Do I need much foundation for a low wall?

dohda

Hi All,

I am planning on digging drainage into my garden and at the same time building raised flower beds out of you standard concrete block. I was hoping to lay these sideways (about 9inches I think) and 2.5 blocks in height above ground. The other side of this raised bed is the main back wall which has those lage supporting posts at each end.

My question is how much foundation do I really need for this. The raised will be 20-25inches high with half a block under the ground and 1 block wide. Will htere really be enough earth in that (20x18x144) (23 cubic feet) to justify digging down a foot, 2inches of compacted stone and then 4 inches of concrete?

Im sorry I dont know the proper terms for anything, I have never even attempted anything like this before and would like to get it right. Its just the maths seem out of whack to me.

any and all advise greatfully received!!

ART6

dohda wrote: »

Hi All,

I am planning on digging drainage into my garden and at the same time building raised flower beds out of you standard concrete block. I was hoping to lay these sideways (about 9inches I think) and 2.5 blocks in height above ground. The other side of this raised bed is the main back wall which has those lage supporting posts at each end.

My question is how much foundation do I really need for this. The raised will be 20-25inches high with half a block under the ground and 1 block wide. Will htere really be enough earth in that (20x18x144) (23 cubic feet) to justify digging down a foot, 2inches of compacted stone and then 4 inches of concrete?

Im sorry I dont know the proper terms for anything, I have never even attempted anything like this before and would like to get it right. Its just the maths seem out of whack to me.

any and all advise greatfully received!!

I would say that it depends on the depth of your topsoil. You are building a retaining wall where it's weight holds back the soil behind it, so with blocks on their sides you would have a lot of weight there. The main issue is likely to be on the long side (12 ft?) since if the topsoil is deeper than a couple of inches (half a block) then you could get sinking and cracking of the mortar joints in places. If I was doing it I would expend a bit more effort by digging down a good six to eight inches and hammering some sand down to bring the depth up to the half block you want, then use good strong mortar in the block joints.

dohda

Hi Art6,

Thanks for the reply. The efforrt nad work involved is not really the problem. I don't mind the hard graft. My problem is it that for me, laying a solid foundation like that turns it from a diy job into something I would need help with.

If I have 3inch topsoil that would be the max. I am also digging drainage trenches throughout my garden as it is constantly too wet for my young daughter to go out and play in. The soil here is all clay.

Am I right in understanding then that a small foundation 10-12 inches of say pea gravel and sand. Compacted down should be enough for a small wall of this size?

I'm not sure I would call it a retaining wall, more a raised flower bed. Your length is correct for the higher bed. The lower (1.5 block aboce ground) would be more like 20foot.

Thanks again for the help.

ART6

dohda wrote: »

Hi Art6,

Thanks for the reply. The efforrt nad work involved is not really the problem. I don't mind the hard graft. My problem is it that for me, laying a solid foundation like that turns it from a diy job into something I would need help with.

If I have 3inch topsoil that would be the max. I am also digging drainage trenches throughout my garden as it is constantly too wet for my young daughter to go out and play in. The soil here is all clay.

Am I right in understanding then that a small foundation 10-12 inches of say pea gravel and sand. Compacted down should be enough for a small wall of this size?

I'm not sure I would call it a retaining wall, more a raised flower bed. Your length is correct for the higher bed. The lower (1.5 block aboce ground) would be more like 20foot.

Thanks again for the help.

The main thing is to get down to the subsoil because that is already compacted and doesn't have organic matter in it that can slowly decompose and lose volume. The soil you put in the flower bed behind the block wall will slowly compact in the wet weather, and will try to spread, putting a load on the wall. That is when it's important that the wall is keyed into the ground and is heavier than the soil immediately behind it. The ground you describe wouldn't need 12 inches of pea gravel. I would just dig down (say) six inches and put a few inches of rammed sand down simply to be able to easily get a level foundation, then, with the bottom blocks buried their full thickness, I wouldn't expect the wall to move.

I correct my earlier post as your soil is not what I had assumed.

dohda

thanks for that. looking forward to giving it a go!

Carlow52

what u could also do is use 9" hollow-block as a normal block and drill holes in your foundations to take vertical lengths of rebar: say 15mm, that will come up through the hole in the 9" hollow and then fill in the hollows with concrete: u could then cap it with 4" solid on flat.

if u go with 9" hollow, try not to have to cut them, they are a pain and the 1/2 ones, called footballs are quite expensive
HTH

on the drainage, how is the water going to get out of the drains u dig?

dohda

Carlow52 wrote: »

what u could also do is use 9" hollow-block as a normal block and drill holes in your foundations to take vertical lengths of rebar: say 15mm, that will come up through the hole in the 9" hollow and then fill in the hollows with concrete: u could then cap it with 4" solid on flat.

if u go with 9" hollow, try not to have to cut them, they are a pain and the 1/2 ones, called footballs are quite expensive
HTH

on the drainage, how is the water going to get out of the drains u dig?

Hi Carlow,

I'm afraid I do not fully understand what you mean in relation to the hollow block? Do you mean lay one of these as teh foundation or build the entire wall out of them?

In relation to the drainage I am going through the backwall as my neighbours have done. We have no houses out the back and this seems like the best solution. The wall is 3 feet higher on the outside than my garden side and I think this may be part of the drainage problem in the first place.

On another note, I live in a mid terraced house and have two options for getting the drainage stone out the back. Wheel it through the house or lift it over the backwall. (there is access). I think I can get the builder to lift it over the wall for me but I would need to buy one of those large 1 ton bags. Does anybody know where I can get these?

Carlow52

dohda wrote: »

Hi Carlow,

I'm afraid I do not fully understand what you mean in relation to the hollow block? Do you mean lay one of these as teh foundation or build the entire wall out of them?

In relation to the drainage I am going through the backwall as my neighbours have done. We have no houses out the back and this seems like the best solution. The wall is 3 feet higher on the outside than my garden side and I think this may be part of the drainage problem in the first place.

On another note, I live in a mid terraced house and have two options for getting the drainage stone out the back. Wheel it through the house or lift it over the backwall. (there is access). I think I can get the builder to lift it over the wall for me but I would need to buy one of those large 1 ton bags. Does anybody know where I can get these?

build ur foundations as u had planned the hollow blocks are instead of ur blocks on flat, they offer the easy option of tying the wall into the founds with some rebar, prolly over kill with ur low wall.

so the ground outside the wall is 3 feet lower than ur garden?

can u not buy the stone already bagged?

the bags can be bought new in places like Grange Builders providers in Baldoyle.
AFAIK they are insured for one use only, from new, so builder may not lift them if secondhand

dohda

hi,

yea can buy them already bagged but at a serious premium. I can collect 2 ton of drainage stone for twenty euro but cannot get it delivered for less than 140 euro in bags. with the raised beds, the drainage the new shed, and the topsoil I was just trying to save on costs.

Yep wall is a good three feet lower on the other side. I do understand now what your saying re the blocks. makes sense but really hope this would be overkill tbh for two reasons.

1 they cost more
2 they are a little wider than the standard blocks on edge and I was hoping to have the wall as this as was possible without being too weak for the job.