Wow, good memeory - luckily, I don't do it all the time, just for myself and have helped neighbours.
Yes, I had to build up my ground some, for two reasons - one - the pool top rail was to sit level with the top of my raised septic bed, we were eventually going to build a deck at that ground level over to the pool - essentially making it look and accessible like an inground. Second, every inch helps because of my creek that comes to within 50 feet of the pool and could flood the entire lawn it sits on when we have storms. I had the clay moved in from another section of my property the year prior to building the pool, this way it had time to settle over the winter.
I used a ring of crusher run about 6-12 inches thick for my wall foundation, no footings, personally, i don't like footings, I've thought this over many times and they just don't make any sense to me, ESPECIALLY in a frost environment.
The clay gets very wet in the fall, winter frost really expands it, essentiall lifting the entire ground by what can be several inches, the footings can be distorted easily by frost - by that I mean they will exert uneven pressure up on your wall and most likely no longer be level once they settle after the frost leaves the ground, at least the screenings are workable, and pretty much settle flat again because they are able to shift with the heaving of the frost, dispersing pressure upwards more evenly.
Another reason I don't see any need for footings is that the pressures exerted by the water in your pool are not directed to the posts and down into the ground through them, here's how the pressures work (more or less):
Most pressure is straight down due to gravity - so it's compressing the dirt under your pool.
Some pressure is outwards, since the wall is the structural component, most of the force there is trying to expand the perimeter of your wall, so the stresss point here would be the seam - thus all those bolts holding it together.
The bottom rail is really just there as a spacer for teh posts and as a flat receiver for the wall edge, it also helps disperse upward pressure from frost more evenly over a larger section of wall.
The posts are just there to hold and join the top rails together and transfer any downward pressure exerted on the top rail to the ground so the wall does not get affected - this pressure would only be external forces such as a person standing/leaning on the top rail. The top rail is also there to give the wall rigidity and stop it from becoming an oval through uneven water motion (kids roughhousing) or people leaning on the wall from outside or inside the wall - however, if you remove all outside forces - whether it's frost or human, your pool is perfectly fine to just sit there full of water even if all you used from the entire kit was the wall, bolts and liner - unless you incurred really high winds which can be interpreted as outside forces pushing on the walls and moving your water more that circulation would.
I hope this helps you some, personally, I would NOT pour concrete, especially with your soil, sandy soil might be a different story since it drains very well and there's less frost heave due to that, but clay is a very uncontrolled environment that works in mysterious ways to somehow spit up solid objects every spring in random patterns.
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