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Foundation - how deep shall we dig?

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The SE suggested digging 900mm deep because the soil survey said we had clay.  And we have trees.

I used the NHBC foundation depth calculator and did alot of reading around foundation digs.  Overthinking it all, alot.

BC said to dig to 700mm and see what was there.  Guess what?  No clay (well, only a tiny patch amongst loads of gravel).

So the SE suggested a new depth of 200mm.  But we are already at 700mm I said.  No problem he said.  Fill it back in, with crushed concrete and then Type 1 on top.

So £5K for muck away and another £7K to fill the hole we didn't need to dig.  Did I mention our contingency was gone?

But we have extra secure foundations.  BC happy.  And we have moved on.


A photo blog seems like the simplest way to show what has happened.  So here we are digging it out.






We even shovelled a bit by hand late one night.  




We laid some ducts.  Thanks @JSHarris for swift assistance on getting those in properly.  For about 10 days we got up at 6am, worked on site for 2 hours, went to work, got home, worked on site til 11pm or later.  Couldn't have MBC turn up without it being ready.




Then we put the crushed concrete and Type 1 back in the hole and compacted it to level.  Just in time.  MBC arrived the following morning.  Its now 4th July.



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Having dug deeper than you "need" I would have just poured more concrete rather than filling it in again.

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1 hour ago, ProDave said:

Having dug deeper than you "need" I would have just poured more concrete rather than filling it in again.



It's a passive slab, though, so no concrete underneath.

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Would it have been much more expensive to fill the hole with EPS? Depends on the area but quick calcs say not a lot in it. E.g., 150 m², 0.5 m depth, £87/m³ → £6525. That's for small quantities of EPS70. Stronger would be more expensive vs a large quantity.

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How could the SE "suggest" a foundation depth without having full knowledge of the actual ground conditions investigation with trial pits?

How does he know the actual bearing capacity of the ground without trial pits and onsite testing?

The correct process would be to

1) Look at historic plans to ascertain what was there before.

2) dig some trial pits to confirm the above and to see exactly what's in the ground. If clay - test for shrinkability to allow the NHBC calculator to be used.

3)Design a foundation. If it is clay have at least the recommended cover/depth over the foundation to avoid frost susceptibility.


If the SE has not done this then change your SE. At present it could have cost you £,0000s

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12 minutes ago, Tyke2 said:

If the SE has not done this then change your SE. At present it could have cost you £,0000s


Assuming a passive slab was planned from the start, 900mm seems an awful lot to be digging down, even if clay was a possibility.


Please don't think we're having a go here, by the way. It's just that someone may read this in the future looking for information, and it will be helpful for them to understand how this all happened (and how to avoid it!)

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