Quantities for garden wall footings

[Tony K]

Hi all. 

I'm doing a small garden wall footing but struggling to get accurate quantities from the various online calculators, which I think are being thrown by my dimensions. 

 

The wall is 18 linear metres, 50cm deep and 50cm wide. 

 

I'm planning on a ratio of 1 cement 2 sand and 4 aggregate. 

 

Anyone any good at QS?! 

 

Cheers 

[Canski]

Are those the dimensions of the wall or the footing ?

If its the footing then 18 x 0.5 x 0.5 =4.5 m3 

[Tony K]

25 minutes ago, Canski said:

Are those the dimensions of the wall or the footing ?

If its the footing then 18 x 0.5 x 0.5 =4.5 m3 

 

It's the footing. Thanks, but I know it's 4.5m3, what I need to establish is what quantities of cement, sand and aggregate I need for that volume of footing. 

[Canski]

Google  says  A bulk or jumbo, loose tonne bag of building material such as ballast, sand or aggregate generally weighs around 850kg, which yields 0.5m3 so you could use 9 bags of mixed aggregate for which you will need approx 54 Nr 25kg bags of cement. or if buying separately 6 bags gravel & 3 sand .

 

If it was me I'd probably go for a volumetric delivery unless access is a real issue.

[Radian]

How did you arrive at the dimensions for the footing? I'm guessing this must be a substantial wall. 2m tall?

[Tony K]

26 minutes ago, Radian said:

How did you arrive at the dimensions for the footing? I'm guessing this must be a substantial wall. 2m tall?

It's only six blocks high, laid on their sides. It retains about 700mm of ground at a boundary where I have dug down. The footings were suggested by a knowledgeable friend, who might have been mindful of the fact that I've dug down into the thick clay. 

 

You think that's over the top? 

[Tony K]

9 hours ago, Canski said:

Google  says  A bulk or jumbo, loose tonne bag of building material such as ballast, sand or aggregate generally weighs around 850kg, which yields 0.5m3 so you could use 9 bags of mixed aggregate for which you will need approx 54 Nr 25kg bags of cement. or if buying separately 6 bags gravel & 3 sand .

 

If it was me I'd probably go for a volumetric delivery unless access is a real issue.

 

Thanks @Canski seems an awful lot for my little project but I can't argue with your sums! 

[Russell griffiths]

Easier to get a truck that mixes-on site, you will need two fit helpers and 3 barrows. 

Will be about £550 plus vat. 

[Tony K]

Just now, Russell griffiths said:

Easier to get a truck that mixes-on site, you will need two fit helpers and 3 barrows. 

Will be about £550 plus vat. 

 

Sorry, I should have said, access is seriously limited so I'll have to mix the footings on site. 

[Tony K]

18 minutes ago, Russell griffiths said:

Easier to get a truck that mixes-on site, you will need two fit helpers and 3 barrows. 

Will be about £550 plus vat. 

 

Sorry, I should have said, access is seriously limited so I'll have to mix the footings on site. 

[Tony K]

Re-reading the advice I was originally given regarding the footings dimensions, I was actually told to go with 450mm deep and 450mm wide footings. I have rounded up the linear metres to 18 to give me a margin for error, but the small amendment to the footing depth and width reduces the number of jumbo bags to seven.

 

If I use a 1:2:4 ratio I'll need 1 bag of cement (or 34no 25kg bags), 2 jumbo bags of sand and 4 aggregate. Unless I've missed something, but I'll find that out on the day! 

 

Thanks all. 

[Mr Punter]

If you are using bulk bags, buy all-in ballast instead of separate stone and sand.

[George]

In terms of the design, this is the BRE standard solution for garden retaining walls: https://imgur.com/a/LJAP6pV

 

You need about 1800kg of ballast per 1 cubic metre of concrete and (depends on mix strength) about 250 to 300kg of cement. 

 

Using the foundation dims on that detail:

0.5 x 0.3 x 18 = 2.7m^3 of concrete

 

Which would take about 5 tonnes of ballast and 700kg (28 bags) of cement. Not allowing for wastage and the extra sand and cement needed for the mortar. 

 

Handy link: https://builderhq.co.uk/guide/concrete-mixing-guide/

Edited July 8, 2022 by George

[Radian]

49 minutes ago, Tony K said:

It's only six blocks high, laid on their sides. It retains about 700mm of ground at a boundary where I have dug down. The footings were suggested by a knowledgeable friend, who might have been mindful of the fact that I've dug down into the thick clay. 

 

You think that's over the top? 

 

Yes. The aim of a footing is to put the load onto stable ground so the depth from ground level to the bottom of the trench is key. It needn't be full of concrete. The depth of concrete is determined by the vertical load which in your case is minimal. It's up to you if you fill it to ground level with concrete or just put in a couple of hundred mm and build up with blocks. It's a pig to lay blocks below ground level so concrete might be the simpler option. My retaining wall is 600mm high and I went down 500mm and laid in 200mm concrete and came out with blocks laid flat. I had spare blocks on site and access was poor so went for this particular balance.

 

Most important for long-term stability will be drainage. A perforated pipe on the foundation and back fill wth gravel one the side facing the ground you're retaining will help keep the wall in good condition. I used a vertical drain sheet on the back of the wall to drain the surface down to the foundation level which, being plastic, also stops water leeching through and messing up the pretty stonework.

 

[Tony K]

Really helpful advice (as always). 

Thanks everyone.