1.4 deep footings

[nathanrobo]

Hi

I have a 12 year old detached house that my neighbours tell me has piled foundations.

I've bought a conservatory (4.2 x 4 mtrs) and wanted to dig decent footings for my dwarf wall so that at some point in the future I would have the option of replacing the conservatory with an extension.

I've started my trench and found that the ground was made up with plenty of builder rubble down to about 1 metre. So I kept on digging until I got to firm clay which is at between 1.2 mtr and 1.4 mtr depth. I was planning to apply for building control (even though conservatories are exempt) so that I could get the footings documented. Question is... given that the rest of the house seems to be on piles will the inspector accept foundations of 1.4mtrs deep for an extension if I've hit good firm clay? Is there a depth limit for footings?

Any advice... I am planning to tie my new foundations in with the house foundations using steel rods and to use steel reinforcing in the trench.

Thanks guys

 

[jeds]

Standard trench fill foundations are normally ok for about 2.5m. I have known them a lot deeper on occasions but it's not that common. My question would be; why is the house on piles if the ground conditions are apparantly standard clay?

 

[nathanrobo]

Hi John

Thanks for that! not sure why the house is on piles. Some say that the land in this area was reclaimed although the houses either side are on standard footings. I think this house which was build later was on a sort of turning circle. A dead end tarmac bit of road with pavements either side.

 

[^woody^]

As a conservatory is exempt, I don't think that you can actually make a b/regs application unless you do something else to make it come under building control ...... and that something has to be something which requires the foundations to be checked

Even so, there is no guarantee that just because they look at the foundations in context of your lightweight conservatory, that the foundations will be acceptable for future building work

You may as well put the conservatory on a raft, and sort out proper foundations when you come to do an extension in the future

 

[nathanrobo]

Thanks for the advice guys... I'd still like to understand what constitutes appropriate ground for proper footings.

I built a 20' x 20' two storey extension with a pitched roof a few years ago and found similar soil at 1200mm. This project was under BC and the inspector came out a few times and congratulated me on the development on final inspection.

In this instance, I will probably want to do a proper building in the next couple of years and ripping up a raft and the dwarf walls seem like a waste of time and money.

If I were to build the dwarf walls to the same standard as a proper building I could just remove the conservatory and then build up. Seems to make so much sense... How do I accomplish this?

 

[jeds]

To get a good foundation on clay it must be at least 900mm deep and the clay should be firm and not too wet. The standard test is to drive a 50x50mm square ended peg into the clay. If it goes in easily it's too soft. If you can drive it in up to a maximum of 150mm but not more it is medium density and ok. If it's a struggle to drive it more than 50mm then it's dense and good.

If you can get that all round I would carry on. Make a record of what you find - including the above tests - and take some photos. When it comes to sorting building regs later it will just be a case of exposing the depth of the foundation.

 

[nathanrobo]

Hey John

That's great! Thanks for the detailed answer, just what I was looking for.

 

[amwc]

Send a sample to structural soils in Bristol its about £70 to give you a bearing capacity just make sure you have a representative undamaged sample.

 

[nathanrobo]

Will Structural Soils provide a definitive analysis that the inspector will accept without question?

How do I achieve an unbroken sample? Do I need a dedicated soil core sampler, or could I use a long piece of steel tube an push out the core? Any ideas on sample depth below the 1.4mtrs I've already dug?

Thanks

 

[amwc]

Hi
I've not had a problem with thier analysis all I do normally is take a large unbroken piece from what is dug out. If you get a fairly high bearing cap then for a single storey it should be fine. if you speak to then you can ask exactly what you require eg shrinkability of the clay. Also remember that if have a say 500mm seam and you found on top of it the load will spread a further 1000mm before it hits the lower soil. it all needs to be taken into account, I would dig a bit further just to see what happens but keep it safe.

 

[amwc]

http://www.isomass-lbu.co.uk/Downloads/NHBC-2006-4-4-page-4.pdf

 

[nathanrobo]

Thanks for that!

I spoke to building control and they say that this would be the first practical step.

Thankfully even though the house seems to be piled if the ground is good enough they are happy to do the footings under building control based on soil analysis.