Have you ever had your liner float? This typically only happens when the pool has to be drained for one reason or another. Basically what happens is that there is water that builds up in the ground around and under the pool. most of the time it is due to rain, but there are other variables that come into play from time to time. Anyway as that water builds up around the pool, the pressure from that water pushing up becomes greater and greater. Usually the water in the pool is heavy enough to keep the water outside the pool back. However if you have to drain the pool for some reason then the weight of the water in the pool decreases and the water behind the pool has the opportunity to flow in behind the liner. I have seen this happen many many times. Fortunately most of the floating liners that I have seen were being replaced anyway so it didn’t make much of a difference. Still it always seems to alarm the homeowner, and when I tell them that it is ground water underneath the liner they always question me saying “well it can’t be, it hasn’t rained since last week”. The fact is that it can be because that is what it is. The ground has the ability to retain a lot of water and being that it is underground, it doesn’t have the sun aiding in evaporation. Then when the water in the pool is drained, it suddenly has a place to flow to.
This happens quite often around here. However it is rare that the liner will float with the pool full of water. Don’t get me wrong, it has happened before and it is because the water pressure behind the liner is greater than the water pressure on top of the liner.
There are some things that most builders will do to ensure that the liner doesn’t float and give the home owner some piece of mind. One is that they will often run a perforated tile around the pool that will collect any rain water and allow it to flow away from the pool. There are a couple places that it will likely go. One place is to a sump pit. This is just like the pits you see in basements. It is usually located just behind the deck of the pool and has a pump in it that will kick on if the water level gets too high. The other way they take care of this issue is by having what is know as a French drain or Gravity drain. This is dependent on the elevation of your yard. It is basically the same perforated tile running around the pool, but instead of it going to a sump pit, they will dig a trench down hill away from the pool. That way any water that the tile collects will be directed away from the pool.
I have even worked on a pool that was built with a line underneath the very bottom that could be used to pump water out from behind the pool. This was a extenuating circumstance though. The pool was built in an area that retained a lot of ground water. For the most part liners floating is not a problem around here, other than when a pool is drained.
There is also a product available that was made to combat this issue. It is something that I have never used and something that I would be cautious about using. Hydrostatic vales were made to keep ground water from ever being a problem. They get installed in the bottom of the main drains on the pool floor. They use the pressure of the water to work. Basically they have a spring in them and if the water pressure under the pool gets too great the the spring assists the pressure to force the valve to open. This allows the water from under the pool to flow into the pool rather than behind the liner. Then once the pressure is equalized the valve closes again. My only concern would be that there is a possibility for something to get into the valve while it is open and prevent it from closing again. If that were to happen you would then have a pool that no longer holds water. I suppose it is not a major issue, as i imagine the hydrostatic valves are not too difficult to replace if needed. Honestly it is up to the discretion of the builder on what they think is best.
If you do happen to see that you liner is floating, it is very important that you get it taken care of right away. Usually the pool will have to be drained and the water will have to be pumped out from behind the liner. Hypothetically if you were to let it go, then you could end up with a much bigger issue on your hands. I have seen one liner float to the extent that the water under it pushed it up so much that it stretched out the screw holes in the main drain. At that point the liner had to be replaced to ensure that it wouldn’t leak. In addition to that the water under the liner can damage the bottom to the extent that some patch work needed to be done.
Ground water is very common when digging down especially eight feet or more. Looking back I think that the majority of the pools I have built have had ground water from day one. It is just a matter of keeping it under control. Usually we would dig the pool a foot or so deeper than the finished depth. Then we would drill small holes into a plastic 5 gallon bucket and bury it in stone in the bottom of the pool. That stone would also bring the depth back to finished grade and allow any ground water to flow through the holes in the bucket so we could easily pump it out. Then when it came time to do the bottom, we would keep the pump in the bucket until we were ready to finish the bottom. At that point we would pull the pump out, fill the bucket with stone and do the bottom over top. Then we would get the liner in, cut in the main drains, and start filling the pool. Basically it is a matter of staying ahead of the ground water. If you can get more water in the pool than there is under the pool there won’t be an issue. It comes down to experience. Most reputable builders know what they are doing and very very rarely ever run into an issue. If for any reason there is a problem a reputable builder will likely take care of it at no extra cost.
The reason that I wanted to write about this issue today is just to offer some information as to why liners float and what can be done, both to prevent it from happening, and to resolve it once it if if does happen. Like I said before, ground water is very common but usually doesn’t pose a problem for vinyl liner in ground pools. Typically the weight of the water in the pool is enough to keep the ground water back. In most cases people with vinyl liner in ground pools aren’t even aware of ground water until the pool is drained.