Outdoor arena: recommended depth of base /drainage

[myblackmorgan3]

Hi…Question 1:

I am putting in a 70’ X 140’ outdoor arena. Topography slopes in 2 directions so, at maximum, I have a good 4’ - 5’ deep, 10’ wide berm ( equally as high, opposite corner) - which varies in angle down to pretty much no slope in one corner. Because of the lay of the land - two sides are higher than the arena surface, one side is lower but that is a dry lot (so don’t want to drain into it). That left one short side. Grading to drain across the entire long length of the arena - and out that short side- was not done in favor of draining from corner to diagonal corner.

Now I’m worried about the flow going to that one corner - with the 10’ wide, 4’ - 5’ high, angled berm. Erosion. I do hope to plant tall shrubs in that area, which would help knit the soil some. We have 16" of rainfall/year - so it’s there but not a ton. Far Inland Washington State.

Would a French Drain help with the erosion - extending a bit on either side of the corner and buried in the berm? Or is it best to have this thing graded differently ($$$)? I am hoping that I can get the excavator back out to show us elevation points with his lazer and discuss…

Question 2:

We have ashy silt loam - well draining, agricultural land. I was thinking 6" of compacted gravel base (1.25" granite minus, overlain by 5/8" granite minus). But this seems to be by far the most expensive part of arena construction - hence many skip it - often to their peril.

Anybody else out there on silt loam? Because it is well draining, does that mean I can have a base of less thickness - say 4"?

Thank you , all!

[dqtastic]

Thank you for posting - i’m currently undertaking almost the exact same thing (though I angle to long side / no berm). I am contemplating a 4" base on top of geotech - as my sandy / silty loam drains well. I am wondering if the geotech will save me a couple inches of rock… We put 6" drain rock / 3" screenings in our paddocks and in retrospect it seems like a lot to place on what was already fairly well draining.

I have not started the process yet, so no direct report for you, simply a high five from another PNW’er trying to figure this all out!

[myblackmorgan3]

Dqtastic -

Best of luck on your endeavor, as well. Things I’ve heard about geotextile is that, if between base and footing, it has a tendency to creep up. Indeed, this happens in our stall runs. There are ways to “sew” the seams but I doubt there’s many folks with the equipment to do it.

If between sub-base and base, it does help with base floating down into sub-base but the fines in the base can clog up the geotextile, hence causing a drainage issue.

That’s just from my research - you may find more/different info.

[SCF01]

Following…Im starting same project although our soil doesnt drain well at all, lots of clay and big rocks. Ours will slope to drain on the short end to the north because of the lay of the land.

Ive also heard not to use fabric between layers.

[ASB_Stars]

I have seen geotextile used, and it creeps up, if your base is not done extremely well.

I did four inches of street compacted granite screenings, and then let it bake in the sun for 30 days in the Summer- now, before this, we stripped the thing down to clay, and that sat for 30 days, also. The ring is 20 meters by 60 meters, drains beautifully, and has never had an issue- for 18 years.

[napalmbright]

I’m hoping to put in an arena in the summer, but I have very wet clay soil. Was originally going to put down geotextile but not sure if that will work based on the above comments.

Has anyone has success in wet clay soils? Would love to know how best to prep. Was thinking 6" of gravel screenings, s thin layer of sand to fill the gaps, and then 1" of rubber footing. Worried about everything slowly sinking into the clay!

[dqtastic]

@napalmbright

,and then 1" of rubber footing. Worried about everything slowly sinking into the clay!

I’m not on clay - but i’ll warn you that the rubber footing can float up and away, depending on how much rainfall you experience!

i have decided to not do geotex, but am doing large drain gravel (3"-) then 1.5-" gravel then screenings for a total of 6-7ish". Will do sand footing.

[S1969]

@napalmbright

I’m hoping to put in an arena in the summer, but I have very wet clay soil. Was originally going to put down geotextile but not sure if that will work based on the above comments.

Has anyone has success in wet clay soils? Would love to know how best to prep. Was thinking 6" of gravel screenings, s thin layer of sand to fill the gaps, and then 1" of rubber footing. Worried about everything slowly sinking into the clay!

I have very dense clay - e.g. you can literally make pottery from any excavation deeper than 12-18".

My arena is a semi-professional job (done by a professional excavator but with only a handful of arenas on his resume). He felt it would be better to improve the drainage through the arena; another thought would be to try to keep water from draining through and divert around/away. We removed about 18" of top soil and installed gravel with drainage in it. Then geotextile and stone dust. Then sand. It’s deep. It drains fairly well but I have a few spots where the drainage isn’t as good; probably somewhere in there is an infiltration of clay/silt in the drainage. Or, just an odd spot in the clay - it’s not consistently dense for some reason.

