Pool Bottom Finishing
- Thread starter KermitThor
- Start date
Oh, never heard it called that. I’d imagine you just put the walls on top of your backfill, but the recommendation for the backfill above is the rock.
There are these people called geotechnical engineers who actually do work like this for a living - and they can actually test the soil (or the compacted area, if any) to be sure it won't settle. Not free, but based on everything I have read in this thread, you need one.
Also - while drain rock is "self compacting", it is NOT true that it doesn't need compaction, or that it won't compress. A 12" lift of 3/4" drain rock (or crushed stone, as it is called up north) will compact as much as an inch and a half.
Also - while drain rock is "self compacting", it is NOT true that it doesn't need compaction, or that it won't compress. A 12" lift of 3/4" drain rock (or crushed stone, as it is called up north) will compact as much as an inch and a half.
That has not been my experience. 3/4" washed stone (#57) resists all attempts at meaningful compaction when/where I have used it. It has a "fluid" characteristic (somewhat like sand). The end result is the rock sort of "squishes out" of the side of the compactor allowing the compactor to settle slightly into the fill. Perhaps that gives the illusion of compacting but nothing is being gained.
My large decking area around the pool was 2500' of poured concrete on top of 3.5" of #57 rock (on virgin soil) and 14 years later, there is not one crack or sign of settling. It would be impossible for me to think of a better subbase for concrete and I do not think it requires compaction.
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We have different experiences in with similar outcomes, @duraleigh. My decks (also totalling about 2500sf) all have compacted drain rock, on a bedrock base, constrained by perimeter walls that keep the drain rock from squishing sideways - and in all 3 cases, the compaction dropped the rock by a half inch or more. Inspectors here (or, more importantly, the geotechnical engineers who have to sign off on grade prep before you can get an inspection) generally require compaction in 6" lifts.
All that being said, drain rock is called 'self-compacting material' and under typical slab or deck loads, it interlocks and won't settle much. But a pool can impose an atypical load, especially on uncompacted or expansive soil - even native 'undisturbed' can settle without proper drainage and compaction. YMMV, of course.
My understanding on #57 is that it lands in a hole at 90% compaction, but there is a lot of bad info out there. I’ve done what I can in the hopper and I think it will hold up. Compacted 6” lifts and then redug the whole thing. The amount of “engineered soil” (if you will allow) is very little all said and done. I’m using a local PB for pool bottom and liner and he agreed this was probably the best solution.
I have no issue giving some light tamping in lifts for #57 backfill. I’m going with pavers so cracking not a concern but excessive settling will be a pain.