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We are looking to build a small pond (9'x18'x6'), but we want the edge to be raised up about 8". I am thinking about using CMUs (Concrete Masonry Units) which are 8x8x16. The entire pond will be lined with an EPDM pond liner.

My question is that since some of the water will be pushing up next to the blocks, how do I figure out to engineer this so that the blocks don't push out?

  • Concrete footing?
  • Rebar in some of the blocks?
  • Rebar in all of the blocks?
  • Fill the openings of each block with concrete?

So, I have a pretty good idea of how to over engineer this, but I don't want to waste time and money though either.

If you don't have any concrete answers, but if you know where I can find this information, that would be great too.

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up vote 3 down vote accepted

Unbelievable good math for a block wall, but I think Jim is only going to have one block on the ground at the top of the pond. The liner itself is very strong, and once laid up and over the block, that block is not going anywhere. Not only would 6 inches of water have to move the block, but stretch the liner the same distance as moving the block. The most i would do is drive a few 3 foot rebar rods in the holes of the blocks and fill the holes with dirt or concrete. We have build 100 foot by 150 foot, 8 inch deep deep skating rinks out of standard concrete blocks with absolutely no extra support, just a little backfill behind the block, laid in the liner, the fire dept filled it up and nothing moved.

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Good answer. Experience trumps a calculator just about any day in my book! :) – Doresoom Apr 21 '11 at 13:44
Ya, but I love a calculator too. just this one is way too simple. – shirlock homes Apr 21 '11 at 23:41
While Doresome's answer is probably more technically complete (glad he provided some translation), I really appreciate your "real world" experience in the matter. Thanks. – Jim Apr 22 '11 at 13:01

This website has a good explanation of hydrostatic forces on a dam surface. You could build your wall with a submerged lip to help counteract the moment applied by the water pressure on the vertical face of the wall.

enter image description here

Image from TheConstructor.org

I did some quick calculations and got the following: enter image description here

So to sum up my incoherent scribblings, assuming a 30 lb cinder block holding back 8" of water and a coefficient of friction between the ground and the blocks of 0.4 (which I think is conservatively low), the blocks will not tip, but they will be pushed backwards by the hydrostatic force.

Reducing the water level to 6" results in the wall being able to withstand the hydrostatic force by a small margin. Even so, you'd be risking high water conditions after a heavy rainfall destroying your wall. It will need reinforcement. You could do that by pouring concrete into the voids and increasing the weight of each block to greater than 46 lbs. You could also drive a rebar stake on the inside of each block against the side holding back the water.

NOTE: I did quick calculations and made a lot of assumptions. This answer is an estimate and should not be considered professional advice. (Translation: Please don't get pissed off at me and track me down if your pond floods your yard.)

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that is a very detailed site. I will definitely look at that one. But, you would mind elaborating a little on what you mean by a "submerged lip"? I did not see that mentioned on that web site (at least not those words). Thanks. – Jim Apr 20 '11 at 19:18
@Jim: Figure 2 shows a sloped upstream dam surface with a stabilizing downward force from the weight of the water column above it. I didn't read your question well enough on my first pass - I was thinking of a stacked wall of several shorter blocks. Making the bottom row protrude halfway into the pond side and using a half block on the outside would have a similar effect as in the drawing, as long as mortar was used to join all the blocks together. – Doresoom Apr 20 '11 at 19:43
Yes, I am hoping that I don't have to build anything near as complicated as in the figure above since by the time I get to grade the wall should only have to hold back 6-7" of water. – Jim Apr 20 '11 at 19:48
@Doresome: WOW. What a wonderfully technically complete answer. I am glad you provided a "translation" since all of this is over my head. I am happy to provide with with an up-vote for answering my question, but the "answer" goes to @shirlock for having real world experience (as you have already noted). – Jim Apr 22 '11 at 12:59

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