I have done several outdoor arenas. Do it right the first time, or it will cost you more $ and it is harder to fix
. There are 2 parts. Base and then your footing.
should be hard. You want a minimum of a 4% slope to where you want water to drain off. Have your "dirt man "scope it." Do NOT
let them "eye ball it!" Trust me your "dirt man is KEY!" Depending on where you live "hard pack" may be called different names.
goes on top of your base. 6" is custom. Reiners, barrel racers, working cow, prefer 6". English riders want about 3 inches. They want bounce. So depending on the discipline is how I set my arena equiptment depth.
You want a "dirt or combination of sand"
that does not pack easily. You want a soft footing to prevent leg injuries. If your base is not compacted hard, you will loose your footing into your base. NEVER CUT INTO your base
with your arena equipment. You only work
your footing. Set your arena equipment to the depth of your footing you prefer to ride.
Your arena base will allow it to drain and your footing will dry out. If you have puddles or mud holes, it is because you have not groomed your arena footing level.
I would not use clay as footing. Hope this helps. Remember it will cost more
to remove your footing and go back and fix your base. (been there, done that mistake)