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Thread: Depression under liner

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    Depression under liner

    I have a 30,000 gallon in-ground vinyl liner pool. We bought the house 2 years ago, and the pool is about 20 years old. I don't know anything about its construction. Just this year we noticed a depression in the bottom of the shallow end- about 8" in diameter. Applying moderate pressure, the liner depresses, but I can't feel a solid bottom. This area is about 2' from the side of the pool, and there is a large concrete deck all around the pool, 10' wide. I have been told by someone that here in Mississippi under the liner is probably sand, not concrete. Could this be true? Not sure, but the hole may be getting bigger. There does not appear to be a leak now, but last season we had a leak in the steps, which are about 10' from this area. No suspicious areas between the steps and this hole. Any comments or suggestions on how to deal with this? The liner seems to be in good shape and I really don't want to have to replace it.
    docelect
    30k gallons in-ground vinyl
    1.5 hp pump 300 lb sand filter
    Polaris cleaner, T15 Hayward SWG
    Jackson, Mississippi

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    Mod Squad JVTrain's Avatar
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    Re: Depression under liner

    Many IG pools have a vermiculite base if they were installed in that time range. This is a mixture of portland cement and vermiculite materials which set up like concrete but remain more porous. It could be that your pool was installed over sand and you have some erosion, sinking or wash-out below the liner, especially if the previous leak caused water to flow through the soil in that area.

    All you can really do is keep an eye on it in that situation. It will be a weak spot and might develop a leak in the liner if it's poked or stepped on heavily.
    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
    40x20 Pool: 32K Gallons * Vinyl * Bleach Chlorination * Hayward S270T Sand Filter * Pentair SuperFlo 1 HP * Teledyne/Laars Heater * AquaVac Tigershark * TF-100 w/ SpeedStir
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    Re: Depression under liner

    Is the depression a perfect circle or "roughly" circular?
    Jeff
    60,000 L inground pool (Installed July 2015
    Cartridge filter
    Heatsaver Liquid cover
    Clear Blue Ionizer

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    Re: Depression under liner

    Quote Originally Posted by jinglesmountjoy View Post
    Is the depression a perfect circle or "roughly" circular?
    Definitely not a perfect circle. The edges feel hard and rough, but not like they would poke a hole. It seems deeper toward one side.
    docelect
    30k gallons in-ground vinyl
    1.5 hp pump 300 lb sand filter
    Polaris cleaner, T15 Hayward SWG
    Jackson, Mississippi

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    Re: Depression under liner

    A leak could be masked by an auto-fill device and could erode the sand under your liner. Do you have an auto-fill?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Depression under liner

    No- no autofill device. Right now in this very hot and totally dry weather, I'm losing about 1/8" to 1/4" per day, probably due to evaporation- but that is all I lose.
    docelect
    30k gallons in-ground vinyl
    1.5 hp pump 300 lb sand filter
    Polaris cleaner, T15 Hayward SWG
    Jackson, Mississippi

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    Re: Depression under liner

    If it is "soft", that means it's water on the underside of the liner. Do you live in a low area?

    Like JVTrain says, there really is not much to be done about it but observe it. If it gets bigger, you will have to drain the pool, remove the liner and find a way to drain away the water.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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