Deep foundation (over 2.5m) cost or advice please

Sistergold

I have an extension planned, also trees in neighbours garden going to affect my foundations. Instead of digging deep traditional foundations, they suggested using piles or something? Piled foundations will avoid dealing with too much soil. Either way due to trees they told me foundations is going to be expensive. Needless to say I have not yet gotten round to doing the extension, still scratching my head before I start. 

Harrywayon

Very expensive may need to move pipes underground if you not careful could damage neighbour's pipes then all the dust , rubble you have to clear.

Section62

Avasmokey said:

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.

...

Is this something that is highly unusual and very specialised?

No, on clay soils, or with trees in close proximity, it is common to have extra attention paid to the design of foundations.  If you need a SE on the project already then it isn't anything to worry about - particularly as they will need to consider how the loads on the steel beams will be taken down to ground level, and that inevitably means thinking about the suitability of the foundations.

One of the potential problems you could have is with differential movement between the part of the extension with 1.5m foundations and the newer part with (possibly) deeper ones.  It isn't ideal to have shallower riskier foundations closest to the tree and more stable ones further away - this is really the opposite of what you'd really want.

Therefore the detailing where the old and new foundations and structure meet will be really important.  One solution sometimes used is to drill into the exposed vertical face of the existing foundation and epoxy in short lengths of reinforcing bar, to provide sufficient 'key' between the old and new foundation.  Another approach is to design in a movement joint where the old and new structures join so they can move independently of each other.  A good SE will come up with the right solution in your case.


The main thing to be concerned about with any deep excavation is the importance of supporting the sides of the trench - the deeper you dig the more vital this becomes. Also, if people need to enter the trench (e.g. to clean/prepare the existing foundation) then it is absolutely essential the trench sides are properly supported so there is no risk of collapse and burying the worker under tonnes of soil. Proper trench support costs time and money and is something often skimped on or avoided completely in order to save cost and hassle.

Although you wouldn't necessarily be held responsible if an accident were to happen, having someone killed or seriously injured on your property is not a pleasant experience. A trench collapse could also threaten the structural integrity of your property.

So if deep excavation is required you might want to have a read up on the Construction Design and Management Regulations so you know who is responsible for what, and the right questions to ask of the people doing the work for you.
https://www.hse.gov.uk/construction/cdm/2015/index.htm

stuart45

This one was done in low shrinkable clay with trees a couple of metres away. Went down about 2.2M below the base of trees. Clay board on the inside of trenches. Original footing only about 2 ft down.

GDB2222

Party wall agreement is in place?

grumbler

GDB2222 said:

Party wall agreement is in place?

Is it really needed? It may be, but not necessarily.

GDB2222

grumbler said:

GDB2222 said:

Party wall agreement is in place?

Is it really needed? It may be, but not necessarily.

Indeed. But, the increase in depth of the foundation may alter the requirement.

grumbler

GDB2222 said:

grumbler said:

GDB2222 said:

Party wall agreement is in place?

Is it really needed? It may be, but not necessarily.

Indeed. But, the increase in depth of the foundation may alter the requirement.

Why? Do you really mean the agreement, not a notice (if the excavation is close to the neighbour's house)?

weeg

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.

GDB2222

grumbler said:

GDB2222 said:

grumbler said:

GDB2222 said:

Party wall agreement is in place?

Is it really needed? It may be, but not necessarily.

Indeed. But, the increase in depth of the foundation may alter the requirement.

Why? Do you really mean the agreement, not a notice (if the excavation is close to the neighbour's house)?

"To fall within the scope of the Act, your proposed works must include excavation within 3 metres of either a neighbouring or shared structure and to a greater depth than the base of the foundations to that structure (this is extended to 6 metres if your foundations are particularly deep – such as piled foundations)."

https://www.peterbarry.co.uk/blog/the-party-wall-act-common-misunderstandings

Not a problem if they have a PWA in place, but they may have thought they were exempt. Hence, I asked:
"Party wall agreement is in place?", as a preliminary.