My round pen was built in an area that use to house pigs. It is fine black dirt, very sooty like. I added some yellow sand that we have on the property to the pen. I put countless bucket loads, and the perimeter was about 1' deep with sand. But the sand always disappears.
What do I need to do to make the sand stay? Do I need to dig out the black dirt and put some type of foundation down before adding sand? Or just keep adding the sand?
Eventually you could add enough sand that it would stop disappearing, but it would probably be easier and cheaper to clean it out and put proper footing in.
If you have a round pen that can be disassembled the easiest thing to do would be to use a tractor and scrape the area down to hard dirt. Once you hit the hard dirt put down a base layer of stone dust and compact it, then top with your choice of footing.
If you have a permanent round pen and can't get a tractor in, you might try something with a bigger particle size so that it won't sink as fast. Maybe a combination of wood chips and river sand.
As mentioned, scrape out what you have as much as possible so the pen footing area is cleaned of the present muck.
We laid the geotextile fabric, smoothed it out, dumped in the new footing. First layer was about 5 inches of heavy gravel, then another 4 inches of medium gravel. We don't have much sand in it, makes the footing too deep. The gravel mix is a "road gravel" that will harden some, prevent hooves going too deep.
On a really good footing, the hoof should only sink in an inch or so, have a FIRM base to push off of for the next stride. Deep footing, sand in depth, is going to be real hard on your horse and his legs. Fatigue comes much faster working in deep footing, and they hurt themselves working when muscles get sloppy, letting legs flop around.
Would you rather run or even WALK on the soft, dry, fluffy sand of a beach, or walk beside the water where that sand is HARD? How fast do you get fatiqued walking on that fluffy sand, and how far is it easy to walk on the hard sand? Same idea for good footing under horses. Those fatigued legs just swing ALL over trying to go forward, giving them soft tissue injuries and problems when working in deep, soft footing. Horse hooves are MADE to go on firm surfaces, using a hard push-off to continue moving.
Our base footing is higher than the ground outside the pen. With use, the pen footing is a bit banked along the fence. You do have to go out now and again to pull the dirt into the pen, out from under the wood fences to keep it where you need it. If we used the pen a lot, I probably would have boards to the ground, to keep the footing inside where it belongs. We don't use the round pen a lot, but when we want to use it, everything is ready to go. Raised footing in the pen lets it drain better, faster, than ground outside. We have clay dirt and it just eats any kind of fill when you don't use the geotextile fabric to keep the layers apart. I have lost TRUCKLOADS of fill before we discovered the fabric and how well it works. With the high level of fill over fabric in the pen, no hooves ever get close to tearing it. Our pen is permanent, wood board sides about 6ft high for our tall horses, very solid looking to be discouraging if they are looking to escape. I think it is 70ft across, so they can use all their gaits and speeds if asked.