The back of my property has a couple of drops down toward the final elevation, sort of a terraced effect. The first of those drops I retained with a timber retaining wall in order to make the backyard area closest to the house a level space. The retaining wall is built using 4×4 uprights set to frost depth (~28") and 2×8 horizontal members attached to the load side of the posts. Excavation was limited to "scraping" the little slope to remove the remains of a flag stone wall from years ago, and to remove a large bush that had grown into the slope. So for most of the length, little soil was removed, save the bush which required a 5' or so round excavation to fully remove. The wall was then built out 2-3' from the previous slope and then back filled to bring everything level with the rest of the yard to the house. This was all completed before the end of September 2020.
My wall is the same as this, only the posts are square:
The height of the wall varies along its run, but is never taller than 36". That means there is probably about 3' or so of disturbed (back filled) soil along the wall's length, and an extra 2' or so in a semi-circle where that bush was.
Now for the question. I would like to place an 18' round above-ground pool on the level space. The edge of the pool would come to around 5' of the wall at its closest point, which means it would end up setting on some amount of the disturbed soil, where the wall was back filled in the area of that bush I removed.
So the question is whether or not this disturbed dirt has been settling long enough for me to reasonably expect it to bear the weight of a pool this size. Or at least the part of it that would be sitting there. It's over-wintered, been driven on at least twice by a Bobcat, and seen rain this spring. But beyond that, I have taken no extraordinary measures to compact it further.
What opinions/guesses could I get as to what the load-bearing status of that soil might be at this point?
P.S. The soil here has a lot of clay in it, some sand. Dig deep enough and it's kind of a river wash consistency.
Best Answer
Suppose your 18 foot round pool is 54 inches deep and is filled right to the top with water. How much pressure would that exert on the ground below? Not much, in fact.
Every square inch of pool has 54 inches of water above it. Thats's a volume of 54 cubic inches or 0.234 US gallons. At 8.34 pounds per gallon, then, we've got just under 2 pounds of water standing on every square inch of soil. That's less than the pressure of an adult standing on her feet.
If the pool were any closer to the wall then we might start to worry about whether the wall can hold back the added weight of the pool. Given the setback is greater than the wall height that shouldn't be a problem.
The soil surface will settle ever so slightly under the pool but assuming there were no large voids in the backfill (ie no large clumps of clay tossed in a group) it'll be fine.