Over specifying Foundation Depths

[Paul-man]

After a visit from my building inspector last week, he has advised that due to the trees we have in our vicinity (An Oak 15m from the front of our works and a Eucalytus 16m from the rear of our works - side and rear extension) we will need to increase the foundation depths to what we have on our plans 150cm.

The BI has said we will need 175cm along the rear (near the Eucy) and 200cm at the front near the Oak) but said that we can come up from those levels in between. The site will be about 10m from front to back.

Apart from reduced muck-away and concrete costs, is there any other benefit in designing our foundations to be a bit shallower along the sides? My understanding is that groundworkers want to be in and out with their diggers as quickly as possible and faffing around stepping the depths up and down could be more costly than just digging the whole lot to 200cm. (or perhaps 175 along the 6m at the back, then 200 for the rest?)

What would you do?

 

[Blagard]

I would do stepped foundations. If you go deeper you know it is more muck away and more building to do.

Your only problem is setting out clearly, so the man on the excavator knows where the steps are and the depths. It really should not take any longer at all, maybe a bit less with less to dig out! You will not have that many steps.

 

[Static]

You also risk undermining your main house foundations if you dont step them..

 

[Paul-man]

Thanks gents.
Out of interest Static, how could it undermine the main house foundations?

One of my biggest concerns is that I have read conflicting views about the impact of Eucalyptus trees. I've read that they aren't as bad as the NHBC make out, but because the NHBC guidance is the only thing that is out there- it tends to be what is used. Therefore, I am probably already overspecifying a section of my foundations and I would also hazard a guess that the Eucy was not in place when the house was first built- so I will still end up with deeper foundations in the extension than on the main house...

I have asked my designer to step the foundations anyway based on Blagard's first post. I have to leave a 30cm heave prevention void under the floor, so there is going to be a lot of muck coming out!

 

[Static]

IF your existing footings are shallow and only 450mm-600mm deep and you suddenly step down to 2m+ the forces from your existing footing will be exserted on the new footing which can overload it.. so worst case is the new footing sinks a little and you crack the existing footing at that spot causing cracking internally to the building..
All of which is a nightmare to stablize.. so step down by 500mm every 1m till the required depth.. its not all that tough to do in clay..

Problem with trees is that they grow.. ecalyptus can grow in excess of 20m high so if left unchecked could have roots out 15m odd from the tree..