Deep foundation (over 2.5m) cost or advice please

Avasmokey

Hi,
We are embarking on a project to replace and increase the width of a current extension.  We have a builder in place who is hoping to start in about 4 weeks.  We also have a building control officer arranged and a structural engineer who is coming at the end of next week to put together more detailed plans wrt steels etc..

We were aware that any trees nearby may affect the depth of the foundations that are required.  We hadn't thought that we would have an issue with this but we had failed to consider a very thin but very real eucalyptus tree close by (in our neighbour's garden).  Having run my very amateur eye over some tables I think we may end up with a situation whereby we are required to have foundations over 2.5m deep, which from what I can gather would have to be designed by a structural engineer.

The existing foundations are 1.5m deep, and run along 4M width.  We will need to extend these existing foundations by another 3 M to give a 7M wide extension (so we will need 3m brand new foundation).  The beginning of the existing extension is the closest point to the tree, with the distance increasing as you move along to the new bit.

I realise this is a very big question but I'm loosing sleep over the implication of needing structural engineer designed foundations of over 2.5m deep.  Is this something that is highly unusual and very specialised?  If anyone could give me a very very rough idea of how much this might increase the cost as I literally have no idea....

I assume the first thing to do is ask the building control person to visit asap?

thank you.

Rosa_Damascena

Its gonna be expensive, very expensive. (Voice of experience although I can't offer a price, it was all included within a bigger project - but I can tell you that a 3m deviation to the floorplan cost and additional £2.4k in foundations alone).

Avasmokey

Hi Rosa, thank you for replying and for your honesty!  In your case was the 3m deviation just an extra 3m of foundation, or was it deeper than your original plan (like ours will be).

Thank you

Rosa_Damascena

Avasmokey said:

Hi Rosa, thank you for replying and for your honesty!  In your case was the 3m deviation just an extra 3m of foundation, or was it deeper than your original plan (like ours will be).

Thank you

It was near my neighbour's boundary and the trees they have never bothered to maintain, so went down 1.8m. One section went down to 2m, nowhere near 2.5m though.

stuart45

Do you know the soil type, eg high, medium or low shrinkable clay?

grumbler

AFAIK, deeper foundations are needed when trees get removed. This is because the removal disturbs water balance in the ground.  Not sure how it is if the trees remain. What happens if the neighbours remove the trees later? Eucalyptus are special - they are know to pump the water from the ground excessively.

Rosa_Damascena

grumbler said:

AFAIK, deeper foundations are needed when trees get removed. This is because the removal disturbs water balance in the ground.  Not sure how it is if the trees remain. What happens if the neighbours remove the trees later? Eucalyptus are special - they are know to pump the water from the ground excessively.

More than willows?

grumbler

Don't know about willows, but

Provided the growing location is suitable for a tree, some species of Eucalyptus are very efficient at draining boggy ground including sites such as a domestic garden, a schools sports pitch, or a farm. They have been used throughout history to reclaim land, or drain marshes to kill mosquitoes.

- a random quote from the Internet.

+ Cultivation and uses  from Wiki

FreeBear

The subject of Eucalyptus cropped up in another thread recently... Whilst they can grow quite tall, in this country, the root system is usually pretty shallow (typically around 300mm down), so tall trees are susceptible to being blown down in high winds. How close to the new foundations is this tree ?

stuart45

Foundation depths are taken from the NHBC calculator. NHBC Standards 2011 (nhbccampaigns.co.uk)
The Eucalyptus is listed as a high water demand tree with a mature height of 18ft.
If you scroll down the tables it gives you the foundation depths needed in different clay types.

stuart45

grumbler said:

AFAIK, deeper foundations are needed when trees get removed. This is because the removal disturbs water balance in the ground.  Not sure how it is if the trees remain. What happens if the neighbours remove the trees later? 

When trees are removed the clay expands and causes the ground to heave, so 3 years is required after removal.
If the neighbour removes the tree later the foundation depths are deep enough to deal with any heave.