Best way to stabilize rocks

[Bread & Water]

I have two places within my 11 foot high waterfall which have vertical spill, on is 12 inches and one is three feet. My concern is how to secure the vertical stones that are behind the waterfall to the epdm liner. I'm considering the black spray foam or mortar or pressure-grouting. Any thoughts or experience out there?

 

[jgrandmont]

I don't know how applicable this is to your situation, but when I built my pond, I made a vertical drop of about 4 feet from the base of the waterfall to the deepest part of the pond (a water depth of 3 feet with about a foot drop to the water's surface). I believe I mortared in the base of the waterfall, but the rocks were just stacked beneath it, angling them backwards as I went. I don't believe I used any mortar in the stack, assuming that the angle would prevent them from falling forward into the pond. I probably did use some spray foam to try to lock them into position. In the picture below, the waterfall is on the far right-hand side. You can see how the rocks start further out into the pond and are angling outwards to a spot under the waterfall.

 

[Lisak1]

Your pond walls MUST be stable dry stacked. Anything else is playing with fire. The wrong foot in the wrong spot and your wall collapses. Pond foam won't hold rocks that aren't stable on their own. Even a "straight drop" needs to have a backwards lean to it. Work with your rocks to find the ones that fit together PERFECTLY and lock in place naturally. It can take a bit to learn the technique, but once you see it happen, you'll know. Sometimes you have to try multiple rocks until you find the right one.

 

[Bread & Water]

Lisak1 is absolutely correct about the need for stability. I constructed the rocks last fall, carrying rocks up and down the waterfall, stepping on the stones for footing, and they didn't budge. And the waterfall has sat dry-stacked all winter through snow and freeze/thaw cycles and the rocks haven't budged. In the attached picture you can see the one-foot and three-foot drops near the top of the waterfall. It's the vertical stones which will be behind the curtain of water that I am most concerned about.

 

[Lisak1]

Same story - don’t put rocks where they won’t stay. Don’t use vertical placement - either one bigger boulder to fill the space or slate stacked walls to fill the space. You definitely don’t want to try to adhere rocks to EPDM. One because it won’t work and two because you don’t want the weight of a stone pulling on your liner.

 

[GBBUDD]

Your pond walls MUST be stable dry stacked. Anything else is playing with fire. The wrong foot in the wrong spot and your wall collapses. Pond foam won't hold rocks that aren't stable on their own. Even a "straight drop" needs to have a backwards lean to it. Work with your rocks to find the ones that fit together PERFECTLY and lock in place naturally. It can take a bit to learn the technique, but once you see it happen, you'll know. Sometimes you have to try multiple rocks until you find the right one.

Buttttt i will disagree to a point about waterfall foam. Now it absolutely is not mortar or grout and I know in time the foam is very susceptible to falling appart due to uv. But i did use it UNDERWATER a good three years ago now maybe four where is just used it to lock in a stone wall laying flat on fabric in the pond. It does not have the strength of mortar by any means but anyone who has recieved something sensitive in the mail that was shipped in foam knows that when it's an expandable foam it does not need to stick to hold something in place. But the complete forming around the item can hold it in place.

Now mortar or grout would be my first choice After a well stacked dry wall. But to be 11 feet tall would need a very large base unless your talking it's a hill that's traverse up and you have stream drops over an 11 foot drop.

Pictures would be helpful for us to help you in the best solution.

 

[Lisak1]

Agreed - locking in place by basically filling void spaces works. Gravel can do the same thing. The guy who builds ponds in Cali - ponddigger maybe - uses pond foam in all his stacked walls for that purpose. But I would never attempt to foam a rock TO the liner and hope it would stay.

 

[GBBUDD]

Agreed I have underlayment that everything sits on the liner is hardly seen anywhere

 

[Bread & Water]

Agreed I have underlayment that everything sits on the liner is hardly seen anywhere

I've been away a couple of days, haven't lost interest. To GBBUDD's comment, my 11-foot waterfall is about a sixty degree slope with two vertical drops at the upper end, one is 14" and the other 36". I'm on board with both GBBUD and Lisak1. My current thought is to use concrete in the more horizontal streambed because it resists erosion from the moving water. I'll use mortar for the sides of the streambed and waterfall because it has strength, sticks to the rocks and its fluidity makes it easier to place in the nooks and crannies. For ease of installation, I think i'll use the spray foam behind the stones which are nearly vertical behind the vertical drops of the waterfall (they lean back a little). I think the foam sticks to the EPDM liner and the stone.
Here's a photo of the upper part of the fall showing the vertical drops. The cords from the chaser lights highlight the future water flow.

 

[GBBUDD]

The problem with tall waterfalls is.
1. the water builds up lots of energy when it has a greater drop it can splash for a large distance looks like your biggest drop is about 4 feet and my guess would be you could have splash that could extend out 12 feet or more.
2. THE WIND it can blow a stream of water way off course this can actualy empty a pond out a lot faster than you might think as the pump will keep pushing water it has to the falls but if the containment isn't excessively over a empty pond can be the result sized

 

[ADHDiy_Guy]

By the looks of your photo, you have already stacked most of the rocks up, but if you place your rocks forward a little from the liner, and backfill will gravel as you stack up, that gives you a much more stable platform for the next row of rocks. Then you backfill with gravel again up to the height of the rocks, and continue on. For a waterfall, you can bib the top with spray foam and underlayment or liner so that the water flows over the top of the spillway rocks instead of down the back through all the gravel.

Here is a YouTube link of me doing this type of technique but on a small scale to stack up round cobblestone for a planting pocket, but the concept remains.

 

[Lisak1]

Excellent demo. For larger boulders you can use larger gravel in the exact same way.

But please notice how @ADHDiy_Guy keeps moving the rocks around to find the natural fit and sometimes even abandons one rock to find another that's more suited. Th rocks have a natural way they want to "fit" - don't force it, but rather work with it and you'll have great results. Even the pros will set a rock and then pull it out when they realize it's just not the right fit. Patience and practice will give you result that are both stable and pleasing to the eye.

Good job!