Foundation question...me again shy

Hi guys

thought i would make a new post as I didnt want to hijack someone elses regarding foundation work.

I guess this post is really for shy and anyone else who wants to input.

I am preparing to build a garage and want to know to be on the safe side how deep I should place my stripfoundations.

The site is going to be 1 metre away from the garden fence, and about 1.5 metres away from where the edge of my foundations are going to go used to be a big 40ft tree, and about 5 or 6 metres away is where another massive 40ft tree used to be. The trees were both cut down about 4 years ago and now all that remains are the stumps.

Now with the soil I have, I can dig a peg into the ground easily up to about 250mm below the grass untill i reach hard soil and can go no more.

On what I have said, I would like to know how deep my foundations should be and also how wide. I know you said before shy that 450mm wide was ok, however I realise now I have left out important information.

Also Ill be looking for a c20 mix right? but what wetness am I after?


Everyone is invited to post back and this is not just a shy topic only

Thank you once again for all your replies

Hi

If you're in sandy soils, then moisture extraction by roots won't be a problem. If you decide to have the root balls out, then that will disturb the ground and you'd need to take the foundations down to the bottom of that disturbed ground, if they will run either through the area of the root ball or within half a metre or so; otherwise put them in at an absolute minimum of 600mm below ground, 450mm wide.

If you want to make sure that the trees don't start to regrow, a very un-eco friendly way of doing so is to drill holes in the stumps and pour creosote down them, which will poison the roots.

If you're unsure as to the bearing capability of the soil, stand on one leg and see if you sink into it by any appreciable amount. If you don't, it's fine to take the load: the pressure you put on the ground on one leg will be more than the building does. Think of the difference between stillettos and snow shoes...

C20 is plenty good enough. If you're ordering readymix, get Gen 1 with an S2 slump. They will wet it up for you on site, if you want or need to make it flow a bit more easily in the trench. Before you pour, spend some time putting some wooden pegs - tiling batten offcuts will do - in to a set level, to make the top of thw wet concrete as level as you can and to give you a guide when you're levelling it off.

Shytalkz said:

If you're unsure as to the bearing capability of the soil, stand on one leg and see if you sink into it by any appreciable amount.

Click to expand...

Careful though in case the ground water comes up and drowns you! lol (could not resist )

I hate the "how far do I dig" questions, as there are a few variables and "my neighbour only went 500 down" does not mean that this is the depth to go on the next house.

If there are sand or loose ground pockets, or its made up, or if there are drains near, or variable clays.

Apart from the stand on one leg test (does it make a difference if the OP is 20 stone or 10?) look for changes in soil colour

loool cheers for that guys

I can stand on one leg and I dont go anywhere and im about 16stone so I guess its good for 600mm depth 450mm wide

Cheers for that

sorry guys one more question

If I decide to self mix the concrete with a mixer Im gna need about 2.5cubic metres of concrete, If my calculations are correct I will need about 30 batches from an 85litre mixer. This means if each one takes 5 minutes for 10 ill need 50minutes for 30 Ill need 150minutes which will be about 2 and a half hours, am i correct in thinking that the concrete at the botom would have started to set already?

If you were selfmixing this amount how would u do it?

Would you recommend getting two mixers in so that I can get double the amount done in the same time?


Is it ok if i finish after two and a half hours or would this risk the strength of the concrete?

thanks

not a problem. you will deal with 2.5 cube with a belle and plenty of graft.

forget acquiring another mixer, get a mate. one to mix the other to barrow and tamp.

you will need just under 5t of ballast and 30 dust.

don't mix it too dry, but make it manageable for the bloke barrowing.

Cheers for that noseall, was wandering when ud post,

What do u mean by 30dust? So yeh about 5tons of ballast and 1ton of cement for a c20 mix right?

30 bags of OPC

lol cheers for that shy

Ok guys, after a little investigation, and a sore back

I have dug an inspection hole roughly 600mm to see what the general condition of the ground is and am posting a few pics to get your feedback on.




