DIY underpinning, how far to dig under?

[nuzuki]

Hi all,
We have a block built summerhouse which was built on hardcore, then 6inch slab, 4course tanking wall (substructure going 70cms underground), at ground level there are 9 (maybe 10) courses of 100mm standard building blocks, then wooden roof structure.

Yesterday I decided to underpin one corner of the building and intend to do the other 3 corners over the course of the summer. I dug down about 30cms under the slab and in about 20cms (hole was 105cms deep from ground level). I just wondered if I dug far enough in under the slab?

Ive done one half of one corner and hammered rebar into the mud so when doing the other half it will bond nicely. I got down to very hard thick clay before pouring a strong mix of ballast and cement (3 or 4 to 1 mix). Ive never done this before and only read up on it quickly. The reason im doing this is not just to make the building last longer but I had leftover materials from another job and also there are trees nearby the building so my theory is that this would toughen the building up a bit.

Any advice would be greatly appreciated, thanks

 

[John D v2.0]

Seems excessive to do all that work when the existing foundation is working OK. You only need to underpin enough to spread the load of the walls over a wide enough area of the ground. So as long as you've gone down below the root zone, for a building that small it doesn't need to be too wide.
The worry would be by underpinning parts of it that those bits will move at different rates.
Have you checked whether the trees are the type likely to damage the foundations?

 

[nuzuki]

I havent checked but have heard of a chart that tells you which trees are a worry and which are not. There must have been around 20 trees on the plot that this building was built on so I was worried about heave. We have cut down most of the trees and pruned the rest. My concern would be that as the roots rot the slab would resettle and crack hence me thinking the underpinning would be a good idea.

 

[tomfe]

I don't really think this is a good idea.
If you have trees which are going to cause problems then 1m down is going to do little.
By supporting the corners more then there might be movement in the middle.
You underpin under the foundation not the slab.
By taking down the trees you might get ground swelling due to the extra water. If you had no problems before managing the trees would have been better than removing them.

 

[nuzuki]

Thats just the thing Tomfe, the foundations are non existent pretty much. Well there is a 30cm by 30cm trench which I managed to locate and dig under. Some of the trees were on the actual plot so had to come out. Others were roughly a metre away and very tall so would of interfered with the building in high winds. The other trees that were left we have chopped enough so they are still alive but not tall enough to cause issues. Does anyone know which trees are the worst offenders? I read somewhere that silver birch was particulary bad.