DIY Doctor

Foundation for single Out building / garage + concreate Mix

Postby abdulm01 » Sun Oct 28, 2007 12:26 am

Can you please advise me on how deep and wide I will need to dig for foundation of a single storey out buildinng at the back of my gardern. I am planning to build with concreate blocks.

The size of the single storey out buildinng / Storage garage :
The Height Will be less than 3 metre
Width Will be approx 6 metre and
Depth 3.5 metre

There is a mature tree approx 1.5 to 2 metre away from my proposed build. The soil is firm clay (After digging approx 600mm). - I was planning to dig approx 600mm deep and 400 wide. or do i need to dig less then this ??

Please also advise me concrate mix for the foundation (Cenemt and Balast) 1 cement to 6 balast??

I am new to this and very much appricate your help and advice on this

many thanks

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Simply Build It

Postby andysarah » Sun Oct 28, 2007 9:35 pm

Clearly the building will be in the zone of influence of the tree, what this means is the soil is likely to be in a desiccated state from the tree, when the trench for the footing is dug it will cut off tree roots and the soil will with time re hydrate and swell. This is where the problems come from, the swelling clay; if the tree was removed you would have the same problem so that is not an option. Building control as I would, would follow the \nhbc guide on building near trees chapter 4.2. with the little information given the following would have to be adopted; the clay soil would have to be considered as high shrinkable soil as not testing has been carried out and the tree would have to be taken as a high water demand tree, so leading to foundations probably in the region of 2.5-3m deep. Coupled with this you would need 75mm claymaster compressible board on the inside face of the external footings and the ground floor would need to be suspended, either timber or precast concrete.

This may be an ideal project to be mini piled with reinforced concrete ground beams etc.

I would contact building control to get their comment on it before you get started, you may need to contact a Local Structural Engineer.
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