As you can see from the first pic, at about 300mm down there is a layer of crushed bricks stone and glass which seems to be from the 50s due to its thickness. Once I passed this the soil became really quite difficult to dig with it being really dense and compact and I couldn't get past the 550mm mark due to this, it was really like clay and was was extremely dense and compact as you can see from the picture the stump i used has made a nice circular mould at the bottom, and would also take your fingerprints rather nicely.


The next picture shows how once i reached about 500mm the colour of the soil completely changed.




After seeing the pics and explanation do you still recommend 600mm? or can i get away with the 500mm mark?


Thanks again

That soil that seems like clay is clay . It looks like it's quite sandy; not sure how it's quite hard to dig, yet at the same time it's easily mouldable, though - one does not begat the other, unless you mean that it sticks to the shovel when you're digging it out?

The stuff above is topsoil and fill and is clearly not what you want to be in.

The fact that you can make an imprint in it shows that it's got a reasonable amount of moisture in it ie it's not dessicated - at the moment. At that level, it is still susceptible to moisture removal by climatic variations (although if we continue having "summers" like last year's, that's not much to worry about!); and any significant vegetation within the immediate vicinity.

What to do, then. If you go about 100mm into the clay, then, theoretically, you are still within the zone of influence of the climate. You should still go down to 900mm depth, as it's sandy and therefore likely to have a low/medium heave/shrinkage potential. However, as it's "only" a garage and outside the remit of BRegs, then you might think ferk it, that's deep enough, I'll take a flier.

Personally, if it was my building and if the clay is moist like that everywhere, there's no significant vegetation within say 15m and you don't intend heavy planting anywhere near in the future, I would take the flier and ensure that I was about 100mm into the sandy clay.

However, I would also stick some loose rebar 75mm up from the bottom, say 4T12 bars running longitudinally and lapped 400mm as required and that will assist the foundation to span over any areas where the clay might subsequently be affected by a reduction in moisture content in the future. You could use B283 mesh instead, the main bars (at 100mm pitch) nearest the bottom and running down the length of the foundation, again lapped 400mm) as an alternative.

thanks for the reply shy


You said it, it is very moist but in its defence it has been raining quite hard for the last few days.

I think I will have to go 900mm, as I do have quite a large amount of vegetation in the garden usually all year round.

Will definitely go for rebar, but i want to know if i dig down 1 metre can i fill it with concrete upto about 600mm and then blockwork the remaining which will be about 1 and a half courses untill ground level then lay my dpc and then continue? or should I lay my dpc 1 course
after I reach ground level which will be about 215mm.

Also where can i purchase the rebar or mesh, as all the big stores near me dont seem to stock any and i cant seem to find any online sellers where i can order from?



thank you very much for all ur help, lol i think when its done i will name it the shytalkz room

I just went back to the beginning of the thread, just to remind myself as to what had been near it - and you mention two 40' trees that we cut down. Do you know what species they were? Whatever species, where you want to put the garage is well within the zone of influence of the tree roots and consequent moisture removal by them from the clay soil; interestingly, though, any roots seem to be up within the fill and topsoil.

Trees and soil, by virtue of mot being manufactured in a laboratory or workshop do not necessarily conform to a set pattern and the guidance for depths of foundations in clay soils, near trees and other vegetation is just that: guidance.

Providing that the soils you propose to found in are not rock hard, that is, they're slightly soft (you can mould the soil into ribbons squeezed out between finger and thumb), then you should be fine, but put some rebar in, just to make sure of support and continuity.

Rebar is not usually held by BMs, you need a steel stockholder. Got to Yell.com for steel reinforcement suppliers in your area. If you're using the loose rebar option, get it supplied in straight lengths, cut it on site. 12mm bar can be bent around something like a scaffold pole, when you need to go around the corners.

600 depth and then blocking from that point is fine.

Hmmmm yes think they were pine trees, no the ground can be molded but wen digging its I guess very compact and sortve sticky, I reckon the roots grow more sidewards but since the tree went about 4 years ago they could have gone over time? Thanks for letting me know about the rebar, I think I'll go with the mesh.



Once again thank u very much, I will keep u posted about progress or any problems

It's good that the trees went a reasonable time ago, as clay takes about seven years to sort itself out after removal of trees, so you're most of the way there.

Can I come to the official opening?