Accidentally building a 12' diameter pond

[Brandyn]

Hi, y'all. I'm following advice from another thread: posting my project to solicit feedback and guidance. I didn't mean to be nose-deep in this project, but my step-father brought a backhoe over at 9:30 one night to help redistribute a few inches from one side of the existing, small koi pond to the other side, where the soil had eroded below the water level. I blinked, and suddenly there was a 12' diameter, 6' deep hole in my yard.

I rolled with it.

The old liner obviously no longer fits. I was not prepared for this situation financially, so I'm diligently keeping the money cost minimal -- without taking the kind of shortcuts I'll regret later. A local pool store gave me a brand new pool liner pulled out of a new pool that didn't fit, and they will call me later this month when they have another one.

The liner is about 15'x20'. I do not expect it will cover the entire surface area, but should get very close. I have pvc adhesive to join it together with the second piece once that's available. The liner is 27 mil. I'm removing rocks by hand, and adding sand prior to putting the liner down.

I'm building two circles, planning for a future pond renovation. The second circle (partial retaining wall tangential in photo) will have raised vegetable garden beds around the sides for now, which eventually may form a step/sitting area underwater. One pond would be the filtration plant pond at that point, while the other becomes a hot tub.

My step-father is coming back with the backhoe this weekend. I'm taking steps to plan more carefully and direct his contributions more effectively this time. There is a small hole in the retaining wall where a 3/4" pvc drain pipe could be placed, but I'll need to dig up the dirt to place it there, then put the dirt back, water it, and pack it with the backhoe to settle. I have a ball valve for that size pipe. Eventually perhaps a pump would be connected there to move water from the swimming area to the filtration area.

I've never done anything like this. Your insight and encouragement is greatly appreciated.

 

[brokensword]

Hi, y'all. I'm following advice from another thread: posting my project to solicit feedback and guidance. I didn't mean to be nose-deep in this project, but my step-father brought a backhoe over at 9:30 one night to help redistribute a few inches from one side of the existing, small koi pond to the other side, where the soil had eroded below the water level. I blinked, and suddenly there was a 12' diameter, 6' deep hole in my yard.

oohhhhh, @addy1 has a sista she didn't know she had!!!

 

[j.w]

@Brandyn

 

[poconojoe]

I don't mean to rain on your parade, but I'm a bit concerned on the strength of those stacked landscaping wall blocks. They need to be functionally strong, not just look good.
I might be paranoid, but there will be a lot of pressure pushing out from all that water.

Most will build with concrete blocks that have voids where you can hammer rebar down through them and then fill those void with concrete. You need the rebar to anchor it to the ground.

Maybe you have done something similar that is not visible in the pictures.
Maybe I'm being paranoid....
Hopefully the "experts" will chime in.

 

[brokensword]

I don't mean to rain on your parade, but I'm a bit concerned on the strength of those stacked landscaping wall blocks. They need to be functionally strong, not just look good.
I might be paranoid, but there will be a lot of pressure pushing out from all that water.

Most will build with concrete blocks that have voids where you can hammer rebar down through them and then fill those void with concrete. You need the rebar to anchor it to the ground.

Maybe you have done something similar that is not visible in the pictures.
Maybe I'm being paranoid....
Hopefully the "experts" will chime in.

totally agree with this wall strength assessment; just didn't want to be the first bearer of bad news! Though, if you have a couple feet of dirt between liner and landscape block, it should be okay; the top row won't get much push if that's the only one exposed.

 

[poconojoe]

totally agree with this wall strength assessment; just didn't want to be the first bearer of bad news! Though, if you have a couple feet of dirt between liner and landscape block, it should be okay; the top row won't get much push if that's the only one exposed.

Yeah, I feel bad for mentioning it, but it would be horrible if it gave way sometime in the near future.

I guess it depends on how much water it has to hold back.

 

[bagsmom]

Pond building is so much fun! I am excited to see the progress on your project! One thing I noticed in your description was that you are using 27 mil PVC pool liner. I don't know the life span of PVC, but I suspect it is shorter than the 45 mil EPDM. Having said that, I totally understand trying to do a big project on a somewhat minimal budget. (It's why my backyard excavation has sat empty since August of 2019! I started digging before I had the money all saved up, then we had a variety of setbacks.) So -- if the EPDM liner is not an option, I would be sure to take every precaution you can to give the PVC the longest life possible. I wonder if some of the other ponders in the group have some suggestions for that?
One final note: if I had a backhoe and hadn't been digging by hand, I can't imagine how big the pond hole would have been. It's easy to get carried away!!!

 

[combatwombat]

So -- if the EPDM liner is not an option, I would be sure to take every precaution you can to give the PVC the longest life possible.

If I wanted to last as long as possible, I would do everything I could to prevent it from contacting soil or being exposed to sun. So, a good underlayment and then an overlayment. I would hesitate to really rock the pond at all as I would not expect the liner to last long, but I would cover the edges above water to keep them out of the sun.

In any case, it should last longer than it would in a pool (about 5-10 years) as you won't be attacking it with the harsh chemicals that you would in a pool.

 

[Brandyn]

Thanks, everyone!

@poconojoe, I do appreciate your raising the question of structural integrity. If I didn't already have a plan in mind, you would have just helped me avoid a major, devastating disaster.

