Foundation depth and options for garage next to trees

[pg325]

I'm looking to build a garage measuring 6m x 3.5m to side of my house.

The garage will go up to the boundry line, to which the otherside has a variey of close trees (1 to 2 metres away). The local council has confirmed that if the trees are oak and Hazel which I believe they are, the foundation needed will be an engineered design solution.

Does anyone know what exactly this is, and more importantly roughly how much this type of foundation will cost for 6 x 3.5m.

 

[jeds]

With an Oak tree one or two metres away you'll probably be looking at piling. Are you on clay?

 

[jeds]

With an Oak tree one or two metres away you'll probably be looking at piling. Are you on clay?

 

[^woody^]

It's not a double garage, John

 

[^woody^]

A raft should be OK?

 

[pg325]

The local inspector has sent me a few details via email confirming the soil in my area is likely to be firm medium shrinkability clay.

Whats the only way be sure, is it a case of submitting the plans and then test digging or just submitt with a Raft design?

 

[JMSD]

NHBC give guidance on building near trees which is widely adopted. If you know the type of tree, distance to the tree and the shrinkability of the soil you can use this guidance to look up the depth of concrete strip foundation that would be required from a table.

As you are close to a large tree in shrinkable soil the tables might suggest that the footing needs to be very deep i.e. 2.0m or more which is obviously not economical or practical to build. If this is the case you might consider a couple of mini piles (not necessarily as expensive as some people will tell you they are) or setting the foundation further away from the tree to minimise the depth and using some kind of arrangement of cantilever ground beams to build the garage wall off.

It may be possible to agree this with the BCO on site, I think it's more likely that you will end up needing some kind of professional advice for this one though.

I would be vary wary of the idea of using a raft.