Retaining wall foundations - What depth of concrete is required

[Neal]

Hi,

I was just about to arrange pouring of foundations for my retaining wall in my back garden. I dug my trenches 600mm deep and was all set to pour 450mm of concrete onto fairly firm clay. That was until I realised I'd need 3.5 cubic metres of concrete and nobody was prepared to barrow it up my fairly steep drive, through the garage and into the back garden.

I don't fancy mixing that myself as it'll take ages and I wouldn't be able to complete the job in one go. Recommendations for foundations for a retaining wall seem to vary between 100mm and 450mm of concrete which is a massive variation. Tony, on this page - http://www.pavingexpert.com/featur03.htm - recommends "150mm of concrete on solid clay or at least 450mm deep". Does this mean that you need 450mm of concrete if you're not pouring onto solid clay, or, 150mm of concrete in a 450mm trench?

If I can get away with 150mm of concrete that'll reduce my volume by a third and make it massively more manageable. Any help is greatly appreciated... Cheers.

[Dave_L]

I say employ some more enthusiastic and work-like people!

3.5 m3 is a fair amount of concrete to barrow though.

Retaining wall founds we did yesterday were 200mm deep.

[lutonlagerlout]

a lot depends on the height of the wall and what its retaining??
i generally go for 25% of the walls height in the ground
when tony says about 450 and 150 he means that you can have 300 of bwk underground==> backbreaking nosebleed for the brickies,better off filling it up to 100mm below ground,if the wall is retaing a bank and is over 1m high we generally put a cage of L-bars in the concrete, then when the wall is built we can shutter and reinforce the back by adding more concrete
it all depends on what its retaining
dont forget to use sulphate resisting cement for bwk that is underground
cheers tony

[Neal]

Cheers lads. The wall will be retaining my lawn which is topsoil and then clay. The wall will be 1m high. I'm basically replacing an older retaining wall as part of some landscaping. The previous wall didn't even have any sunken foundations, it was just built on top of some paving flags. It didn't appear to have moved any since it was built - by the previous owners - years ago so maybe I'm getting a bit hung up on the foundation depth. I just want to do things right that's all, unlike the bloke that lived here previously.

So, if you reckon 25% of the height of the wall that's 250mm of concrete near as damn it, and just a bit more than Dave recommended. I've ordered some hardcore to raise the level of the trench. That means I'll need just over one and a half cubic metres, half the concrete I was originally planning on which is a bit of a relief. I'm pretty sure I can get that up the drive with some help, especially after my workout, filling four skips with clay and rubble.

I was planning on having three courses of engineering bricks sub-surface. Tony, would you say that's excessive?

Cheers, Neal...

[lutonlagerlout]

i would build the whole wall in engineering or stock bricks if i was you,or at least do the earth facing skin in concrete block sor engineering bricks
it breaks my heart when i see ppl using LBCs on retaining walls,you need a brick that is very very strong,also it helps to hav epiers or some kind of return to give the wall a bit more oomph,what subsoil have you got??
chalk is great,clay is not good
alos do not forget to put drainage pipes through the bottom of the wall to allow water though
cheers LLL

[Neal]

I was going to build the wall using an English bond so it wouldn't be two seperate skins and therefore would preclude using blocks for the rear face. If I used a good F2:S2 brick with low water absorption, wouldn't this be good enough? The wall is facing onto my patio so I'd prefer something with a bit of character as opposed to a plain looking engineering brick. Also, referring to your earlier point, would you still use sulphate resisting cement with an S2 brick?

The subsoil is clay, I'm in central Scotland here, there aint much chalk about. Ive just taken delivery of the hardcore which I'm about to pack into the trench ready for the concrete arriving tomorrow, so the footings will be on a sub-base of this hardcore sitting on the clay

Also, I was going to use a perforated pipe at the back of the wall and leave some weep holes at the bottom. The longest bit of the wall is only just over 3m so I didn't think there was any need for any piers. Would you agree.

Cheers,
Neal...

