How to alleviate hydrostatic pressure under liner

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I am currently digging a pond approximately 10 feet by 10 feet and 3 feet deep. I live in an area where the ground is mostly clay. I get a lot of ground water in my lawn after a rain. It was suggested that I install perforated pipe Along the bottom of my pond and vent it out to An opening so I can relieve pressure. I'm Trying to understand this process.I guess this is not like a French drain that is supposed to drain the water out from underneath the pond.Can someone please explain this to me so I install it properly?
 
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Any arrangement where you can siphon or pump rising ground water from under a liner will relieve it from ballooning up

Finding the lowest point will help
 
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I had the same issue. Clay soil and I hit water at 2’ down. What I did was put a pipe with slits in it, much like what you would put in the bog, at the lowest part of the hole. I actually dug down another 6” and put in a 2” bed of small blue stones, then set the drain pipe in and covered it with more stones until it was level with the bottom. I then ran the pipe up and out the side of the hole. Because I hit water, I ended up building a wall around the hole at ground level so my pond is half in and half out of the ground. I backfilled around the wall with the dirt from the pond. This works great because the drain outlet is lower then the pond surface level.
This is the important part. For the water under the liner to escape without a pump, the outlet must be below the pond water level. With this design, the water pressure in the liner, forces the water under the liner up and out the drain. Hence your reference to hydrostatic pressure, if the water under the liner has no way to escape, it will balloon up the liner and force the pond water to spill over the edge.
Since I had clay on the bottom of my pond hole and it was very soft and sticky after I pumped out the water, I used concrete to cover the bottom of the pond at the lowest level and the sides where the water had eroded the clay. Only about an inch of cement since it was just so I could get the underlayment in without getting stuck and making footprints, I also cut a Lowe’s blue tarp to fit the bottom of the hole so I could walk in there without the clay mess. It worked great, got the underlayment in and the the liner without a clay mess. And now with the pond filled and doing great, when the rains come and the ground water rises, the drain let’s the water under the liner run out.
Hope this helps.
 
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I had the same issue. Clay soil and I hit water at 2’ down. What I did was put a pipe with slits in it, much like what you would put in the bog, at the lowest part of the hole. I actually dug down another 6” and put in a 2” bed of small blue stones, then set the drain pipe in and covered it with more stones until it was level with the bottom. I then ran the pipe up and out the side of the hole. Because I hit water, I ended up building a wall around the hole at ground level so my pond is half in and half out of the ground. I backfilled around the wall with the dirt from the pond. This works great because the drain outlet is lower then the pond surface level.
This is the important part. For the water under the liner to escape without a pump, the outlet must be below the pond water level. With this design, the water pressure in the liner, forces the water under the liner up and out the drain. Hence your reference to hydrostatic pressure, if the water under the liner has no way to escape, it will balloon up the liner and force the pond water to spill over the edge.
Since I had clay on the bottom of my pond hole and it was very soft and sticky after I pumped out the water, I used concrete to cover the bottom of the pond at the lowest level and the sides where the water had eroded the clay. Only about an inch of cement since it was just so I could get the underlayment in without getting stuck and making footprints, I also cut a Lowe’s blue tarp to fit the bottom of the hole so I could walk in there without the clay mess. It worked great, got the underlayment in and the the liner without a clay mess. And now with the pond filled and doing great, when the rains come and the ground water rises, the drain let’s the water under the liner run out.
Hope this helps.
Thank you for your response and that all makes sense. My problem is that my pond is 3 ft deep and my yard is flat so I don't have anything lower to run any drainage to. someone suggested that I put perforated pipe along the bottom of the pond and then run it up to the surface of the grass so that it has an escape for air. Maybe I'm just not understanding this properly. I've also seen online where people have to dig a sump pump deeper than the pond in order to pump the water out.
 
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I think the sump pump pit is the only answer unless you do what I did and add a wall so that you can get the drain outlet above ground and below the water level. I considered the sump pump pit but I was not keen on having to depend on another pump. Maybe someone else will chime in with some other solutions.
 
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You can have that problem with a gunite swimming pool, but only when the pool is empty. The only thing that is done is to put a pressure plug in the drain. When the pool gets emptied the plug gets removed, if not the pool can lift out of the ground because it is one solid mass. I don't see that happening with a pond. But I could be wrong. I can not see how the pipe that you mentioned would do any good.
 
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If you get water under your liner without a way for that water to escape your liner will balloon up. It is not like a pool.
 
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I may be out in left field, but a 10'X10'X3' pond is 300 cubic feet. A cubic foot of water weighs roughly 62.5 lbs so each column of three feet weighs about 180 lbs. Am I to understand that rain/ground water has enough pressure to overcome this at the three foot depth?

What am I missing?
 
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There is little doubt that water under your pond liner will cause a floating liner that some call a bubble or whale. You can go to Youtube and search on pond line balloon or digging a well point and see the effect of water under the liner.

You can do an experiment to prove to yourself that this happens by putting a plastive bag in a container, attaching it to the edge of the container and then filling the bag up so that the level of water in the bag is at the top of the container. Then using a hose or aquarium pipe start adding water between the bag and the container side. You will see the water in the bag will begin to overflow as the water level in the container rises. The bag will compress and if you continue the bag will almost empty. You can put some rocks in the bag and repeat the experiment and the rocks, if they are heavy enough will keep the bag on the bottom and the sides of the bag will compress. This is a simple exercise that demonstrates what happens to the liner when water gets under it with no way to escape. The Youtube videos show several examples that show that this can be a real problem. They also show several after the event solutions. I believe it is way easier to put the drain under the liner to begin with if you even think you may have this issue. This only works if you can place the outlet to the drain BELOW the water level of the pond. Otherwise you need a sump pit and pump to pull the water out.

I hope this helps.
 
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Thank you Dan, I'm not trying to be controversial and certainly can't argue with science, I'm just trying to reconcile this in my mind. I understand the concept in your illustration, but in a container the "ground water" outside the bag is constrained by the container walls and with just ambient pressure the water can't overcome the static pressure of the water within the bag. In the earth there is the potential for lateral movement. Granted, an empty liner in a hole would be the path of least resistance hence the bubble on the bottom, but once the pond is full of water the forces should cancel each other out. I'm trying to figure out why thousands of full, established ponds haven't been pushed out of the ground based on the premise we're discussing.
I did notice that most of the bubbles in the video's were at the upper areas of the sides. The video that made the most impact to me and I would think be the easiest to accomplish in an established pond was the dry well approach.
Please don't consider this an affront, I'm just a thick-headed old duffer that has to think things over and over until the "light" comes on over my head - it'll come to me, it just may take some time....... :)
 
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I'm of the opinion that you are overthinking this. Have you hit water at 3 feet?
No I have not hit water. I have clay soil which tends to Hold water and the local pond shop recommended that I put perforated pipe under the liner to allow air to escape.
 
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After speaking to a local pond store they said that the purpose of this is to vent air from under the liner.That would make sense with my drawing because obviously water is not going to push out above ground level. I guess there are circumstances were air pockets can Occur in clay soil under the liner and cause issues.The local pond shop has installed many ponds in the area and they had several floating liners This season due to lots of Rainfall. I guess is better to take a precaution and put the pipe in because it's not going to bother anything even if it doesn't work. It only cost $16 for the pipe too.
 

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