Foundation depth for conserv on clay plz

Hi all,

Been digging all morning at these footing im down to 1000mm from ground level ive dug out approx foot of clay and theres plenty more on top of this one side i will be having full hieght wall just at that side then 600 hieght the rest of the conserv question is do i need to go deeper?

One other question with that side been 1000mm does the other sides after be the same if clay is not presant and its just soil? there currently at 700mm.

PS if i go any deeper i will not be able to get out maybe the wife wants that





Thanks for your time

Lee

It really depends on the type of clay and the distance to, and type of, trees in the vicinity.
You are deep enough to not have to worry about frost heave.

If you can find out the type of soil in your area (either from a geological map or you could ask Building Control ), then look at the tables in NHBC guidelines on foundations (I've got a copy I can email to you if you want it).

Having said all that, if the tables suggest 2.5m foundations you might be tempted to take your chances with something shallower. BC aren't going to care but then, do you want your conservatory to shift on its footing?

Thanks for the info theres a conifer about 1 meter away about 12ft height there was a 18 ft conifer i removed in the position im digging apart from that nothing else.

It was quite dense clay im currently trying to find what clay it could be will let you know if i need the chart thank you for that.

Lee

As long as it's not a Leylandii or other Cypress your conifer won't cause a problem as long as you keep it trimmed back and don't let it get much taller.

I'd go down another couple of hundred mm just to be on the safe side.

Are there any other trees within 10m or so?

Hi Ronny,

Thanks might aswell show you a pic its easyier than the first tree will be coming out its as good as dead to be honest the other coniferes not sure what they are The conservatory will be goiing from house up to the drain shown in pic.

Thank for you time



Lee

I'd bin both those trees, the second one is dead from the waist down anyway. As said, invest in a decent hedge trimmer and use it a couple of times a year, don't forget to trim height as well as girth.
Lots of trees next door, do the neighbours have any biggies?

Hi deluks

Ok was thinking of binning them trees, the niebours trees are about 12 meteres away they were big but got halfed in size over 12 months ago.

cheers for your input


lee

Conservatories are normally exempt from building regulations.
If this is your case, then you can go to whatever depth of dig that you require, but it is generally recognised that in clay, providing that there are no trees in the immediate vicinity you should go to 1 metre deep below ground level to allow for high volume change potential.
However in your case you state that you have removed a 5400 tall coniferous tree, that was growing in the position that you are digging in, and that you intend to remove the 3600 tall dead conifer nearest to the proposed conservatory.
I am trying to keep this simple, so that you will understand the point.
Any tree whether it is deciduous or coniferous takes vast quantities of water out of the ground each year. This presents no problem whilst the tree is growing as the ground acclimatises with the growth of tree, but when the tree is removed this causes a volume increase in the ground causing it to swell and heave to compensate for the additional water content that it is not used to. Over a period of time the ground will go back to its original condition.
I am not saying that the ground will swell or heave, but if it does, it may lift the foundations when they are only a metre deep, push the foundations sideways or lift the over site concrete.
In my opinion and experience all the guidelines with regard to working near trees are over the top, but these guidelines have been put together by people more clever then myself, so we normally work to them.
With regard to neighbours trees, even though they may be 12 metres away, depending on the species and the height before they were crowned, they could still be within the zone of influence on your works.
If you are not exempt from building regs, and the BC officer knows that you had a tree in the middle of your foundations, then you could have a shed full of problems.
oldun.

Your trees are definitely a type of Cypress - not sure if they're the dreaded Leylandii - which makes them a high water demand tree. As Old Un said, you could have a problem with heave as the soil rehydrates.
You can avoid issues of lateral movement by putting a compressible material such as clayboard on the inner faces of the footings.
Installing a suspended beam and block floor will prevent heave of the slab.

The only way to prevent potential heave from your strip or trench fill foundations is to dig deep enough to get below the influence of the roots.
Alternatively you could use a reinforced concrete raft slab
. This will prevent differential movement of the structure but it might end up tilting...
Or you could use piles but now it's getting expensive

Or as I said before, you could take your chances as it's not subject to Building Regs anyway.

Hi theoldun and RonnyRaygun,

Well thank you for the great right up i fully understand what you mean, im exempt from building regs i did phone to make sure so that aint an issue thank god, im not quite sure how far to take this then i did remove some roots 3ish at about 1 ft below half inch diameter branching from the tree i removed and they were the last of its roots.

