrometer)as clay sized particles, since the actual minerals and flat shape of some clay result in a dramatically different behaviour than what would be expected from simply smaller sand type particles. Therefore what we call arena sand can include materials which range in size from even fine gravel down as small as to include tiny clay sized particles.
These clay sized particles can have both advantages and disadvantages for arena properties, as mentioned earlier. There are different opinions on how big this fraction should be. Some expert consultants have a guideline percentage of about 2-4% percent of filler. Some commercial producers recommend the use of fine sand with some 15% of clay/sediment particles, which will increase the tendency to compact. Crushed materials also have a proportion of “filler” consisting of particles less than 0,063 mm in diameter.One aspect of the finest particles is that, overtime, they can cause clogging/blockage of the arena drainage.
What is “washed sand”?
This is sand in which the smallest particles have been washed away. Washed sand is very useful if, and when, anyone wants a specified grain size to be included in a surface, or to add to an existing one. The disadvantage,mentioned earlier is that washed sand is used on its own, when the smallest particles are missing and all grains are about the same size,the surface will provide less grip/traction. Crushed material can also be washed.
Water is the single most important factor that influences the properties you want from an arena. Water is also discussed in the chapters on Construction and Maintenance. Moisture to a high degree determines the properties of the sand, and therefore also of the arena. A good example is a beach by the sea. Compare walking or running at the edge of the water, where the sand is even and saturated by water from the waves, or away from the edge where the sand is dry and often deep.If the water content is too high the sand will start “floating” and the surface will again be too loose.
A good supply of water and a good watering system are, therefore,very important to consider when installing an arena. If the stable/establishment has a limited water supply there are alternative solutions:
- Buy water in a tank
- Organise collection of surface water, for example, rain water (in some countries this can be subject to laws and regulations)
- Consider a waxed sand surface, which requires less added moisture
- Subsurface watering also called “Ebb and Flood” systems savewater by adding the water under the arena surface and thus reducing evaporation
One solution for riding surfaces that has been developed in continental Europe where water is the main ingredient is the Ebb and Flood system.The top layer consists of sand only or with a proportion of fibre, that is kept moist from beneath. Arena surface properties are controlled by the moisture level, and with the right construction the whole arena will maintain a consistent degree of moisture. The system requires specialist know-how to install, including careful selection of the sand. The so-called“flow point” varies between different types of sand, and is especially important when the sand is kept thoroughly moist as in this system.
Worldwide there is great variation of different materials used in surfaces for equestrian sport, mainly mixed with sand, but also as the sole material. In the following we will discuss a couple of materials more commonly used in mixtures with sand. Remember that with the correct maintenance (control of moisture content and mechanical interaction) different materials could be used to achieve the desired properties of an arena. In a global perspective we also have very different prerequisites concerning environmental factors such as rain, humidity, temperature and wind,which could substantially influence how different materials behave.
Fibre and textile
Fibre and textile have been used in riding surfaces for more than 20 years, with the aim of improving functional properties, increasing shear strength and stability and reducing maintenance requirements. The effect of adding fibre to sand based sport surfaces is well established in the scientific literature but the effect on the sports horse is not well documented. Suggestions below are therefore based on the practical experience of international arena consultants.
The effect of textile fibre is explained in terms of increasing the binding between sand particles. The sand particles dig into the surface of the fibre creating a surface is somewhat like sand paper, which resist the sliding of the neighbouring particles and thus will increase grip and friction. The market for equestrian surface fibre is not standardized, and thus it is important to specify materials in a manner, which ensures that the surface gives the desired characteristics.