[lutonlagerlout]

we call hardcore in foundations plums , its bad practise to be honest neal i dont know where you got that one from,but if you do try and make sure the concrete is all around the "plums"
english bond is fine witha strong brick,but for a bit more decoration flemish looks the bollox,i hate 2 skin 9" walls but a lot of ppl do it like that
and yes i would use sulphate resisting cement,i was around a guys house we did a wall for 5 years ago yesterday (with sulphate resisting cement) and theres not 1 crack or dodgy bit ,it really does work for retaining walls
regarding piers are there any angles in the wall??
if so piers may not be needed mate
good luck
LLL

[Neal]

Cheers LLL, are you saying that putting concrete footings on 'plums' is bad practice??? I only went down this route to cut down on the amount of concrete I'd have to pour and barrow up the drive, 250mm as opposed to 450mm. I've actually had to postpone todays delivery of concrete due to it wazzing down with rain all day yesterday - the joys of living in the west of Scotland. I'm originally from the north east coast of England where it's a lot drier. Anyway, I underestimated the amount of hardcore slightly so I'm waiting for another delivery tomorrow. What do you mean when you say make sure the concrete is all round the plums? I got the idea about putting foundations on hardcore from a number of sources. One of my mates is a site manager and he said it'd be OK... and also from Tommy Walsh - I bet you guys love him to death!!! Surely if you can't get a firm base you have no choice but to put some hardcore down.

In answer to your other question, yes there are two flights of steps so the wall makes four right angles to form the retaining walls for the side of the steps. You posted on my other thread about stepped foundations for these side walls.

Finally I was actually going to use Flemish bond but I thought I'd go for the stronger English bond as it's a retaining wall. If you reckon Flemish bond will be OK I'll gladly revert back to that cos you're right, it looks well smart.

I was intending to post this last night but I got a bit carried away with the booze after yesterdays setbacks!!!

Cheers for all your help and advice by the way, it's much appreciated. I'll make sure I use SRPC in the mortar.

Neal...

[lutonlagerlout]

yes it is bad practise to use hardcore in foundations,esp in a load bearing structure like yours
if building control caught us doing it we would be in it big time
if your footings are deep enough your better off with just 225 of concrete in the bottom of the trench then more brickwork
its the depth of the dig that counts not the amount of crete
withthe steps you should be ok re: piers
your site manager mate is talking from where the sun dont shine,if he says its ok
hope this helps you
my mums is from greenock ,it rains 99/100 days there lol
cheers tony LLL

[Neal]

Ah, I've got a decision to make now then. Do I go ahead and compact the hardcore with the compactor I've arranged for tomorrow or do I shovel the hardcore out of the trench??? What would you do? Obviously I don't want to shovel a ton of hardcore out of my trench but if it's necessary I'll do it.

If I went with 300mm of concrete in an empty trench. That'd leave 300mm to make up with over four courses of brickwork underground. The lower level of foundations also throws out my spadework for the stepped foundations for the side walls of the steps.

So, which way forward? More beer tonight will be a start but I obviously need to make a pretty quick decision.

[lutonlagerlout]

its in your court neal, good luck
cheers LLL

[Neal]

Seems like you had a late one last night

Anyway, I had a go with the compactor and there was too much water in the hardcore to do it properly. Obviously it rose to the top and then it all started sticking to the plate.

I've now shovelled all the hardcore back out of the trench - nice easy job - and I'm just gonna put the concrete straight in. A full 450mm if the guys are up to the job or 250mm if not. Hopefully it won't rain between now and Monday. I'll let you know how I get on.

I'll be glad when this is all done that's for sure.
Neal...

[lutonlagerlout]

sadly its one of those things neal where if you do it right once,you only have to do it once
i made the same mistake at home not painting the floors in my 1922 house with centaproof befor ethe oak floors were laid
now every room the floor his risen up by 50 mm in the middle as the timber is absorbing water through the oversite
i should of listened to the chipie when he advised me but i said and i quote"water will never come through those floors !)
oh yes it did

LLL tony

[Neal]

Just a quick update. The guys turned up to pour the foundations today and basically weren't prepared to barrow the concrete up my drive. I'd already warned the main man it was an awkward job and he said it'd be OK but would just incur an additional cost. The guys that turned up just weren't interested and just drove off when I said I wasn't for having it dumped in the street. Not when they were already four hours late and there was a monsoon approaching from the west. It wasn't premixed so it's not like they've lost anything on the load, but their attitude sucked from the minute they got there. I'm away now for a fortnight so I guess I'll just have to get busy with the mixer when I get some free time after that.

[lutonlagerlout]

who were these clowns neal,3.5m2 is a mornings work at most for 2 blokes especially on founds
maybe they just fancied the monday club
you do not want to leave your footings open for 2 weeks mate,they might collapse
cheers tony LLL