Conservatory Foundations

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4 Dec 2004
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United Kingdom
Sorry to repost this, but I am desperate to sort it, as the footings are due to be poured first thing on Tuesday, so I will have to speak to the builder tomorrow if there are any alterations.

Conservatory footings have now been dug to support a dwarf wall 450mm high. Footings have been dug to 580mm deep and 400mm wide all round. There is 450 mm difference in height from top to bottom of slope, over the 6.2 m length of structure.

Is this depth and width OK? (my plan spec reads 1000mm x 450mm) The difference in height will clearly affect the footings depth, as, by my calculation, given level foundations, the depth of concrete on the high end of the building will only be 130mm, unless the footings are stepped up, which I hope is the plan. The floor level will be 210mm above the top of the trench at the high side. The floor spec is 100mm hardcore, DPM, 100mm steel reinforced slab, 75mm Celotex insulation, and 75mm (give or take) screed. say 350mm. The wall is to be brick and Celcon block, with a 50mm cavity.

Any help out there tonight?

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450 wide is fine, unless it's cráp ground, when any width would be unlikely to work.
1000 deep is generally too much, but it all depends on ground condition.

You may hit firm ground at 300/500/700 who knows? The existing house foundation level would be a good indicator

If you have a 450 difference in levels, then its fair to say that there would not be some magical step in the ground. So for instance, soft ground at 300 deep at the lower level could mean soft ground at 750 deep at the higher level.

The ground at the lowest level should be the reference, and then if conditions allow step it up, or make it level if need be.

I would be allowing 3 steps of 150 if stepping it is required
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I would be allowing 3 steps of 150 if stepping it is required
Two of 225, with 600 overlap is fine, providing the footing beyond the step is min 225 thick.

Don't get the comment about good and bad ground, Woodster: if the ground was ok at the lower end anyhow, then there's no reason to assume that the higher level is over rubbish, it's just a case of stepping up to follow the existing ground contours.
The ground all looks firm, although it was made up about 30 years ago, sub-soil is mainly chalk with some clay. I'll ask him to put in two steps, I take it by overlap you mean where it will be double thickness for 600mm run?
The wall it is going against is an extension, footings much deeper as originally made up ground.

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