Help!! Is this retaining wall going to support an AGP?

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Hey folks! New here to pools... My backyard is slightly sloped, so I did my best to dig down instead of build up but in the end I did purchase a hundred or so retaining wall blocks and built a retaining wall (about 12 inches high in the back), and then filled it with about 12,000 lbs of mason sand.. Now that I think about it, I am wondering if the retaining wall will be able to support the weight of 63000 pounds of water! Anyone done this before? Thoughts?

I bought an 18' round prism frame Intex Pool that is 4 feet high, so should be roughly ~63,000 lbs.. I did plan on using cement pavers and burying them to ground level. The pool is in place but I have not started filling it yet until I get some advice on what I have done and if it is going to be safe and work.

EDIT** The retaining wall blocks do have a lip on the bottom to help prevent sliding off of the block underneath it.. I just don't know if that will be enough

 

[Texas Splash]

Welcome to TFP! I think what will be the deciding factor is how well that sand is packed down. Did you use a plate compactor? My concern is that if it's not hard-packed well enough, that the pavers underneath the vertical posts might start to sink which could displace the sand a bit. That's bad enough on its own. Whether or not it would encroach on the pavers you have going around the perimeter is hard to tell. But I think the big question is how stable and compacted the base will be.

 

[Newdude]

Hey PL and Welcome !!!!

I see two major flaws with the plan.

1) a flash flood will wash the base through the cracks between the stones

2) you have 10 full sized pickup trucks parked on top when it does.

We always recommend to dig down and never build up.

 

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Thanks fellas for the replies! I did not use a plate compactor - wishing I had, but I did pack it down pretty good manually using my own weight as well as driving a heavy lawn mower tire over the area. As for the sand washing out between the cracks, we already had a torrential downpour and I lost no sand through the cracks or otherwise - its packed pretty good and doesn't seem to want to move. I do realize that my own weight and a lawn tractor pales in comparison to ~32 tons of water, but I also read that sand doesn't compact very well, so hoping I did a good enough job.. I am thinking about just going for it and watching carefully during filling to see if anything shifts. If it does start to shift then I drain and start over. If it doesn't I may just let it ride and just let the thought of it collapsing later down the road keep me up at night..

As for the pavers sinking in, maybe I should avoid burying them level with the sand and just keep them laying on top?

 

[Texas Splash]

As for the pavers sinking in, maybe I should avoid burying them level with the sand and just keep them laying on top?

Hard to tell. Some pavers stay flat and even with no issues, but some owners don't have them perfectly level and once the weight pushes down they tilt and dig-in. But if your base is solid they should be okay. Interested to see how everything goes. You'll have to give us a follow-up later. Good luck!

 

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Well I just got off the phone with my brother, who happens to have a plate compactor that just needs a new carb. I ordered the carb from Amazon, and should be here tomorrow... Now I just need to figure out how to get a 176 lb plate compactor over to my house and into my backyard without killing myself. I am thinking maybe its better if I just wait for the compactor before I start filling this pool up. As long as the pavers don't shift I think this plan will work. Now I am really wishing I had dug a trench or glued the pavers together

 

[phonedave]

Time for my PSA on soil compaction

Soils (including sand) will only compact to their maximum density when they are at the correct moisture content. Too wet or too dry and it will not compact. In order to figure out what that moisture content is, you need to run what is called a Proctor Test on whatever you are using to fill the area. Then you need to measure the water content of that fill, then you need to measure the in place density of the soil to make sure you have compacted to at least 95% of its maximum dry density. That soil will get wetter or drier, and with a surcharge as heavy as pool, it WILL settle unless you have compacted it correctly.
As far as compaction tools go:
- Hand tamper - useless
- Lawn tractor - next to useless
- Jumping jack - close to useless
- A light vibratory plate (like 176 lbs) - Sketchy at best
- A substantial vibratory plate (like 700 lbs - 45Kn of compaction force) - Ideal

And any compactor needs to be used with 4" (maybe - MAYBE) 6" lifts. Not the full depth all at once.

This is why we recommend not using fill under a pool. Pools are HEAVY, and are basically a big bag of water. Fill a zip-loc bag with water, and put it on an incline, and watch how easily it moves.



This video does a good job of why you can't just "spray the sand with a hose and then pack it real good" and especially the issues with compacting pure sands with little to no cohesive soils in them. This is why you read that you "cannot compact sand". It is not that it does not compact, so you can just put it in there. It means it is a royal PIA to compact correctly and it will shift and settle very easily.