I’m no “expert” per-say, but I do have a fair bit of experience and understanding of these things. If this was installed with reasonable expertise and it looks and sound like it was. IMO and experience the last thing you want to do is “disc” it. The last thing you want to do is risk the chance of the compacted base being “disturbed” or parts of it. Unless it is perfectly level and very few if any rings with age are going to be especially ones that are crowned. You run a very high risk of “digging” into the base in some areas and not in others. Even if the operator sets the disc to “float” mode. So unless you go over the entire surface checking for “soft spots” and compacting them the base is going to be inconsistent. Which IMO is worse then what you already have. Which doesn’t look bad to me based on the picture. I assume this is for personal use not to have high end competitions on where it will get lots of use and abuse.
I use 2 types of drags on mine. When it footing gets compacted from lack of use and or not dragging on a regular bases I made a pretty aggressive drag to break up and get down almost to the base. An 8 foot “box” made out of 4X6, reinforced corners, bolted together with carriage bolts and some lag bolts. With a “weight box” on top that can be filled with stones, cider blocks, whatever. I bought 3/8 rebar and cut 12" sections, drilled holes in the frame sized to the rebar for a “snug” fit. The holes were drilled in a “V” pattern. I think around 4-6" spacing. Pounded the rebar through and “exposed” enough to get through the footing, around 4± inches. As the rebar wore down over the years I just “pound” the tops to “even up” the bottom to the others. I attached a chain so it would drag on the “diagonal”.
After working up the base I “finish” with a chain drag. If it is dragged on a regular bases weeds and grass will be kept at bay. If not use a “ground clear” product that will usually last through the “growing season”. Spray when it has been hot and dry.
IMO and I am saying this without the benefit off inspecting the project, I would just add more footing and smooth out. But you will needs some kind of equipment to do this. A tractor with a bucket and a decent operator can do a pretty good job. And or a 3-point hitch scraper blade on the rear. Those combined have worked for me. Than drag and drag until it becomes pretty consistent and even.
You can do the math to determine how much footing/sand by using any number of online calculators. Or just measure it call whoever you are going to buy the footing from give them this dimension and tell them how deep you want it to be. They will do the math for you. When the trucks come be prepared to start moving the footing around as soon as the truck leaves. Than have them dump the next load in a different area of the ring. It will be a PITA to try and work around the various “piles” if you try and do it after the fact.
The “go to” area guy around here told me the best bang for the buck that gives a nice riding surface and tends to retain moisture and keeps the dust down is to add saw dust/shavings also. Yes it will break down over time but is worth the effort to add more when needed. You’re in Colorado with a dry climate so it should last a lot longer than it does here in the wet, humid mid-Atlantic region.