Like sand, there are great variations between the materials and shapes of the texti-Fibre and textile can be an ingredient in the top layer.equestrian surface background 65le additives for arenas. Some vendors provide shredded carpet for this application, with others having a similar product which is cut and with the backing material removed. Single fibres and yarns are also used with both new and recycled materials used for the fibre for riding arenas. In general, longer fibres are used in smaller quantities, but can be more difficult to distribute and make maintenance more difficult. Small rectangles of material can also be used which also require less material.The shorter single fibres or yarns are typically easier to maintain but requiring larger quantities. Both the stiffness of the fibre and the strength also affect the quantity and durability of these additives. The pricing also varies greatly, from 0.20-0.30 euros to 1.50euro per kilo (in continental Europe).
Three important criteria to consider when ordering fibre or a fibre mix:
- Is the textile/fibre clean? Rubber or glue backing material (in recycled carpet)can be present, and some people question the environmental effects.
- Size? Textile and fibre are sold both as pieces and as fibres or yarns. Pieces of textile bigger than 3×3 cm, or 3 cm long for a fibre thread, make the surface more difficult to maintain.
- Sensitive to ultra-violet light? If a material is not resistant to ultra-violet light from the sun, there will be rapid degradation with dust formation, which will shorten the lifespan of the arena.
Proportion of textile and fibre
The proportion of fibre can be measured as kilos per ton of sand or per square metre. The quantity of fibre is dependent on the type offibre and the shape. In addition the shape ofthe sand will also have an influence on theeffect of the fibre on the final surface material.
The advice given here is again based onpractical experience, and effects have notbeen mechanically tested.
- Low proportion of fibre (suitable, for example, for riding schools and other users that do not wish for a high degree of grip/friction but want to take advantage of other effects of fibre: 10 kilo per ton of sand or 2-2.5 kilo per square metre of sand (see advice on choosing sand below).
- Higher proportion of fibre (for example, for competitions with better grip for higher speeds): 12-16 kilo per ton sand or 3 kilo (range 2.5-3.5 kilo) per square metre.
An even higher proportion of fibres, such as4.5-7 kilo per square metre of sand or 40-45kilo per ton of sand, produces a very high degree of grip with correspondingly increased load on the leg and is rarely used for riding arenas.
Choice of sand for fibre surfaces
Indoor: Fibre is best suited for mixing with fine sand at diameters of 0-1/0-2. The fine sand adheres better to the fibre compared to coarser sands. Another recommendation for sand used with fibre is to have a low proportion of filler (maximum about 5 %). The sand used by commercial producers of fibre sand arena surfaces at an international level is typically very clean with little or no silt and clay and with a very high proportion of silica. Silica sand is an example of a durable (and more expensive) sand. Durability means that the sand is not being crushed by the wear from the hooves.If the sand grains are getting crushed relatively more fines will be produced, which could lead to decreased vertical drainage capacity.
Outdoor: If the fibre sand arena is installed in an outdoor location it may be required to withstand heavy rain. Coarser sand can then be used, and there should be almost no fine material in the surface to maintain horizontal drainage. Often in an outdoor arena a slope is also used so that in the case of very heavy rain both vertical and horizontal drainage will occur.
Installation and maintenance of fibre surfaces
For a fibre sand mixture to function correctly it is important that the sand and fibre are mixed and properly watered. Mixing can be done at a factory or done with a rotavator/rotary tiller at the installation.If the sand is allowed to dry out, the fibres can accumulate on the top surface during maintenance or in dry conditions without watering. Remixing of the material must then be done carefully, especially if the material has moved, to avoid damaging the foundation material.
A fibre sand mixture requires considerations about final disposal at the end of the useful life. A well maintained fibre sand arena can last for 20 years, but will be classed as waste when it is time to replace it, and this can be costly. Methods to shift the sand and fibre for recycling do exist and are also being developed.
Woodchips, sawdust or other biologically degradable material is a main ingredient in many surface mixtures around the world. Woodchip only surfaces are seen occasionally but not discussed here, although some aspects such as degradation are relevant in both cases.
