Question about structural integrity of concrete blocks and mortar

justplaindonJune 19, 2010

My present plan is to build a rectangular pond with a liner that is partly below ground level and partly above ground level. The pond will be made of concrete block (8" x 8" x 16") and mortar. The pond will be approximately 8 feet wide and 24 feet long.

Due to the location I think that I may only be able to go about 20-24 inches deep. As such I expect to have 16 inches of pond wall below ground (about 2 concrete blocks) and about 24 inches of pond wall above ground (about 3 concrete blocks). My concern is the sheer weight of water on the 24 inches above ground that pushes outward. Will it need some type of extra reinforcement to contain this amount of water and prevent the mortar joints of the pond wall from cracking/breaking?

If I can manage to get 3 concrete blocks below ground then there would be 16 inches of pond wall (2 blocks) above ground. Would this be able to hold this weight of water or would it also need reinforcement? In both cases this is a LOT of water.

I would prefer to not fill the hollow concrete blocks with concrete and rebar ($$$$) to reinforce them, but will if anyone has experience in this area with the materials. If anyone has any suggestions for a lower cost means to reinforce the wall IÂd love to hear it. NOTE: For aesthetic reasons due to location, between 16" to 24" must be above ground.

Your thoughts on this are welcome.



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ccoombs1(7B SC)

Concrete blocks are very strong against downward pressure. mortor is's only purpose to keep the blocks together. The kind of forces you are looking at in pond construction are not downward, but outward. And the force will be very great. There really is no way to use concrete blocks safely without filling them with concrete and rebar. But have you thought about using landscape blocks above ground? They have a small lip on the back edge that is designed to lock against the block below it. They are designed to be used in retaining walls and would be very strong. They dry-stack together so it's pretty easy to build with them. Cost wise....I could not tell you which is cheaper.

    Bookmark   June 19, 2010 at 12:27PM
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Thanks for the follow up. This is pretty much what I figured, but didn't know for sure. Since I already have the concrete blocks I'm going to go on as planned with the blocks along with some concrete and rebar. It won't be cheap, but it should last forever.

Thanks again,

    Bookmark   June 19, 2010 at 1:15PM
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My pond is 18" above ground and 18" below and I'm using the interlocking concrete blocks with no cement. I have noticed the long straight side has bowed in the middle about 3" in 5 years. Here is a picture

    Bookmark   June 19, 2010 at 4:40PM
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Our pond is on a slope and the back wall is curved- 30 feet long and up to 40 inches tall.

We built a footer and did concrete blocks, vertical rebar and horizontal rebar.

We filled all the blocks with concrete.

For good measure, we did concrete block pilasters behind the wall. That was probably overkill.

We back filled dirt against the bottom of the wall on the pond side before we placed the liner.

The deepest part of our pond is about 4.5 feet in the middle.
My dad is a structural engineer and helped us design it.

Here is a pic of while we were constructing the wall. It took us a whole summer of evening and weekends.At the time of the photo, we had not back filled the wall yet.

Here is it after we finished.
Unfortunately, this photo does not show all of the back area.

    Bookmark   June 19, 2010 at 5:02PM
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WOW! Great pics everyone! I love all the great advice people are willing to share here...I think it's better than most the "pond" builders that want you to hire them!

    Bookmark   June 19, 2010 at 5:32PM
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I'll second the WOW!! To be honest I hadn't thought of adding horizontal rebar. I guess I figured that the combo of vertical rebar and filling the blocks with concrete would make a rigid enough structure. The combo of horizontal and vertical rebar seems like the best way to go.

Thanks to everyone for their help,

    Bookmark   June 19, 2010 at 6:09PM
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My top pond is cinder blocks, concrete, and rebar. Fairly cheap. BUT I've got to say that I wouldn't do it that way again. I folded the liner where the water level is and did not realize that I would be able to see the liner. If I had it to do over again I would have done it differently- preferably would have found another way to water proof it.

    Bookmark   June 19, 2010 at 8:02PM
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ccoombs1(7B SC)

You won't need horizontal rebar at every course. I'd do the first course that is above ground level and another on the top. And you don't have to buy the blocks that are already notched out. Get a masonry blade for your circular saw and make 6 cuts in the block, so you can create a trough for the rebar to sit in. Once the cuts are made, just tap it with a hammer and the chuck falls out, leaving a nice grove in the top of the blocks. I have horizontal rebar in about every 3rd course and at the top, and vertical rebar and concrete in every hole. It would take a tank to knock this thing down!

    Bookmark   June 19, 2010 at 8:09PM
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Man, I love this forum!

ccoombs1, it looks like some blocks have mortar and some don't. Since I would be filling up the inside with concrete and using rebar, do you think I could get away without using mortar IF I coat the block with something like stucco that will prevent water from getting in any cracks (to prevent water from freezing and crackng seams in the winter)?

Thanks to everyone,

Here is a link that might be useful: Currently in bloom

    Bookmark   June 21, 2010 at 1:26PM
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ccoombs1(7B SC)

None of them have any mortar. Mortar adds no strength against outward forces anyway, so it's not needed. I just dry stacked them and filled them all with concrete.

    Bookmark   June 22, 2010 at 6:47AM
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pat_c(5/ N W OHIO)

Kalevi - That's exactly what I want to do in my new house with pavers! I love it! I'm moving from a large yard with 2, 1000 gal ponds and assorted other water features to a much smaller yard. I certainly still will want a pond and plan one like yours. Thinking about 18" below ground and 12" to 18" above. Same shape because I will back it up to a garage on the long side. A couple of questions:
Does the paverwork go all the way down below ground level or can I dig down and line the below ground portion and start the paverwork at ground level? What would be stronger?
ANyone else who has done something like this, feel free to join the discussion!

    Bookmark   June 24, 2010 at 3:11PM
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I left a plant shelf at grade level. I dug a small trench that I filled with stone dust for the blocks to sit on so the blocks are at grade level. Then I had about a 1'wide plant shelf sticking out after which I dug down to 18' deep. I laid all of the blocks except the top row, then put in the liner and laid the top row on and then trimmed. My pond came out to about 1100 gallons. It is a 7' radius half-circle.

    Bookmark   June 24, 2010 at 5:56PM
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pat_c(5/ N W OHIO)

Terrific! Love the plant shelf idea too. How deep did you dig for the crushed stone base? You're colder than I am. Any problem with heaving pavers with the rock base?

    Bookmark   June 24, 2010 at 7:41PM
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Pat: The crushed stone go about 4" deep and 1" wider than the blocks on both sides. No heaving. The pond does not freeze all the way down so I can winter my fish outside and the plants which I sink into the deep part for winter.

    Bookmark   June 25, 2010 at 6:04PM
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pat_c(5/ N W OHIO)

Perfect! Thanks so much for the info. I have been scouring sites to find the right design and materials to do just what you did. Glad to know it doesn't freeze solid tho' @ 30" it shouldn't here either. As much as I am going to miss my 15 year old ponds here, I am excited about a new start, especially with your info. Thanks, again.

    Bookmark   June 26, 2010 at 1:13PM
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This works OK for a water height of 18" above grade. I would go with some reinforcement if you want to go much higher with the water level above grade.

    Bookmark   June 30, 2010 at 5:18PM
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I am building a swimming pool in the beach. I will use a solar pump to bring the water from the ocean. I was going to use 8" blocks and make it about 4 to 5 feet deep. With the water pressure inside and the sand and water pressure outside, would rebar vertical and horizontal and filling the blocks with concrete be good enough

    Bookmark   August 31, 2013 at 4:33PM
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