Graded Filters: From Surface to Subgrade

This article is a crash course in arena design, from top to bottom. We discuss choice of surface, how this affects the design of the graded filter, and how to work out the depth of the drainage layer for lateral drainage.

Choosing the Surface

Essentially, the choice of riding surface is between a firm surface and a fluffy one. Our type Medium Prime (FMP) sand provides a fluffy surface, suitable for breaking falls. This would be most suited for inexperienced riders, where rider falls are most likely. Our Manege Sand (FFF) provides a firm surface, suited to most uses including dressage and jumping.

The Manege Sand is a fine, washed and graded, subrounded sand, which is virtually free from clay, silt or grit. It is a 'single sized' sands -- i.e. most of its grains are the same size. This ensures good drainage, and avoids the concretion of the surface of the arena (excluding freezing conditions).

Design of a filter layer for Vertical Drainage

Each layer in a filter, although coarser than the one above, must be fine enough to prevent material from above being washed through the gaps between the grains.

The d15 and d85 measurements of a sand are, respectively, the interpolated size of the grain that passes through a sieve at 15% and 85% percentage passing marks on a grain size distribution curve. The d15 is sometimes known as the permeability protection limit, and the d85 as the piping predicting limit. It has been shown through experiment that a filter (the material doing the draining) is suitable if its d15 size is at least four times as large as the d15 size of the protected material (the material that is being drained), and not more than four times as large as the d85 size of the protected material. Where this condition is not met, a geotextile would have to be used in between the layers

Below are tabulated the typical d15 and d85 measurements of the riding arena construction products produced at Cardigan Sand and Gravel.

From the table, we can work out that a Manege Sand can lie over a Coarse Prime sand:
Manege d15 = 90;
Manege d15 x 4 = 360;
Manege d85 = 250;
Manege d85 x 4 = 1000;
The D15 of Coarse Prime Sand, which is not less than 4 times Manege d15, and not more than 4 times Manege d85, thus it can underlay a Manege sand.
It cannot lie over either a Medium Prime sand (D15 of 235) or a Fine Concreting sand (D15 of 150) without a geotextile laid between them. The FCP sand could lie over a 5mm single size gravel, which in turn could overlay a 40mm single size gravel, which could sit over a 150mm graded shale (not tabulated).

If a geotextile is used, then the Average Opening Size (AOS) of the geotextile would need to be greater than the d15 and less than the d85 of the overlying layer (i.e. d15<AOS<d85). Terram 700 Fabric has an AOS of 180 microns and Terram 500S has an AOS of 350 microns.