Advantages: Renewable resources. Easy to dispose of at end of use, due to the non-synthetic contents. The wood aids responsiveness/elasticity, and helps the sand to be stabilised, but it is important to combine it with the right type of sand. A surface containing wood has a shorter ”lifespan” than fibre sand, but is substantially cheaper to install and serves as an alternative that with good maintenance will function well also for competition(though not the highest levels) and training.
Disadvantages: Naturally bio-degradeable, can then get slippery and needs top-ups/replacement, resulting in shorter intervals for end of use or renovation. With removal of manure the risk of slipping and lack of grip are reduced.
Like sand, different types of wood have different properties. This will influence the lifespan of the riding surface, as different woods will degrade slower or faster. One guideline for sand-wood surfaces is that the top layer will need replacing every three to five years.
Oak, for example, will last longer than pine, and pine will last longer than fir. This is due to the trees’ strategies to resist attacks from fungus, but also provides resistance against mechanical wear from hooves. Sawdust from pine has been easy to source in Sweden. As it is degrades quite quickly there has been a tradition to do “top-ups” every year. Meanwhile the remains of the old wood are still there, and in combination with old manure in arenas that are not mucked out, this can result in a slippery surface from the degraded organic material. However, as is pointed out elsewhere, if the arena is mixed deeply through deep harrowing and air is introduced into the surface then this problem is avoidable.
Examples of more durable woods are larch and oak. Based on experience woodchips that are the size of match sticks work particularly well but this is not a standard order. The ease of disposal of wood sand mixtures is an advantage of these types of arenas,since it can then (with permission) be put out on farmland for disposal unlike the sand fibre surfaces.
Choice of sand for wood mixtures
The sand that is mixed with wood should preferably be fine natural sand, at 0-1 mm. You can choose 0-2 mm, but then the proportion of 1-2 mm should be no more than 10 percent. The proportion of filler should be low, maximum 5 percent. The proportion of wood in the whole mixture should be about 30 percent.
Wax-coated sands are used more commonly in some countries and are typically seen when particular performance characteristics are desired. These surfaces also provide reliable vertical drainage.
Advantages: A choice that requires less watering than others. A good alternative for anyone looking for strong cohesion and friction in the top layer without lots of filler particles that can compact into a crust deeper down or clog drainage. An outdoor arena with wax-coated sand can absorb a lot of rain if the drainage is correctly installed. Maintenance can be less intense than for other surfaces.
Disadvantages: The material can be difficult to dispose of at end of use. Waxed surfaces can also be more sensitive to changes in temperature, getting harder in cold weather and very soft/loose when warm (which can be addressed with maintenance). Like fibre, waxed surfaces are included in the discussion on increased friction on the hoof. The wax can wear away from the sand overtime and the sand may need “rewaxing”, which can be costly.
Apart from wood, fibre and wax other materials have also been used as additives in riding arena across the world. In many cases it is recycled materials, most often rubber, often from recycled car tires. Rubber has also been tested beneath the top layer. In Germany recycled products, such as polyethylene, are used in sand mixtures. In the past recycled, shredded cable has been used but is no longer allowed in many parts of Europe, due to risk of environmental contamination from metal residues.
UNWANTED MATERIAL – MANURE
Horse manure collects more quickly on an arena than most people realise, and becomes an ingredient in itself with a negative effect on properties. In a sand-wood mixture the manure will mix with the wood into a compost-like organic material. When sandwood arenas are said to be slippery the reason is often a layer of manure below the top surface. In a wax-coated sand arena manure and urine will alter the behaviour of the wax, and in fibre-sand mixtures manure also shortens the length of use. If a stable, or establishment, finds daily mucking out of the arena difficult to arrange, the suggested solution is to replace the whole top layer perhaps as often as every three years.
Remember that outdoors other organic material such as leaves can also have unwanted effects. If left on the arena they will degrade and change the composition of the surface.
In some countries including, for example, Sweden natural sand is rated as not a renewable resource. There are therefore government policies to reduce demand, for example, through higher taxon natural sand compared to crushed rock.