Cinder Block Foundation - Clay or Crushed Rock/Sand

[Theguitarman]

I finally got my small pond dug out, and I put up a test water feature made of concrete blocks that I have been really enjoying. Will post more pictures of the final when done, but I want to make my water feature a bit more stable and permanent. I apologize if this is a bit pond-adjacent, hoping someone can still help.

I dug down 20" below grade to some pretty hard clay, and started building my stacked concrete block water feature on that. I has been sitting for a month, letting things settle. My question is, should I add a layer of crushed rock and sand, or is it more stable to lay the base layer directly on the hard clay bottom? Once constructed and level, I will add some rebar and crushed stone into the cavities of the cinder blocks to help minimize movement.

Any help would be appreciated!!

 

[GBBUDD]

FIRST QUESTION IS WHERE ARE YOU LOCATED?

 

[Theguitarman]

Great question, yes, I should have included. I am located in the suburbs of Chicago, IL USA.

 

[zeuspaul]

Crushed rock mixed with sharp sand will provide a better more stable base.

 

[Theguitarman]

Thanks. How much of a base would make sense for this? And just compact it as much as possible with a hand tamper?

 

[zeuspaul]

I would do at least two inches. If there is any chance the clay could get wet and soften I would want at least six inches.

 

[GBBUDD]

Short of a foundation below the frost line your build will flex due to frost and freeze thaw. It really doesn't matter when it comes to frost what the base is made from when it is above the frost line. Unless you get below frost line it is going to flex, and as your probably aware concrete is not very good at flexing. Thus a liner lined wall with rebar within the blocks is a way to combat not getting below thye frost line and if i had to guess in Chicago i'd say the frost line is 42 inches.

 

[Lisak1]

Chicago i'd say the frost line is 42 inches.

Nailed it.

 

[Theguitarman]

Thanks! I would rather try my best to do it right the first time, so I really appreciate the help.

So I have dug down to 42” now, and the bottom is hard clay. @GBBUDD Am I correct that I should now put 6” of crushed stone on this, then some sand, compact it, and then build my cinder block wall on this? Is any type of sand ok? I have some play sand laying around.

 

[GBBUDD]

Crushed stone works but if you've taken it that far I'd just pour a simple footing with two number 5 re bar. That way you can drill into the footing and set your re bar into that spaced so there's a bar I n every other bay of cinderblocks. Then you pour the block wall solid with concrete ..That won't go anywhere . Make sure to stager your block.
Could you place the block on the crushed stone yes will it settle into that a little probably . Block have no weight disbursement. But if poured solid it's less likely

 

[Theguitarman]

Crushed stone works but if you've taken it that far I'd just pour a simple footing with two number 5 re bar. That way you can drill into the footing and set your re bar into that spaced so there's a bar I n every other bay of cinderblocks. Then you pour the block wall solid with concrete ..That won't go anywhere . Make sure to stager your block.
Could you place the block on the crushed stone yes will it settle into that a little probably . Block have no weight disbursement. But if poured solid it's less likely

I fear you have vastly overestimated my competence! Haha. I understand very little of what you said.

To be clear, I am building a 2ft x 2ft wall, that will be about 4ft above ground height. I don’t really want to pour concrete because I don’t know what I am doing enough, and concrete is harder to undo. And this is a very small “wall”, more of a tower.

So my current plan was to put crushed stone down, tamper, put some sand, tamper, and let settle for a week or two, tampering here and there

For the actual tower, I was going to just stack concrete blocks, with a vertical piece of rebar in each void, and fill the void with crushed stone. Recognizing that this will not be as strong or permanent as poured concrete, does this seem reasonable, or am I missing anything?

Really appreciate your input.

 

[zeuspaul]

I would mix the sand with the crushed rock instead of placing the sand on top of the rock.

Builders sand (sharp sand) would be better than play sand but in your case play sand should be ok.

I don't know what you gain by putting rebar in crushed rock?

Crushed rock and sand won't settle so you are good to go after you tamp the base.

 

[GBBUDD]

if you dug down 42 inches why would you not use that for the pond?

Ok if you challenged by mortar and concrete i can make it easy for you it's not as strong but for a small feature it will be plenty. crushed stone yes no sand spend a little time to level it out as best you can .

build you walls lay them out where they are staggered from one row to the other. like a stair case make it easy on your self and make the walls in increments of 16" so you don't need to cut them.
lay your first row and like you said fill each bay with a piece of rebar the height of your wall plus a foot driven into the ground.
then stack the second row like a set of stairs so the block on top lays on top of a seam below. if you go to a masonry store you can even get 8 x 8 blocks so theres no cutting at all.

after you dry stack them an have ther rebar in each bay pour dry concrete into the bay mist the concrete in lifts like every 4 inches in time that concrete will harden on its own it may take a month but it will

 

[Theguitarman]

I would mix the sand with the crushed rock instead of placing the sand on top of the rock.

Builders sand (sharp sand) would be better than play sand but in your case play sand should be ok.

I don't know what you gain by putting rebar in crushed rock?

Crushed rock and sand won't settle so you are good to go after you tamp the base

I thought having the rebar driven down even further would give it some additional stability, but you might be right.

Thanks so much for the help!

 

[Theguitarman]

if you dug down 42 inches why would you not use that for the pond?

Ok if you challenged by mortar and concrete i can make it easy for you it's not as strong but for a small feature it will be plenty. crushed stone yes no sand spend a little time to level it out as best you can .

build you walls lay them out where they are staggered from one row to the other. like a stair case make it easy on your self and make the walls in increments of 16" so you don't need to cut them.
lay your first row and like you said fill each bay with a piece of rebar the height of your wall plus a foot driven into the ground.
then stack the second row like a set of stairs so the block on top lays on top of a seam below. if you go to a masonry store you can even get 8 x 8 blocks so theres no cutting at all.

after you dry stack them an have ther rebar in each bay pour dry concrete into the bay mist the concrete in lifts like every 4 inches in time that concrete will harden on its own it may take a month but it will

Thanks so much!

Not sure what you mean on the first line. I dug out a 1.5-2 foot deep pond, and the feature I am building is just a stack of cinder blocks next to it. So right next to the pond, I have a 2ft x 2ft hole dug down 42" that I will use for the base of my feature. So yeah, the base of my feature is deeper than the pond, but I wouldn't know where to put all the clay from digging the entire thing 42" deep!