Concrete foundation notation

[oculushut]

Hi we have some structural calcuations and drawings which ask for the following:

• "Minimum foundation formation level 1000mm from ground level"
• "New 450 M/C Trench Foundation"

Does this mean that we need a trench foundation like the following?

Thanks!

 

[Bluepeppo]

Not quite. 1000mm is quoted as a minimum to prevent heave. It is important that it is regarded as a minimum as the final depth of the foundation must be determined by the ground type. It is normal to finish the concrete one or two block courses below ground level. I go for two (450mm), especially where there are internal soil pipes.
Blue

 

[Notch7]

1000mm depth is just a guide.

Actual depth depends on ground conditions and influence of trees.

Foundation widths are almost always either 450mm or 600mm as that is the standard size for digger buckets. Very roughly single storeys are fine with 450mm and 2 storeys 600mm

 

[^woody^]

I'd wait to see what the inspector says.

450 is not wide enough for a 300 wall, and the depth will always depend on the soil type and site features such as drains and trees.

 

[noseall]

450 is not wide enough for a 300 wall

I agree. We have always been asked for/specced 600mm wide for 300mm wide walls. BCO's always stipulate 150mm toe each side of the wall.

 

[Bluepeppo]

I'd wait to see what the inspector says.

450 is not wide enough for a 300 wall, and the depth will always depend on the soil type and site features such as drains and trees.

The width of the foundation is dependent on the ground conditions. See Table 10 of document A. I think it's safe to assume, given the terminology, that the original posters calculations drawings and calculations have been done with a degree of competence. As such I think it's therefore also safe to assume that 450mm is adequate. It's difficult to get a foundation in the perfect position so the extra width just allows for proper setting out.

I agree. We have always been asked for/specced 600mm wide for 300mm wide walls. BCO's always stipulate 150mm toe each side of the wall.

I've never had a BCO stipulate that. But then, I have no idea what's underneath the grass in Staffordshire.

 

[noseall]

The width of the foundation is dependent on the ground conditions.

450mm for a 300mm wide wall? Where are the brickies going to stand?

If you are going to pinch a 300mm wide wall onto a 450mm wide footing, be sure to spend double the money you save on an accurate digger driver.

Only ever use the 450mm bucket for single brick walls.

 

[Notch7]

450mm for a 300mm wide wall? Where are the brickies going to stand?

If you are going to pinch a 300mm wide wall onto a 450mm wide footing, be sure to spend double the money you save on an accurate digger driver.

Only ever use the 450mm bucket for single brick walls.

I have to say Ive done plenty of orangery jobs with a 450mm footing, I realise most oangeries arent as heavy as a single story extension with tiled roof but they often have full height walls.

I agree that 75mm each side doesnt allow for much error but on a flat site it should be possible to get a foundation trench within 25mm.

Of course in crap ground or wet weather if trench collapses happen the foundation can easily end up 800mm wide. That period between digging out thevtrench and castibg is always a bit of concern -its nice when the concretes done.......

 

[Bluepeppo]

450mm for a 300mm wide wall? Where are the brickies going to stand?

If you are going to pinch a 300mm wide wall onto a 450mm wide footing, be sure to spend double the money you save on an accurate digger driver.

Where will they stand?
On the last domestic extension I did there was over 1.2m of uncompressed fill before I got down to the original top soil. It ended up at nearly 1.8m deep . 12m of strip meant "pinching" 300 onto 450 was the only affordable solution. Like a true Yorkshireman.

 

[^woody^]

The width of the foundation is dependent on the ground conditions

Oh I see. So the op can sit his wall on a 300mm wide strip then if the ground is ok, and ignore the edge shear from the wall.

 

[^woody^]

I have to say Ive done plenty of orangery jobs with a 450mm footing,

And you've gone down at least 1000mm or more to suitable bearing soil where necessary? Or are these done without building control?

 

[Bluepeppo]

Oh I see. So the op can sit his wall on a 300mm wide strip then if the ground is ok, and ignore the edge shear from the wall.

Yes the OP could indeed if he/she chose to ignore whoever has done the calcs and drawings. Line 1 of Table 10 of Doc A. https://www.planningportal.co.uk/info/200135/approved_documents/62/part_a_-_structure.
Edge sheer? This could only ever be an issue if the compressive strength of the foundation was lower than the walling material.

 

[cve80671]

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[Leofric]

^woody^ said: ↑
450 is not wide enough for a 300 wall
I agree. We have always been asked for/specced 600mm wide for 300mm wide walls. BCO's always stipulate 150mm toe each side of the wall.
Correct.