Planted pond - clay, ferro-cement, or both? & brainstorming first pond

[makuck]

I have a 10' deep pond (10x10x10 approx.) under a 2-pane cold-frame greenhouse (RIGA V "The Onion").

Temperatures here range from -40 to 100 degrees, with a frost depth of 4ft, though in the greenhouse frost-depth is more like 1-2ft.

I want to do a planted pond in the greenhouse. I am still at the stage where I could dig it deeper than 10' if needed.

My ideas for the bottom are to use a mix of my sandy loam soil with 100% clay unscented kitty litter (cheap). Make the clay bottom deep (is a foot deep enough?), and that way I can puncture plants into the soil, and they can get bigger that way and just easier to grow.

Or is my pond too small to use clay at all? Should I just suck it up and do the whole thing in ferro-cement and put my aquatic soil-bed on top of that?

I have the option of coating the inner walls with pond-guard epoxy paint. I am assuming the walls don't really need a coating though. Does ferro-cement hold up okay to freezing and thawing?

I could even do pond-guard coated 3/4" plywood, but then I have to go with a geometric shape and I have a feeling that wont hold up as well as cement.

All these materials are on-hand for me and cost isn't much of an issue.

The pond will be surrounded by plants, mostly high-yield annuals.

So what do you guys think is clay more trouble than it's worth in a small pond? Anyone have advice in keeping ferro-cement in best shape through freezing temps?

Thanks in advance for any advice. Any and all thoughts good and bad appreciated.

 

[MitchM]

Welcome!
How will you maintain plant growth 10 feet down?
If you find a plant that grows successfully, and are able to get sufficient light that deep, the plants will need to be pruned at some point.
What are you going to do for water circulation?
Can't help you with the cement.

 

[Lisak1]

Deeper than 10 feet? Why? What do you plan to grow?

 

[Waterbug]

I assume ferrocement would be fine if done correctly, but I've never heard of it being used for a pond. It isn't a very forgiving technique, so hopefully whoever does it would have that skill. And of course today there are so many recipes it's hard to guess, but assuming whoever formulates the mix knows what they're doing.

I personally don't care too much for concrete pond structures because of cost and risk.For any coating to work the structure has to be very stable. Coatings can't generally span cracks, or at least cracks increase risk of failure. There can also be problems with the coating staying attached which isn't always the coating adhering but the top layer of cement delaminating.

I like a liner and then concrete/mortar/mortared rock/stucco over the liner. For ponds primarily for plants, meaning no pots and I'm going to be digging every year, I pour 2-3" concrete bottom. The sides get a thinner stucco or mortared rocks. It's the bottom that I see at most risk from digging and me walking around.

For 10' deep you would of course need a concrete wall to hold back the soil. Because so much concrete and steel would be needed I can see why you might want to skip the liner. In this case I suggest building a swimming pool type structure, that's how cement type ponds are built with a few changes for bottom drain system and such. You'd be taking advantage of swimming pool construction knowledge which is vast. Lots of pond DIYers like to think they can invent a cheaper, less material, solution and sure that may be possible, but pretty difficult. If swimming pool builders could shave even 5% off materials they would. I've read lots of posts in different pond forums about people thinking a little cement/concrete should work, and if not they could always add some coating. Throwing bad money after bad money.

I was thinking of a small pond with a green house on top so the water would be a thermal mass. I'd heard of this being done with above ground water tanks...not sure it would work the same in ground.

If you're making the pond 10' deep to try and keep the water warmer I think you may be disappointed. I think (pretty sure) heat would escape the water faster than it could transfer thru the soil. Soil is a better insulator. I suggest maybe adding solar water heating in addition to the greenhouse. Getting the temp up during the day to carry you thru the night. Sizing all this is really tricky, but solar can normally be extended.

Last thing on freezing...not an issue for properly built cement/concrete. Millions of concrete pools and ponds in the world in freezing climates. Biggest risk for ferrocement, because it's thin and in ground, is the steel rusting. That would blow apart pretty fast.

Sounds like a fun project.

 

[makuck]

Great insight Waterbug, thanks, agree on all counts. That has given me more to think about.

