The Brew Cabin

I'm planning to build a 60x20m outdoor arena for she-who-must-be-obeyed. I've looked at several designs and examples of professional installations and, frankly, I'm a bit concerned that sub-base and drainage arrangements may not be adequate - hollows filled with additional riding surface (sand) to compensate for collapsing sub-base seem all too common.
Our land is a hill farm on heavy clay and I've started by using cut-and-fill to create a large enough area with 2% slope. I was then planning to install Terram 700 followed by 100mm perimeter drains in 450x600mm trenches of 14/20 gravel, fed by 80mm drains in 300x300mm of 14/20 at 5m intervals. On top of this I was hoping to put 150mm of compacted DTp1 or 0/32, followed by 150mm of equestrian sand.
Would the compacted DTp1 be sufficiently permeable to allow the sand to drain properly, or would I be better off using 40mm drainage gravel? If I use the gravel, how can I get the strength of DTp1? Or can anyone (Tony?) offer a better idea? Thanks!

Given all the drainage you're putting in, I'd hazard a guess that the area will drain fairly well and that the surface water will find its way through the DTp1 without too much trouble.

If you wanted to consider alternatives, then you might look at using a sub-base material with fewer or no fines (the 'drainage gravel' you mention), and then using another layer of the Terram (700 or 1000) between that and the Eq. sand (which seems inordinately deep to my non-equine experience) to prevent it being washed into the 'open' sub-base, or, more expensively, using a drainage composite to intercept the surface water as it percolates downwards, and so deliver it directly to the land drains.

What type of DTp1 are we looking at? If it was a limestone or a crushed concrete material, I'd be more concerned about possible blockages in the longer term than I would with, say, a gritsone or a granite.

Very many thanks for your helpful and prompt reply.

I'm told that the riding surface is usually 150mm - either all so-called 'equestrian sand' (single size, fine, washed, subrounded and virtually free from clay, silt or grit - presumably also in several flavours..) or 100mm sand plus 50mm special topping of sand mixed with PVC or rubber granules. Ideally, the sand should be firm, like a foreshore, given the correct water content.

Re. geotextile between the sand and the sub-base, it seems this is controversial. Apparently there have been problems with horses putting their feet through it (despite the 150mm cover) - at best defeating the object and at worst causing trips and broken limbs (horses and riders).

The DTp1 (or 0/32 - 0/40) would be granite.

Do you think I've gone overboard with the drainage?

(Excellent site - I'd be lost without it!)

Regarding the depth of the Eq sand - keeping the moisture content correct is going to be the difficult part. The construction you've envisaged is fine for ensuring the sand doesn't become waterlogged when it's lashing down with rain, but, in the dry summer months, this system could 'parch' the sand, reducing the moisture level to well below the ideal, turning it into a dust bowl rather than a riding arena. How this affects horses, I don't know, but it may be that you have to hose the area a couple of times a day in the dry months to make the surface rideable.

For the geotextile - if there are incidents of hooves ripping the membrane, then I would suggest either a stronger membrane, say Terram 4000 instead of Terram 700, or possibly the use of a composite which is much, much tougher, or, even overlaying the membrane with a reinforcing mesh. There is no reason for horses, or their riders, to be endangered by such a construction if a bit of thought is put into it.

Using a granite sub-base material should be fine. It's a stable, inert material and less likely to cause a problem with the sub-surface drainage.

As for going over the top with the drainage, I think you may have done, but without seeing full plans and understanding a bit more about the requirements of a horse riding arena, I couldn't be sure. Does the British Horse Riding Club (or whatever it is) issue any guidelines?

Many thanks once again, Tony. I'll try to follow-up on the questions you raise.
Incidentally, did you manage to sort those b......s that stole your intellectual property?

It's ongoing, but Northants Trading Standards have been pestering them and my pet solicitor is about to have another go at them. Their web hosting company closed down the site featuring the stolen material and quite a few folk have told me about their "experiences" when dialling the freefone number. :)

I would think that the issue with the membrane will mostly be an issue if the surface you put down is not up to the job (as most that I have seen aren't). Common sense dictates that a ton of horse turning on a small surface area (its hooves) at more than walking pace exerts huge sideways pressure on the surface. If you are using the arena for any form of jumping or lunging the majority of surfaces will dig up very quickly - rubber or no rubber. The only way I have seen around this is to pay for a properly bound surface material that is supportive to the horse yet gives when taking off / landing. There are some about, but like all good things in life you pay for what you get. The surface we used for our arena of 7 years is brilliant, but it cost more than everything else put together. We spent ages toing and froing on this issue, but have never regretted the choice we made. Most of the reputable top surfaces have some form of vaseline coating to provide this extra strength - hence the cost.
The vast majority of people do not go for these surfaces (due to cost) and then act surprised when their lovely new area looks like someone has been at it with a spade.
If you are saving on construction then I would spend the money you saved on a top surface. My experience is that you will never regret it.
Moral of the story is that horse maneges used for jumping etc take a beating - you pay for what you get.

Last month I had a meeting with one of the technical experts for the largest supplier of construction membranes in the UK, and he told me that the British Horse Society (is it??) issues a specification for riding arenas that shows a membrane being used but in the wrong position, ie, at too shallow a depth and without sewn lap joints.

There was also some discussion about the type of membrane being specified and the suggestion was that a typical geo-textile is insufficient in such circumstances and that a composite product would be a better option. However, there is no 'official' approval of such a construction, and so it remains the musings of two chaps over a couple of pints. The Tech Rep said that, if he got the chance, he'd sketch a spec and submit it to his bosses and the Horse people for their comments, but I've heard nowt since.

Do you know of a spec. from this Horse Society crowd, Chris?

I asked about and no one knows of one. This does not mean that there isn't.

You could phone the BHS (British Horse Society) www.bhs.org.uk

I'll check with the good folk at Terram.