My current plan is to build a raised garden bed along the exterior side of the wall. Digging that out with the backhoe to ensure a good, deep foundation for the mortared stone wall. It goes about halfway up the height of the wall you see here, and will be a natural stone wall with Type S mortar (1750 psi tensile strength). I'm hopeful that this plus 24" of soil between the walls will be sufficient to support the roughly 33,300 lbs of water in the pond. The retaining wall would only have about 4-5 blocks exposed toward the top; all the lower ones would be buried behind the garden beds.

Also I'm trying to get a piece of natural stone slab long enough to run across the retaining wall blocks in the corner like a hypotenuse distributing the weight between the sides more consistently.

But if anyone knows something about structural engineering and wants to tell me my plan won't work, I'm definitely listening!

@combatwombat, thanks for that! 5-10 years does sound tempting in an "I'll replace the liner when I can afford to" kind of way. I'm considering just springing for a 45 mil EPDM liner now, to the tune of about $1000, but I just can't bring myself to put it on the credit card until I know whether I'm getting the job I've applied for. Maybe I'll be called in for an interview Monday and get to celebrate new employment with a fancy 45 mil pond liner on top of the pool liner!

 

[Phaewryn]

Thanks, everyone!

@poconojoe, I do appreciate your raising the question of structural integrity.

But if anyone knows something about structural engineering and wants to tell me my plan won't work, I'm definitely listening!

Retaining walls need to slope at a minimum rate of one inch for every foot of rise. Also at least one row of blocks should start below grade (preferably two below grade). Do those have the lip on the back side to hook over the block below it? Generally speaking those landscaping blocks are only rated to stack 4 high maximum in retaining wall applications. The ones with pins that drive through them can go higher (if the wall is otherwise engineered correctly), but not the ones that just have the lip on the back, and if yours have no lip, then definitely not - those are for landscaping only - not intended for retaining walls at all.
You can learn a lot by watching this guy's channel: https://www.youtube.com/watch?v=f9ziFv6Q-6k
He also is using the correct type of block for taller retaining wall applications so you can see what it looks like. He goes into using geogrid and why you need it to build taller walls, and how it's used, and when you get to that point, you will understand immediately why your wall will not work. You can't install geogrid into water behind the wall, it has to go into soil. You might be able to get some into soil if you plan to bury the front of the wall with soil, but it won't work for a garden as you won't be able to till over it for fear of snagging it and ripping it up. Grass should be fine. But you appear to have the entirely wrong type of block.

 

[Brandyn]

Retaining walls need to slope at a minimum rate of one inch for every foot of rise. Also at least one row of blocks should start below grade (preferably two below grade). Do those have the lip on the back side to hook over the block below it? Generally speaking those landscaping blocks are only rated to stack 4 high maximum in retaining wall applications. The ones with pins that drive through them can go higher (if the wall is otherwise engineered correctly), but not the ones that just have the lip on the back, and if yours have no lip, then definitely not - those are for landscaping only - not intended for retaining walls at all.
You can learn a lot by watching this guy's channel:

He also is using the correct type of block for taller retaining wall applications so you can see what it looks like. He goes into using geogrid and why you need it to build taller walls, and how it's used, and when you get to that point, you will understand immediately why your wall will not work. You can't install geogrid into water behind the wall, it has to go into soil. You might be able to get some into soil if you plan to bury the front of the wall with soil, but it won't work for a garden as you won't be able to till over it for fear of snagging it and ripping it up. Grass should be fine. But you appear to have the entirely wrong type of block.

Well poop. Thank you.

They do have the lip in back. I suppose when the backhoe arrives tomorrow (I hope, since my stepfather never showed up today as planned) we can dig all the soil back out from the wall and install a support system, then re-bury it.

 

[Phaewryn]

Well poop. Thank you.

They do have the lip in back. I suppose when the backhoe arrives tomorrow (I hope, since my stepfather never showed up today as planned) we can dig all the soil back out from the wall and install a support system, then re-bury it.

If it's the type with lips, then I don't think it will work with geogrid, and you're just stuck to only building 4 high. I would go with doing that. Just reduce your height to the height the block manufactured recommends (I'm assuming it's 4 because those look like the kind sold at Home Depot and I'm pretty sure those say 4 high on the label). Call them and ask. Each block manufacturer has their own recommendation for max height for each type of block they make. Your pond depth can go deeper by digging out the lawn below, you don't need to make a tall wall to have a deep pond.

 

[Brandyn]

The pond depth isn't the issue affecting wall height; the slope of the yard is. The wall must be the height it is, or the water level will pour out from the lower end of the pond -- which is what started this whole mess. The current pond depth just happens to be as deep as the retaining wall, but the wall cannot be made shorter.

That is why I planned to build around the outside of the retaining wall so it is fully supported with soil on both sides, plus a stronger masoned stone wall for stability, so the above-soil height of the wall is only 5 blocks. I could probably change that to 4 though.

 

[Phaewryn]

The pond depth isn't the issue affecting wall height; the slope of the yard is. The wall must be the height it is, or the water level will pour out from the lower end of the pond -- which is what started this whole mess. The current pond depth just happens to be as deep as the retaining wall, but the wall cannot be made shorter.

That is why I planned to build around the outside of the retaining wall so it is fully supported with soil on both sides, plus a stronger masoned stone wall for stability, so the above-soil height of the wall is only 5 blocks. I could probably change that to 4 though.

Ah, it isn't clear from the photos that the yard has a slope.

 

[Brandyn]

Sorry. I'll take some new photos and upload them soon showing the outside of the wall.