Pond Boss Forums POND CONSTRUCTION Building a dam New Cinder Block pond

Pond built of 8 by 8 by 16 cinder blocks with mortar. Inside dimensions are 11.3 ft by 4.6 ft. and the part that the water will hold is about 30 inches high. So it will hold around 1000 gallons of water. I have rebar in each corner block and one half way along the long walls that are filled with concrete. My question is will this hold the water once the underlayment and rpe liner is installed. If not, do all the cinderblocks need to be filled with concrete? I have about a third of them filled in one long wall so far and could do the same on the other long wall. I have the center filled in the short wall. Would filling with dirt achieve anything or does it need to be concrete?

I know the safest way is to fill all hollows with concrete but money is getting low, so I was hoping someone knew the math and strength of blocks with mortar. Right now the long wall contains 9 full blocks plus end block making 19 holes of which 6 are filled with concrete. Was planning on doing at least that on the other long side and on the 4 block side, the corners and middle block is filled.

Liner will definately make it hold water..

I can't answer your question, but I remember a swimming pool my Dad built out of plywood and 2x6's. It looked good (lined) and he started filling it. There was a loud noise in the middle of the night, and it had let go. It broke the window in our and our neighbors basement, and both basements had a bunch of water in them....


He bought a metal pool with a vinyl liner after all the mess was cleaned up.

The liner will deal with waterproof, but the weight of the water concerns me. I know there is a formula for calculating the weight of the water but don't know how to know if the mortared cinder blocks will hold the weight.

I don't know how to calculate it either. Water is approx. 8.3#/gallon, and it exerts approx .5 psi for every foot of depth on the bottom of the pond. I don't know how to calc the lateral forces off the top of my head.

There are some pondboss members that work with concrete/block for a living, at least that's what they have said. Perhaps if you change the title to asking help with concrete, they "might" be more likely to see your need and help.

Esshup, was Fatih a concrete engineer?

I would feel a whole lot better if the blocks were filled with concrete. They're not designed for lateral loading, at least not in the way you describe. If you were to backfill up against them it would be another story, but freestanding, just relying on the mortar joint to hold? Sounds risky.

Twelve inch blocks ( 8x12x16) would've worked better for your application, but I understand the need to work with what's available, and stay within budget.

Not quite, an architect.

I am not a concrete or dam expert but I do have an engineering opinion ...I believe the problem to solve first is to calculate the Hydrostatic Force on the cinder block walls applied by the water. Hydrostatic Force formulas for a dam could be applied here (I believe). Once those forces are calculated; determining if the wall will hold up could then be determined.

Seems like a hard calculation to make and I think beyond my capabilities from looking at the formulas. I do think that is what I need.

the deeper the water the exponentially higher the outward force is on the walls. google pressure-depth calculator to get an idea of the psi you need to account for.

this is more of a decorative pond/fountain than what most people on this forum are familiar withas a "pond". You may find more information for this type of build on a koi pond forum. I would call this a fish tank.

personally, i would drive rebar into the ground through the cinder block holes and and then protect it by pouring cement into the holes.

You can seal it with a fish safe epoxy or pond liner.. or I used "super theroseal" on my 700 gallon ferrocement tank and i like the results so far.

Head pressure is about .4lb per ft up and down not on the sides as much that's why large oil tanks are very thin on the sides . Remember that that's pound per sq inch on bottom no sure pressure on sided but I think it's less because of gravity is down


Pat