foundation depth

8 Jun 2010
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United Kingdom
Can anyone confirm this is correct?

I am digging out for a trench fill foundation. My understanding is depth is always measured from Natural ground level (NGL) down.

I have a slight slope (270mm over 6300mm) so I've sketched out the diagram below showing a 1m foundation.

Its not to scale!

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I would think the bottom of the trench should be horizontal. A sloping base would create a sideways force that could result in the foundation trying to slide down the slope.
The slope is 0.04% so very small but i could introduce 2x 15 mm steps instead of a constant slope. To be honest I think the whole 27cm would get 'lost' somewhere along a 6.3m run anyway.

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Don't expect to hide a slope from gravity. You may have lost it from the naked eye, but gravity works the same whether things are obvious or not. That's why we always put the handbrake on the car!. For the sake of a bit more concrete and digging, do it properly.
In principle, of you hit good ground at 1m at the high end, that same good ground will be level and so at 700mm at the low end, and this should not affect its performance.

Alternatively, depending on the ground conditions, you could go 1150 at one end and 850 at the other, or thereabouts as long as the base is level
Ok somewhat confused now.

Are we saying that the bottom should be Level and therefore we accept that if we dig down 1m from the 'high' end we only have to dig below ground to 700mm at the low end as the foundation would still be a total of 1m although 300mm will be above ground?
Very interesting question. I was thinking if I was a little bit of concrete some where in the foundation block, what would cause me to try to move sideways away from the house? Needs a sideways force and I can't see where its coming from. If the foundation was laid as a wedge, then there is more weight of of it over the "thick" end then the thin end, so its like a see-saw that is overbalanced and its trying to rotate its balance point over the central point of the pivot in the the ground. I reckon foundations are stepped so each step has its bit of weight balanced over the pivot point so each step wants to stay horizontal. Imagine a plank laying on your lawn with a pile of bricks on one end of it. eventually the heavy end will sink, because its balance point will be way off the central support point, so there will be a large rotational force on it. So the real question is "how out of balance are the forces on your foundation block?". No one seems to worry about the weight of the back wall of an extension (doubling or more) the ground pressure under the foundations under the back wall. I suspect that the bits do move around (settlement) but only by fractions of a millimetre until the forces balance out again. If you had a precision measuring device you could actually measure this. On very old castles etc. you can see walls that are out of plumb, yet have stood for hundreds of years.
I would make a decision about what the minimum thickness the foundation should be, lay the block to this and step the top so make your block laying easier.

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