Foundations for porch in clay

[Mag2.0]

Hi all.

Started to dig the foundations for my porch over the festive period.
Im currently down 500mm below existing ground level and into firm clay. Its diggable if loosened with a pick or can be 'sliced' in small sections with a spade vertically.

Top of concrete will be 3 bricks below DPC so that means currently the foundations will be a total of 275mm thick.

Construction will be medium dense concrete blocks (rendered) outside + 65cavity + thermolite inner with a small lean to/hipped roof so not a huge loads involved.

Building regs states that for medium plasticity clays a depth of 900mm is required. However I'm not required to comply as its external to the house with existing door remaining.

My concern is with clay swelling / shrinking and causing movement later down the line.

Has anyone undertaken such work before and if so what depth did you take foundations to? Any problems later down the line?

I'd rather not create work for myself if its not needed so thinking I can get away with a lesser depth? Any previous experience much appreciated.

 

[^woody^]

The house foundation depth may be a better guide.

 

[KenGMac]

The link below should allow you access to the Institute of geological Science, there are "maps" of all areas of the UK and you can find from them what the geology is like in your area

The image posted looks like it is fairly stiff.

https://www.bgs.ac.uk/discovering-geology/maps-and-resources/maps/

 

[Mag2.0]

The link below should allow you access to the Institute of geological Science, there are "maps" of all areas of the UK and you can find from them what the geology is like in your area

The image posted looks like it is fairly stiff.

https://www.bgs.ac.uk/discovering-geology/maps-and-resources/maps/

Thanks Ken. Yes firm with pockets of stiff.
It's just the settlement issue I'm bit concerned with rather than Ultimate bearing capacity.

The house foundation depth may be a better guide.

Yes that's one indication. Thanks

 

[^woody^]

It's just the settlement issue I'm bit concerned with rather than Ultimate bearing capacity

It's neither of those with clay, rather the susceptibility to seasonal movement, or disproportionate movement from slight moisture changes.

 

[KenGMac]

Are there any trees or large shrubs in the vicinity?

During your excavations have you seen any root systens?

 

[Mag2.0]

Are there any trees or large shrubs in the vicinity?

During your excavations have you seen any root systens?

Thanks for the response KenGMac. No large trees. A hedge 1.8m tall and 2m distance away No signs of root system.

 

[KenGMac]

The lack of any indication of root systems is good in that the Clay will not have trees Etc. drawing moisture from the ground.

The [so called] London clay is very highly susceptible to water being drawn out of it by the action of trees and large bushes. in certain conditions, long hot DRY summers London type clay will shrink this effect is exacerbated by any trees and such within an alarmingly large area.