Liners are cheap so it wouldn't make sense skipping that, especially for a pond this small, less room for error. On that note, maybe instead of ferro cement I should just use 50% Portland cement 50% quickcrete at 4-6" thickness throughout. Still wouldn't be more than a few hundred bucks. I like to keep it granola as possible, but since it is in a greenhouse want to make it a bit more formal, and potentially bring the walls of the pond above-grade. Since it will be poured against virgin dirt, outward tension should theoretically be reduced. No chance of rusting then and I can leave the inside uncoated.

Solar heating greenhouses is right up my alley. And yes part of the reason for the pond in the greenhouse is thermal mass, but I am also going to use it as nutrient solution for hydroponics. I've been brainstorming ways to make a DIY passive solar water heater to pump the water through before it hits the filter. I'm thinking just pumping the water through pvc pipes painted black in the back of the greenhouse. Would have to be fairly extensive run on pipe to effectively heat enough water that passively, but my pump is a 4200gph, enough to filter my tank every hour or so, should have plenty of lift power. Needs to have a diverter for summer days.

Thanks again for the well thought reply.

 

[Waterbug]

This is a fun project.

I think raised is an excellent idea. You can put boards across, plastic pallets maybe, to set plants so you get a bench almost for free. Above ground should help heat transfer too. Also above ground eliminates risk of the pond floating up and popping out of the ground.

I'm not clear on the below ground...10' deep, or even 7-8' if 2-3' above grade, would require a proper retaining wall. You have the greenhouse and you'll be walking around, a cave-in would be bad. Personally I'd look at how concrete swimming pools are made and copy that exactly. They have a lot of experience and methods are well document. Could save you some money even. They do some things that can appear counterintuitive, like a massive beam around the top edge while the thinnest part is the bottom. DIYer might think the top could be thinner and bottom thicker.

Liner under the concrete I don't think would be a good idea. 30'x30' liner, 900 sq ft, about 270 lbs for 45 mil. To say that's no fun to deal with is an understatement. Poly is lighter but is easy to puncture despite claims to the contrary. IMO poly is even less fun to work with. A proper retaining wall requires steel which would have to be laid over the liner. Lots of rebar, wire ties, walking on the steel...chances of the liner getting holes I think would be pretty much certain. Instead you can built a swimming pool like structure and hopefully no problems and you're done. If leaks should appear you can always lay liner inside the pond. At 10' deep you will not be adding soil and plants because no plants can deal with that depth (even submersibles). So I don't see any reason to protect the liner, though you could certainly add concrete over the liner, wouldn't have to be too thick.

Not sure why the 10' depth is important. If you had a very big solar system for heating the water then sure, good thermal mass. But being in the ground isn't helping. I think above ground tanks against the north wall is a better option. Years ago on "This Old House" I think, they showed a greenhouse in some cold climate in winter with a couple above ground tanks and they said that carried them thru the nights. And/or, 3' above and 1-2 feet below. Sometimes when I design stuff I get some idea in my head, assume I "know", and later wonder why I didn't think it thru a little more.

Insulation...normally the worst thing a person can do is insulate a pond from the ground. In a normal outdoor pond you want the warmth of the soil coming into the pond. Greenhouse is different. To work the water temp has to be rather high to get thru the night...like 70-80F. The ground is way, way under that and so you'd be heating soil rather than your greenhouse. So I think the below ground should have at least some insulation. Also why I think people like tanks for this kind of thing, no insulation needed.

I think painted black PVC is going to be disappointing. At -40F I'm pretty sure you're going to be chilling the water rather than heating it. You could have some complex system for temp monitoring and auto turning the system on and off, probably needed no matter. But still, daytime temps are still going to be what, -20F? Still chilling the water. When off the outside system would need some way to auto drain or it'll freeze. So probably a closed system with oil or something would make more sense. And enclosing the pipe in it's own "greenhouse" would be needed. And if going to all that the PVC will seem like less sense when a better heat transfer material could be used. Unfortunately a project like this you are up against some laws of physics. Tricky stuff. But a lot of info out there on greenhouses, thermal mass and calculating what's needed to meet your goals.

I envy you and this project. Few things better imo than being in a greenhouse in winter.