Laying foundations for brick garden wall on sloping ground... Advice please.

[DIYNotIan]

Hi all

I've finished digging out a trench around what will become a veg patch. Next job will be to lay the footings. Never done that before so it's back to the trusted forum for some help! I've got several questions, but don't want to take the p!ss, so even if you can just help with some of the queries that would be a massive help.

Plenty of photos and a diagram with dimensions at the bottom.

The wall will be a low one, ranging from around 2 bricks high to 4-5 bricks high (the site is sloping). Just a single run of bricks.

OK, so....

1. Does the trench look ok and ready for the concrete? The sides are a little crumbly in places, but if I remove that I think it will just continue to be crumbly and the trench will just get wider.

2. As the photos show, I have put some brick "steps in" so that the trench doesn't get stupidly deep (it's on a sloping site). Did I do right? Also, as one photo shows there is a section of rock along the trench which I have left as is thinking that it is nice and solid and I can just pour over it. Is that ok or does it need removing?

3. Can I lay the footings in two separate sessions? I was thinking of doing the deeper sections up half way first, and then when set coming back and doing the whole lot up to the same level? Can I or would I be best to do it all in one go?

4. How much cement/ballast will I need? The dimensions are below on a diagram, but to save looking... The total length is 17 metres. The depth differs because of the sloping site, but ranges from 100mm to 240mm.

Many thanks

Max

 

[hansPulesh]

Hi,

1. It seems fine to be honest, but where the trench is 100mm deep, can you remove a little more soil to make it slightly deeper? Generally 4" of concrete is a good depth - even though there wont be too much weight on it. Leave the crumbly bits, the concrete will just fill up to the soil.

2. I don't get why you've put the bricks in? Are they to fill in where you've dug out too much? If so, smash the bricks up into smaller pieces and use them to fill these gaps. With the rock, if you have a drill with bolster action on it, use that to trim some of the rock down so that you can lay 4" of concrete above it.

3. Footings are best poured in 1 shot. You want the whole thing to be 1 piece of concrete. Although this is not as bad a situation as having 2 pieces on concrete next to each other, I'd personally just pour it all in 1 go.

4. How strong do you want the mix to be? Google concrete calculator and you'll be able to see exactly how much of everything you'll need.

Hope that helps.

 

[DIYNotIan]

Hi hansPulesh

Many thanks for the helpful response.

1. Fair point. I had aimed for a 100mm depth minimum. I think the issue is that the surrounding ground is too low at that point, so I might still put in 100mm even it it brings it up to level. Once the wall is done, I'll probably bring up the ground a little in that area before putting the shingle back down.

2. I was told that I needed to keep the bottom of the trench reasonably level. If I did keep it level throughout, with the lowest point being 100mm (as discussed above), the highest point would be somewhere around 350-400mm. Maybe more. So I put the bricks steps in at any point where the depth got to around 200mm, which then reduced the depth for the next section. Was that a bad move?

3. That's fine - I'll aim to do it in one go then.

4. I've no idea!!! My dad - who will be laying the bricks - said to go for 1-cement to 5-ballast. Don't know if that answers the question?? I've run the calculator and it says I would need 0.51 m3. I can only find what this translates into with ready made bags. Can anyone tell me how much cement ballast on a 1/5 ratio I'd need for 0.51 m3?

Huge thanks

 

[hansPulesh]

2. It's fine in that case.

4. http://www.diydata.com/general_building/concrete_calculators/concrete_calculator_2_m.php

 

[DIYNotIan]

Great - thanks.

I've got to admit I have been totally naive! I guess that's what happens when you've never done it before! I thought I'd be able to pop down to the builders merch tomorrow morning and get what I need. How wrong I was. I didn't think for a second that I'd be needing around a tonne of ballast and over 200kg of cement. Lesson learnt! A much bigger project than I had anticipated, but hey, it will give me more opportunity for practice!

Should I carry on with the plan as is, or - where the trench is at its deepest (240mm) - should I maybe part fill with some hardcore (to reduce the amount of concrete needed)?

