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Re: swimming pools

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        You and both!!
        Except in clay soils the clay could dry out and not contact the outside of the pool wall so I would design it for pool full with no soil contact as well.
H. Daryl Richardson
----- Original Message -----
Sent: Saturday, July 29, 2006 12:40 PM
Subject: swimming pools

I have been fortunate enough in my forensic work to look at many pools with problems, the biggest being about 20' x 60' at a hotel, most being residential. This is all in FL which is mostly sandy soil, though there are pockets of clay and even more in North FL. We also in many areas have a very high water table, sometimes only a couple of feet down from grade during the summer (seasonal high).
I have seen in sandy-clayey soils with high water table push a pool up out of the ground two feet, and take the deck with it. One in particular was pretty fun just to look at, the person's pool was flooding during a hurricane and they pumped the thing half down and right after that they pool started to pop up out of the ground. THat made my job easy - high water table, not enough weight in the pool to counterbalance.
Other causes of pool shell and deck problems I have seen are poor soil preparation (mucky, organic, non-compacted or saturated soil under the floor), poor rebar placement, wrong size and spacing of bars, thickenss not per dwgs, low PSI concrete, soil settlement under the floor and behind the walls, and improper backfilling. I have not investigated pool dwgs or designs and run my own analyses.
In FL pool designs must be signed and sealed, but in my opinion this industry seems like tract housing and trusses, a bit on the shady plan stamping side (and I see engineers being disciplined by the board fairly often for pool designs).... I know a good structural engineer who designs some pools, and he said that if it is a kidney or oval shaped pool, you rely on water pressure to counteract the soil pressure, and if there is not enough soil pressure the pool walls act as a shell or like a water storage tank and it goes into tension and the reinforcement kicks in. Opposite when you pump the pool down, the curved shape walls act like arches in compression. And I supposed you get a cantilever retaining wall affect as well from the vert reinforcement, such as in a rectangular shaped pool. He also stated there is a bit of art to it and most pool designs could probably not be completely justifiable by calcs. But they MOSTLY seem to work, I cannot say for sure if any of the failures I investgated also suffered from poor design, we were not asked to check the design or perform calcs.
If I had to sign and seal pool dwgs, I would have to go with the cantilver retaining wall design method, using the floor as a footing. Run calcs with no water, high water table and soil; and again with full water in the pool and maybe passive soil pressure. I also bet the pool contractor would not like my dwgs!! Also if I owned a pool I would not pump it down if I could avoid it, and I sure would do it during the dryest season.
Andrew Kester, PE
Principal/Project Manager
ADK Structural Engineering, PLLC
Lake Mary, FL