Digging a deep pond, clay soil, water filling the hole - do I need to plan for drainage below pond liner?


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Hi there,
I've started digging out a pond, aiming for 2m deep x 1.2m wide x 1.2 length - I live in a colder area and am trying to get enough depth for trout to survive winter (will be connected to an aquaponics gardern). I am planning to insert wooden posts, and wooden slats shaped a bit like a box to hold back the soil, and use a pond liner. I have started digging out, and we have clay soil - and I am noticing that water is collecting in the hole.

I am assuming this is happening at the moment because it is the lowest point in the garden, and the water is seeping in from the surrounding wet clay that I can see.
Once there is pond liner and the weight of the pond water in the hole, will this prevent the water from the surrounding soil collecting in the hole? Or do I need to plan for some sort of drainage? Because the hole will be so deep, it won't be easy to get this water to run elsewhere/to a deeper point. But I could try a french drain below, and will probably at least leave a PVC pipe into the cavity so that I can pump water out if there is a disaster....

Also any other suggestions for materials for the retaining wall apart from what is suggested above?
Thanks..
 
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addy1

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Welcome to our forum!

If enough water is seeping in you will want a pipe or something to be able to remove water, ow you can get what they call a hippo in your pond.
 
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Mmathis

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Hello and welcome. High water tables and clay soil — a unique challenge!

Is part of your pond going to be above ground? If you can keep the water level slightly above ground level (even by just a few inches), the water pressure of the pond will prevent the ground water from pushing the liner up (water hippo). We have water table issues here in the southern USA, as well as clay soil. When we dug our pond, you could literally see the water seeping into the hole as we were excavating. We had to re-dig the pond after a year as the clay was causing the walls to collapse. When we did the renovation, we made the pond edge about 12” above ground level so that the water level would be above ground level, and would push against the high water table. Worked like a charm!

Agree with @addy1, as well, regarding the underground pipe to facilitate drainage. At the time we built our pond, I hadn’t done enough research to know about this solution!

I hope this all makes sense. I’m up earlier than normal, and haven’t had my coffee yet, :sleep:

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Thanks everyone, I was literally just watching some videos on youtube that suggested having the pond water level above ground will eliminate the issue of the pond liner bubbling up - I was about to come here to ask and confirm and Mmathis has beaten me to it! Yes I was planning an above ground finish. Does this mean I can avoid sticking a vertical pipe in below the pond liner just in case? Or maybe I should do it anyway while I'm in the mud.

Does anyone have any (cheaper or easier build) alternative suggestions for building this "water proof box below the ground + a little bit above", beyond:
- dig a hole
- wooden posts + wooden slats to retain the soil where it is (although by the looks of the clay, it's not moving)
- styrofoam layer to protect the pond liner
- underlay liner
- pond liner

I've considered an old plastic rain water tank; concrete or bricks? but how to make sure the pond liner doesn't tear.... or do I stick with my original plan.
 

Mmathis

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@Mhwolog When we did the rebuild, we “installed” a collar of cinderblock around the perimeter edge. That helped us get the extra height we wanted, and it stabilized the pond edge. It prevented the clay from becoming squishy and oozy, and was a nice foundation (even though it was above ground, not below) — it wasn’t going anywhere. At one point, before we decided on the collar, we tried using boards to shore up the sides, but that didn’t work at all, at least not for us.
 
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