Was I right in saying that the bottom of the trench needs to be level? Or is that unnecessary? Obviously if I go down the hardcore route the trench will no longer be level at the bottom.

Sorry for all the questions and thanks a million for the help.

 

[hansPulesh]

No its a good idea to have the bottom level as it means that the 'structure' has less ability to move or slip - not that it will anyway but still. its fine having it like it is.

by my own calcs, you need more like 1m2 of concrete to fill your trench. You could use hardcore, but if you do, then make sure you ram it down good and hard into the ground or you might get cracking later on as it moves under the weight of the concrete mix above.

I'd go into the builders merchants and get them to calculate it for you again to make sure and then order it and get them to deliver.

 

[DIYNotIan]

OK, thanks. I'll double check my calcs later and will also ask the builders merch to calculate it for me to make sure I haven't made an error. I think the issue is that because the trench has not got a consistent depth it is hard to be accurate. I was averaging out the depth to around 150mm. The width varies a little but is mostly at around the 200mm mark.

The problem (if it is a problem?) with using hardcore is that the bottom will no longer be level if I do. I cannot use it in the shallower sections. So if I use it in the deeper sections I will no doubt make the bottom of the trench sloped as a result. Or am I missing something here??? I have got hardcore on site, so if I can use it and get away with less cement then great. But only if it doesn't compromise things...

Yeah, I'll definitely opt for delivery. I can hear my Skoda Fabia sighing with relief as I type that!

Cheers

 

[hansPulesh]

I wouldn't worry that much about having a perfect level to that much accuracy. The way I work it as a rough guide, is as long as there is a top level of at least 4" of concrete then it's ok as the concrete will then not crack or snap.

How much hardcore do you have? You could use it at the bottom of the whole trench as long as there is that 4" of concrete above it. Also, cement will be delivered in bags and if unused speak to the supplier and see if you can return it to them in return for bricks etc which you can then use to make your walls with.

 

[DIYNotIan]

Hi hansPulesh and anyone else who is reading....

I had to shelve this project for a short while because other more pressing jobs came up. I'm back on it now though....

I think I've got my head in a real muddle over the the prep for this wall. Never mind, it's a learning curve! But anyhow, without going into detail, I'm going to have to have quite a shallow footing of about two and three quarter inches (2.75"). I know that is below the recommended minimum of 4". What I'd like to know though it what would be the potential problems of having a thinner/shallower footing? What might go wrong?

Please bare in mind when answering that:
a) The bottom of the trench will be totally flat and level.
b) The bottom of the trench is nice and firm. The area used to be a driveway, so the ground is very hard.
c) The wall is going to be short... A maximum 6 bricks high where the ground is at the lowest. 4 bricks where the ground is at its highest.

One other thing.... Because the first run of bricks will be at least partially beneath ground level in some places (and totally beneath ground level in others), I could - if it would help - pour some concrete along the sides of those bricks after the wall has been built if that would help. I.E. on this cross section, where the orange parts are:



So if anyone could tell me what could potentially go wrong with doing it in this unorthodox way, or if you think I can get away with it, that would be extremely helpful.

Many thanks

Max

 

[Footsoldier888]

Epic build Max!

About the concrete idea at the base, I don't know. I believe a single brick wall should have a pier every 3m if it's over 300m high. However you are building a triangular structure so perhaps a strong bond will be ok. I take it you will be using angled bricks at the corners?

The potential problem of a shallower footing is that it won't be able to resist ground movement and will crack, causing the wall to also crack.

Hey it's not too late to knock a few bits of timber in instead!

 

[r896neo]

Concrete at less than 3'' is too thin to resist cracking with even the slightest bit of movement. To be honest with it being so shallow it makes it even worse as even the smallest amount of frost heave will buckle it.

 

[hansPulesh]

Best case scenario - it will be fine.
Worst case scenario - it will crack and so will the wall above it.

Is there no way that you could dig a little deeper - it will literally only need another 2 inches of soil removed or so? Or as another person suggested, wood?