Deep foundation (over 2.5m) cost or advice please

Rosa_Damascena

weeg said:

I wouldn't be losing sleep over it. See what your SE says, but in most clays the ground is pretty self supporting, so you are just looking at digging a trench and filling with lean mix concrete.  Your SE will (in all likelihood) have factored in a foundation check as part of their quote.

No one on the internet can really give you a ball park figure as there are too many variables we don't know.

Au contraire, I would be if I didn't know what I was doing / was going it alone. I don't, but my builders did and also worked well with BC who to be fair were very helpful. It's pne part of the council machinery that actually works.

FaceHead

Yes a 2.5m foundation would be expensive. The direction I would look at it from is not 'how much will that cost', but 'what is the maximum I'm prepared to pay, over which I'd want to reconsider this project.' Your builder will eventually tell you the answer to the first one (if applicable) so you only need to answer the second question, by considering the value it adds to the house, local property prices and how much you want to do this. 

Without having seen the site or even a plan it's impossible to say what depth of foundation is required. For a not-immediately-obvious 'thin' tree a 2.5m foundation seems excessive. Get your SE and BC in a room and you'll soon have an answer, and then you'll know what you're asking a builder for. 

Is this single to two storey? Even the 1.5m should be suitable in many circumstances, so I would be lobbying (if it seemed appropriate) the SE to retain that and dig a new run of matching 1.5m for the new part. 

weeg

Rosa_Damascena said:

weeg said:

I wouldn't be losing sleep over it. See what your SE says, but in most clays the ground is pretty self supporting, so you are just looking at digging a trench and filling with lean mix concrete.  Your SE will (in all likelihood) have factored in a foundation check as part of their quote.

No one on the internet can really give you a ball park figure as there are too many variables we don't know.

Au contraire, I would be if I didn't know what I was doing / was going it alone. I don't, but my builders did and also worked well with BC who to be fair were very helpful. It's pne part of the council machinery that actually works.

You're right - I phrased that poorly. 'Try not to worry about it, you've already made plans to get the right advice' is what I meant.

It's funny for me to think of a BC being the part of a council that actually work. Mine are totally understaffed and a bit of a shower. They were being forced to outsource to a nearby council for a while.

stuart45

In my experience a BCO would more or less stick to the NHBC guidelines in the past, although they have got their own calculator now which allows some flexibility with the plasticity index, Even if there is a building next to it with a depth of 1.5M without any problems.
If you look at the photo of the foundation the old footing had no problems but Building control still insisted on the new depth being 2.2M.
You used to find an experienced BCO who looked at a soil test where the plasticity index means that although it's a highly shrinkable clay it's close to being a medium one, and the tree is 8M away so in theory it should be an engineered foundation, but he would accept a strip one of 2.5M
These new depths for foundations near trees on clay soils came in after the 1976 summer. They are based on high deep the trees can affect the clay to change, so in reality they are often much deeper than is really required, but it's difficult to convince a BCO in many cases as they just refer to the calculator.