I was going to buy some reinforcing mesh anyway and place around the footings and main slab for extra security i will look into this clayboard Am i right in thinking the clayboard is placed into the trench slab wall side and then the cement poured in also for the floor then do i build on hardcore 150mm then sand layer, dpm, insulation then 100mm reinforced concrete layer and this leans on top of the footings concrete thanks very much for your input.

Lee

Lee,
Thanks for letting us know that you have read the posts. I am very sorry and I feel sure that Ronny Raygun will fill the same way at having to give you bad news. However you now know in your own mind that you may, and I repeat may, possibly have future problems.
I am not prepared to tell you what to do, as I am not a SE, but I will tell you what I would do if I had the same problem.
You may not understand all that I write but will try and keep it simple, any problems then ask.
I would bottom out at 1000 to 1200 below ground level. If your trench is 450mm wide I would put a reinforced 300mm square cage all round the founds consisting of 4 no T16 rods with 8mm links at say 300 to 450mm centres. Place cage in trench, drive 16mm bar down each side of cage at say metre centres and tie cage to bars so that is suspended 50 to 100mm above bottom of trench. Allow to put 750mm trench fill concrete into founds.
With regard to oversite we only use Clayshield to side of trench when we are below 1500 deep dig, however, to ease any possible pressure to underside of oversite 50mm of Claysheild round the inside of the trench will not come amiss. Make sure that if you do use it you have enough width on founds for your footings.
I would also run three course of Bricktor in your footings brickwork.
Ronny Raygun has made a very good point with regard to pot and beam floor. Whilst they are more costly in small quantities, and a pig to lump about we use them now wherever we can with a 225mm void below to compensate for any heave.
This is what we would do, a SE may say differently. As Ronny Raygun has said you could take your chances and just concrete your foundations without doing any of this additional work and never have a problem. We agree, you may never have a problem, however Mother nature and ground have been full of surprises to me over so many years, but I feel that these days we go over the top in everything we do. Fifty years back we would have scratched that bit of trench out 450mm deep, thrown a bit of conny in gone up the pub and never given it another thought.
Your choice..
oldun

I'm not an SE either, at least not chartered. But I do work at a Structural Consultancy, graduated three years ago after working in steel fabrication until the age of 30.

So at the moment I'm what you might call a "Graduate Engineer" or a "Technician".
I'm also a competent DIYer who has renovated two houses and done the vast majority of the work myself.

Anyway, have to say one of the main benefits of hanging around this forum is gaining knowledge from the likes of you, Static, and many others.
Often, the solutions that you guys come up with haven't even entered my mind, and I tuck them all away for future reference.
And at other times the questions give me something new to get my head round when I've been doing the same old calcs for weeks on end (there're only so many ways you can design a beam!).

So thanks Old Un, and thanks to any of the other regulars who are reading this

Your both welcome and thanks for your support and time for replying, im of to do some shopping for tools tomorrow so will pop to the builders merchant and take a look at there reinforment bars etc and take your advice on board.

Im currently researching about the flooring method you have discribed but having trouble finding how to do this im sure i will get there

Big big thanks

Lee

Make sure you dig down below any roots you find in the trench (normally about 300mm below should be plenty)..

Would totally agree that removing that 1/2 dead conifer would be a must.. but you risk a very strong potential for heave having removed them both unless you dig 2m deep foundations around the area (3m radius of both removed trees) and as above below any roots found.. theoldun has given a fairly good description of clay heave.
Claymaster (or a compressible boarding) on the inner sides of the footing should be added to all footings below 1.5m from ground level and a void below the ground floor slab..

Now all of this is how to do it properly.. you will still need to keep the other conifers under control or you may experience problems in the future from them..

In the end it is just a conservatory, its not covered under building regs as it is basically a greenhouse stuck onto the external side of your house.. its not a habitable space and not an extension with heating etc..

Whilst the conservatory would not normally require building regulations approval, any new structural opening into the conservatory will require approval - even if the conservatory itself is an exempt structure. Glazing and fixed electrics may also require approval.

Please also think about the practicalities of deep excavations. Do not enter the excavation under any circumstances unless it is adequately shored. This point cannot be overstated.

Was a raft foundation considered?