Is it safe to use concrete in a koi pond?


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We are digging a 3 ft. deep pond with an 18 inch shelf around it. We plan to slope or tilt the sides back so we can stack rocks up the sides on the shelf. Should we use concrete to secure the rocks (if so what kind of crete would be safe for koi?) or buy spray foam to secure them? We are using a flexible EPDM liner.
 

Julia

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Pond liners are not easily accessible here so me and my friends, equally fish and pond enthusiasts, have been using concrete. Most of my kois are now almost 5 years old. The only complaint we have are leaks. So when I built a new pond I had the surface plastered with epoxy coat. So far so good.
 
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Mortar is used to secure rocks. Concrete has pebbles that make laying rock difficult. Cement products are safe for pond use. Some people with expensive Koi prefer smooth surfaces so the fish can't cut themselves.
Thanks for your advice. We are really worried about the sides not holding and falling in. Hate to have another expense but I think we are going to pour a cement footing on the 18 inch shelf and use blocks with cement and rebar just to make sure our sides do not collapse.
 
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Structural stability is a separate issue. If you like you can post your plans and people can make suggestions. I have a fair amount of experience mortaring rock over liner.
 
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I have posted a few pictures of the construction. The side boards are holding back packed fill dirt. We were planning on cutting back into the that fill dirt about 6 inches or until we didn't have dirt caving in. Now we are thinking about blocking the sides but then we would have trouble stacking rocks on the collar and coming up 18 inches. Whats your thoughts on securing the sides so that they do not collapse? Just don't want to do all this work to have the sides to cave in later.
 
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You have lots of choices and all of them relate to risk of collapse. And there are different kinds of collapse.

There's the basic big section of a wall slumping down behind a liner. Sometimes severe enough to lower the top allowing water out and behind the liner and making the problem even worst. Sometimes just a bulge.

Another type is sharp soil edges eroding over time. The rock in Fig B can end up sliding into the bottom of the pond.



The simplest risk reducer is a single block color with the blocks filled with soil. The walls are sloped, kind of pre-eroded. For less risk bond beam blocks can be used with horizontal rebar and filled with concrete. Another option is to pour the collar using rebar and forms and no blocks. Bond beam blocks basically act as forms so they're less work. Also cheaper for DIY builders. Pros can reuse forms over and over so their cost is cheaper for them.


Another option is no collar or retaining wall behind the liner. Collapse risk is highest with A, but still better than just soil and liner. By sloping the wall in B the soil is less likely to collapse and you end up with a collar on the inside of the liner. Because of the center of gravity the mortared rock has it wants to fall back, against the soil. So although the end result is visually a vertical wall there's little chance of it falling over into the pond like A might. In C the blue is some reinforcement mesh and the mortar is extended over the liner top and down into the soil. That can provide a little more strength, but probably not needed.

Notice in all cases the vertical rocks rest on top of a foundation rock, or concrete can be used.

Also the rock doesn't have to be vertical, it could just be mortared against the liner so it was sloped like the liner is. The center of gravity is still the same, pushing against the liner. Purely whether you want a vertical or sloped look.


Here's a block retaining wall with a mortared rock veneer over the liner. There is still a soil shelf with a sharp edge that will probably erode over time but there's no rocks or plants on the shelf to end up at the pond's bottom. The shelf is wide enough that hopefully the erosion is never bad enough to under cut the blocks.

This shows the rocks mortared long wise, like tile on a wall. The rock could also lay flat and be stacked. Just depends on the look you want. Notice there is concrete at the base of the rock as a foundation.



A full retaining wall system can be used to virtually eliminate collapse risk, but of course at a higher cost. The red is reinforcement mesh.
 
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I'm hoping Waterbug is still here to help me understand these diagrams, or anyone else who can help!

I'm planning my first large (ish) pond, and I am researching all my options. I want a cement base to keep rodents out and also to have a sturdy footing. Over that I will put a flexible liner, and then a stone and cement collar around the perimeter. I also want a shallow shelf just inside the pond with flat stones stacked and mortared in place so it appears that the stone continues into the pond (to camouflage the pond liner.)

I understand the first few diagrams above, and I hope my cement base will make that erosion a non-issue. I understand the next diagram, with the stones mortared in place like tile, but I don't know what the blue line is.

The last couple of diagrams....are those cinder blocks outside the pond? I'm just not clear on all the steps and details of the drawing. Why is there a little step in the 2nd one? What is below the pond?

Can you direct me to a more detailed description of my options?

THANKS to anyone who can help!

Ann
 
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Just as a mention, your liner will eventually have some growth on it and won't appear that noticeable as it matures. Plus the black color of it also helps. My pond is a dug hole with underlayment and liner. No concrete, glue or any adhesive. My shelves are pitched a little bit toward the outside to prevent any stones from falling in. I use only large stones as flat and large as I can find. All the stones are natural and only smooth ones with rounded surfaces. No sharp edges what so ever get near the pond. I stack the stones in a brick like manner so they are interlocked. They are stacked on top of each other in a stepping manner, stepping away from the pond. It takes several builds to get it right. The same with the waterfall. I might build it one way, then make some corrections or completely redo it. When stacking stones, sometimes it takes several different stones to make tight fits to the previous ones. Turning and flipping or trying different ones until they fit tight enough. It's basically a puzzle. Rarely does anything fall into the pond. I have had a few over the years, but since nothing has sharp edges, no damage to the liner has occurred. They can usually be netted out. Or if you really have to, you can just go in and get them! It's only water. But the liner can be slippery, so watch out!
 
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Welcome to the GPF @Dogcatlady ! I think you might have better luck getting answers to your questions if you start a new post - this one is very old and waterbug hasn't been around for a while.

I can try to answer a few of your questions though. For example:
but I don't know what the blue line is.
Waterbug explains the blue line is some kind of reinforcing mesh. I don't know what or why, but that's what he included in his explanation.

I haven't seen many ponds built with cement that is lined with EPDM (pond liner) but one concern I would have is if you get water between the liner and the cement you could end up with a floating liner. Your pond bottom will be stable just by the sheer force of the water bearing down on it. And if rodents are a concern (under the pond? seems unlikely, but maybe. Chewing through the sides, yes. That can happen.) you can line it with a product that is basically a mesh screen. Some people call it gopher shield or rodent mesh. Then you put down your underlayment, then your EPDM liner and the rodents won't be able to chew through. Cement is a whole other ball game.

I also am not a fan of mortared rocks. If you take your time and dry stack them properly there's really no need to use mortar. A well stacked pond wall won't move. I think waterbug is describing a pond that can be built to be secure in an area where the ground may not be stable or perhaps a pond that exceeds the normal depth of 3 or 4 feet where stability of the walls may become an issue. Most ponds do not require reinforcement or concrete blocks.

As far as hiding the liner - concealing the edge that is above water level is important and there are several ways to do that. Are you planning a more formal pond? Then your cement collar and coping stones make sense.

Again - start a new post, tell us what you're working with in terms of space, experience, equipment, etc., what kind of pond you hope to end up with - goldfish? koi? plants or no? - and we'll chime in! This is the time of year when most of us are champing at the bit to talk ponds as we wait for spring!
 
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Unless you have sandy soil you shouldn't need any cement. Just be sure that your shelf is wide . Mine is about 12" wide and has a row of stone weighing several hundred pounds, maybe 500 or so pounds on it. Don't forget that a shelf is nothing more than the ground so if a rock doesn't collapse the ground it won't collapse a shelf . Here is a picture on Youtube
 

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