I think you’re right that things will sink into the clay. You will definitely want to remove some of the topsoil and consider having the clay compacted (which I did not have done.) I think that would have improved my arena.

I’d get quotes if you can and compare resumes with clay soil. It’s tough working with this kind of soil, as you must know by now. I also have fence posts that need replacing even though they should have a 30 year guarantee. I think because the clay contracts and expands as it gets wet/dry. Wet = soup; dry = concrete. It’s a lot to stand up to for a fence post.

[napalmbright]

@S1969

I think you’re right that things will sink into the clay. You will definitely want to remove some of the topsoil and consider having the clay compacted (which I did not have done.) I think that would have improved my arena.

Thank you for the input! Will definitely have it compacted when the time comes. There’s so much to consider!

[OzarksRider]

We recently built a barn with the same drainage issues you describe for your arena. You wondered if a French drain would help with erosion. We installed a French drain and it works like a charm. It’s just a channel filled with 2" rock. Water runs through the rock into the trench and the top stays dry. We also put out seeds to try to establish a cover crop over the muddy areas but I don’t think the weather will cooperate. If anybody has other ideas how to stop erosion while waiting for spring growth, I’d love to hear them.

[mlb722]

I had about 8” of well draining loam, it’s an old hay field. Removed all of it to get to clay. Did 3” gravel, 3” stone dust then 2” sand. It does drain out of one corner. Imo 4” is not enough for a well draining base.

[TrotTrotPumpkn]

My natural soil is a loam. My arena was prepped for 90 x 200. I would suggest going wider than you want the riding surface to actually be. It drains north to south and west to east. The soil was removed and the subbase is compacted clay. The base is around 5" of compacted/rolled quartzite. 1/4" minus. I added 3" quartzite stone dust this summer. I may or may not compact that to add sand–undecided–currently it is my footing.

I think the base answer depends on your sub-soil saturation potential, rainfall, the topography of the surrounding land and what aggregate you have available. If you are wiling to wait to ride after rain, that is also different than if you want an all-weather arena. You can get away with less base if you are on top of a hill and cut the peak off to build your arena vs. an arena that sits the same height or lower than the surrounding natural land. 6" is typically suggested as a minimum.

My arena has 20’ swales on the long sides, the north side is built up higher than anything, and the south continues to drain with the natural flow of the surrounding ground. It sucked, but I am very glad we put the base in last year and didn’t add footing until this year. The base had to be raised higher this spring in the southwest corner and the swale was cut deeper. We have had record rainfall two years in a row and it slowed down construction, but I think it was a blessing in disguise.

If I were doing it over, I would start with clay, then 2-4" larger rock, 2-3" 1/4" minus and then a thin layer of stone dust. I think my rock would have bound/compacted better if I had large rock underneath instead of just clay. Also, make sure your base rock is “dirty” (has something to bind it) unless you are installing a french drain system under the arena. A clean rock base will not compact hard enough together, but is what you want if you are doing an arena mat system over the rock base with French drains underneath.

[TheJenners]

I have clay as well, not pottery quality but doesn’t perc at all. My arena will be done this way in the spring once we dry: excavate down, removing the thin soil and then into clay; compacted; six inches (compacted depth) screenings; then 2-2.5 inches footing, probably just angular sand. I had a specialist come in for the design and give the directions to a good dirt work guy. I plan on stopping before the sand and sprinkling it a lot for 30 days or so depending on weather. I had wanted it done this fall so it could be rained on all winter but oh well.

My arena will be placed with short sides east and west, and there is a drop to the south and southwest. The dirt guy did my septic and finish work around my house in Feb and March, so he knows exactly how nasty wet I get, so he will be installing french drains along the north long side and winging out from the short sides.

[TheHorseMutterer]

Question for those on clay that have done clay > larger rock > geotextile > footing base > footing: I have been told (and seen from my own experience) the larger stone sink into the clay and disappear fairly quickly. If you’re putting the geotextile above the larger gravel (below the screenings), will you eventually assume you will “lose” the drainage?

Perhaps I’m over thinking this and it’s that an arena has a limited lifespan from the get-go, but I’m curious as I’m about to embark on a DIY arena on GA clay here shortly. My current plan, based on several conversations with other local folks who have lesson barns with high traffic arenas (mine will be very limited personal use), was to simply put down 4-6" of M10 (which is “screenings”) and let it compact naturally before using. I’m adding in some ground grid over geotextile on the rail on the perimeter to stabilize and reduce the wash-away effect, but no real prep to the clay other than leveling and sloping.

I’m scared as hell, but I’ve ridding on several arenas that claim to be made exactly this way here and they’re rideable about 95% of the time, which is far more than I need.