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Thread: Empty pools & hydrostatic pressure?

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    Empty pools & hydrostatic pressure?

    I am looking for advice on how to repair a large hole in my fiberglass pool. I can see the dirt and the water has drained significantly. I would like to be able to drain the pool so that it can be properly repaired, but am worried about the hydrostatic pressure causing the pool to pop up. I have read some posts about a valve that relieves pressure...is this valve accessible? If so, how can I find it? Thanks

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    mas985's Avatar
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    Re: Empty pools & hydrostatic pressure?

    If the pool has one, it is usually plumbed in the main drain of the pool. They are suppose to open automatically if the pressure below the pool exceeds the pressure in the pool. However, from what I have seen, they are not very reliable so many stay closed when they are suppose to open.

    If there isn't a hydro-static valve, then sometimes a plug is plumbed into the bottom of the pool so after the water is removed, the plug can be removed so water can equalize on both sides of the pool.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Empty pools & hydrostatic pressure?

    There might be a pipe going under the pool that is used to control ground water when draining the pool.

    Another possibility is a hole dug near the pool that is used for the same purpose.

    Do you know the level of groundwater under your pool?

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    Re: Empty pools & hydrostatic pressure?

    Thank you for your suggestions. I live on Miami Beach so I think it's safe to assume that the groundwater level is high. We have also had a lot of rain recently and I don't think I can assume that the city's drainage systems are working perfectly as they are fairly old. I do have a drain in the deep end of the pool, but due to the water loss I haven't been able to use the motor and the pool is green. I am trying to avoid the high cost of hiring a pool company and I bought all types of materials to make a repair, but I have yet to execute a plan that accounts for relief of the hydrostatic pressure. If there is a hydrostatic valve where the drain is located, how difficult is it to open it? Could I do it myself?

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    mas985's Avatar
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    Re: Empty pools & hydrostatic pressure?

    As I posted before, the hydro-static valve is usually located in the main drain. In order for it to work it must be in the lowest part of the pool.

    If you don't know if your pool has one, you need to dive down to the main drain, remove the cover and check to see if you have one. Otherwise, you would be taking a risk of the pool rising. If it doesn't have one, then check for just a plugged pipe in the same location. The plug can be removed so water flows in both directions.

    I should also mention that a hydro-static valve is not something you turn on or off it is a relief valve which opens automatically when the pressure under the pool is higher than the pressure in the pool.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Empty pools & hydrostatic pressure?

    Most likely your fiberglass pool does not have a hydrostatic relief. Not very many fiberglass pools even have main drains. You need to put well points around your pool to control ground water levels.
    Over 30 years in the pool business
    We build vinyl, fiberglass, stainless steel pools
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    Re: Empty pools & hydrostatic pressure?

    Do you mean inside the pool or around the exterior? Any idea of what this should cost if I called a pool repair company? Thanks

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    Re: Empty pools & hydrostatic pressure?

    The well points would be placed outside the decking. I would guess they could run around 500 a well. That is just a guess.
    Over 30 years in the pool business
    We build vinyl, fiberglass, stainless steel pools
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    Re: Empty pools & hydrostatic pressure?

    Looks like you were thinking we would need more than one...? I have read about installing plugs but can't figure out if this is a simpler option or just a different suggestion. Is there any simple solution?

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    Re: Empty pools & hydrostatic pressure?

    Where is the large hole located? If it's on the pool floor, it will probably function as a relief valve.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Empty pools & hydrostatic pressure?

    It is on the floor and a little closer to the shallow end of the pool (than the deep end). I assume that it has been functioning as a relief valve because we lost a lot of water initially and then it seemed to have leveled off without adding more water. It hasn't rained in 5 days and the water in the pool is now looking lower. The deepest part of the pool is probably still 4 feet. It is too green to be able to see if there is a hydrostatic relief valve in the main drain or to unscrew the drain cap. I have brick pavers going around the perimeter so I haven't attempted to dig a hole outside the pool. Do you think it is safe to drain it? Thanks for your help.

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    Re: Empty pools & hydrostatic pressure?

    Has the water drained down to the level of the hole?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Empty pools & hydrostatic pressure?

    No. The entire floor of the pool still has water...but it's low.

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    Re: Empty pools & hydrostatic pressure?

    If the hole is in the shallow end you could probably to drain it down to that level for the repair and leave the rest of the water in the deeper area to alleviate the popup issues. Most fiberglass pools are not that deep (Mine is 5'6") so chances if yours is of a similar depth you are not very far into the water table if you are at all. How deep is your pool?

    As far as the repair you can do it with basic fiberglass products and methods like you would use on a boat. I have done tons of fiberglass work although fortunately haven't had to on my pool! If your pools is white get a basic white gelcoat for the final layer. You can also pick up tinting for white gelcoat to match tinted pools.

    Got a pic of the pool that shows the crack?
    6000 gal inground fiberglass
    1.5hp pump / sand filter
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    Re: Empty pools & hydrostatic pressure?

    I would guess my pool is the same depth, maybe slightly deeper. I am going to try on Saturday...I'll let you know what happens! I will upload a pic of the hole (way bigger than a crack) later today when I am home. Thanks again

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    Re: Empty pools & hydrostatic pressure?

    No. The entire floor of the pool still has water...but it's low.
    I have to assume it drained to that level and stopped because it reached equilibrium with groundwater on the outside. You could reinforce that idea by pumping out a couple of inches of pool water and the groundwater should then re-enter through the hole.

    If it does, you will not be able to easily lower the pool level to repair the patch without reducing the ground water outside the pool.

    Your description of the hole is still vague. Can you give a definitive size or pic that compares it to something like a ruler?

    Just curious, do you know how the hole got there?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Empty pools & hydrostatic pressure?

    The hole looks like it is about 8 inches in length; it is widest at the center (probably @ 4 1/2 inches) and then narrows towards each end to be about 2-3 inches. It actually looks as if someone stepped on that area and their foot went through. I am not sure of the exact cause...we have lived here 11 years and have never had to do work on the pool (except change the motor). This past year we had problems with algae and the pool had to be shocked several times. My hypothesis was that perhaps the chemicals were not adequately mixed and it penetrated the surface, or it's just worn thin. I broke a piece off from the area where the hole is and it is so thin there is barely any fibers....

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    Re: Empty pools & hydrostatic pressure?

    That is certainly large enough to allow water from outside the pool to enter and water inside the pool to exit.

    So, if the water in your pool has stopped lowering, it's a pretty safe bet that water inside and outside your pool are at the same level.

    It sounds like you will need professional help to effectively patch that hole.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Empty pools & hydrostatic pressure?

    Thank you so much for your advice!

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