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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 4x4 uprights set to frost depth (~28") and 2x8 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: enter image description here

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.

DonBoitnott
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  • How deep is the pool? What's the bottom made of? – jay613 Jun 02 '21 at 18:28
  • @jay613 It's a hard side, 54". Bottom would just be the liner on sand, the walls/uprights are supported by a steel ring. – DonBoitnott Jun 02 '21 at 19:38
  • As the answers below suggest: the weight of the entire pool is not too critical. As a rule of thumb, staying well within the 45deg or 1:1 ratio (*more* than 1 foot away for 1 foot of height) is fine for disturbed but re-tampered ground. – P2000 Jun 02 '21 at 22:10
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    Do also check your local legal requirements for pool fencing - which vary considerably around the world. Maybe nothing, maybe you need a 6 foot fence with no horizontals and gates that can only be opened by adult-sized people, Or you may need X metres of space between pool and fence. Best to check before commiting to a plan. – Criggie Jun 03 '21 at 09:16
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    @Criggie OP's profile says Ohio, so at very least it needs a fence and a locking gate. – J... Jun 03 '21 at 11:05

2 Answers2

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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.

Greg Hill
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  • 5’ away I agree there should not be a problem but I would expect uneven setting or more settling where the fill is deeper, not an issue with the wall at 5’ but after a few years it might need to be drained and releveled but that is not a show stopper in my opinion I have had to do that on my own pools in the past a load of sand did the trick for me (sand is how I prep for an above ground then cheap harbor tarps underneath extends the liner life. But I agree I would put the pool up. – Ed Beal Jun 02 '21 at 20:24
  • @EdBeal interesting you had success with sand. I have a pool like this, and was advised by the manufacturer *not* to prep with sand, not even 1/2inch pocket fills. I did patch small spots anyway, and after 2 seasons of PWC rain I saw why: parts of the sand get washed away from underneath, leaving small pockets and streams. It's not a serious issue though. – P2000 Jun 02 '21 at 22:05
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    @p2000 I have a pool sitting on sand right now I put it on top of grass , dirt or asphalt then the tarps as I said and it works great perfectly level excess sand outside the premier of the pool moved but not under the pool. – Ed Beal Jun 03 '21 at 12:40
  • @EdBeal This probably warrants a new question, but I'd love for you to elaborate on that tarp thing...seems like something I might want to do. – DonBoitnott Jun 03 '21 at 14:05
  • @DonBoitnott yes please do. Actually if you don't ask I will. My pool came with a tarp to protect the liner but I don't quite understand how and I was considering posting too – P2000 Jun 03 '21 at 14:50
  • I started using tarps in the past to protect the liner from rocks & when gophers tunnel under the pool the tarp takes some of the stress and the liner has to stretch less, on asphalt the tar base can age or harden the bottom so if the pool is taken down the bottom may be less flexible because of the tar. The cheap tarp helps extend the life in all the above cases especially if the pool is taken down for the winter, where the liner had stretched not be at quite the same place so now a new place has to stretch, with the tarp(s) under the pool the tarp takes the damage and the liner last longer – Ed Beal Jun 03 '21 at 16:22
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With the recent backfill and about 4 feet of water depth my concern would be very localized subsidence, ie mini sink holes, that would not be a problem with a solid bottom pool but a vinyl bottom would sink into these, stretch and maybe break.

My suggestion would be to prepare the ground first by removing any sod, renting a vibrating power tamper, going through a couple of cycles of flooding, drying, tamping. This will hopefully cause small cavities to collapse and create a solid base.

Then you can google "prepare ground for above ground pool" and follow generic advice .... ie, levelling and covering with crushed stone.

Another way to solve this would be to cover the ground with something solid. I had a temporary soft-sided pool that I placed on large foam tiles (the kind used in gyms) to avoid this kind of problem. It worked nicely. I don't know if it's recommended for a more permanent installation. I only needed mine to last a few months. Anything that would withstand subsidence of, say, a 2 inch hole. Some people put a concrete base covered with a soft lining for their larger above ground pools.

jay613
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  • Agree that local pockets are maybe of concern, not the general load of the pool. If it rains a lot, sand is not a good option for pocket filling, and tampering & crushed stone like you write is the correct method. – P2000 Jun 02 '21 at 22:08
  • Purely anecdotal experience with mini sink holes: the last time I put up my pool, also 18 ft x 54 in, we had a gopher problem. Through the course of the summer sunken spots developed where gophers had attempted (and failed) to route a burrow up and make a new mound under the pool. I know they weren't there prior to the pool erection because I leveled the location with a skid loader (cutting and adding soil) in preparation for the pool. Fortunately the liner sunk and stretched into those collapsing burrows but didn't tear. – Greg Hill Jun 02 '21 at 22:52
  • @GregHill second gopher story this week on DIY! Other guy was a driveway with three holes. – jay613 Jun 03 '21 at 02:04