Building extension, existing house footings v deep, options?

[coxsox]

Hi,
Now on my second extension to the house. With the first extension BC requested 1.4m deep footings, then on inspection stated they needed to be the same as the house, which I knew could be possibly 3m ! as the house is built on an old orchard. After some arguing I was cleared to proceed with 1.4m footings.

So onto the next extension on the same house, BC have been round and are more wiser this time. They have stated I need to go down as far as existing footings, which I don't think will be practical. I've dug down 1.5m and can not find the bottom of the house footing. I could go on digging forever, but the further down I go, the more concrete I will need to fill it all. I questioned BC about other options and he stated I could tie in the shallower extension footings to the house footings with steel bars drilled and resin'd in, but would need a structural engineer report for this.

I rang a structural engineer and suggested this to them, which they replied they would still need to know the depth of the footings, so I may as well keep digging. They also didn't think the steel bars option would be wise.

Does anyone know the practical options for footings, when the existing development goes quite deep, apart from raft, pillar etc. Trying as ever to keep costs down as extension is only single story measuring 6m by 5m. In the meantime I'm going to see how far down the digger will go in an attempt to really discover how deep the footings are, but I do remember the builder mentioning some were 3m's down! just hope its not my house.

thanks

 

[garyo]

Three metres? Good god. You have my sympathy!

 

[Static]

If the footings are deeper than about 2.5m then mini piling may be an option.. but you will still end up with a fairly hefty bill for either route..

Better soil testing means you can prove the ground pressures and design shallower footings without the need for matching existing (within reason)..

 

[^woody^]

I would not just think of trying to get a cheaper foundation and avoiding the potential higher cost of piles

If the foundations are no good and the whole lot moves, then you will really be into some expensive repairs

Any more than 2m and traditional foundations become impractical and not financially viable

 

[coxsox]

thanks, have a SE coming round today to carry out a soil test and will know more then. Have calculated that approx 18m3 of concrete for 2M deep foundations is gonna cost approx £2k, which is still the cheapest option at present. Hoping SE test clay and can advise further, I think any other option looks like it will be 3-4 times the cost.

 

[^woody^]

What about shoring the trench, props, cost, risk, rain, pumping out, time, labour etc?

Are those factors, or is it just the cost of the concrete?

 

[noseall]

2m deep footings are unlikely to be nice neat straight walled trenches.

Any disturbed ground, i.e. where drains may have been dug will collapse in.

And, as Woodcock has said you will need shoring.

2m is a nightmare dig and will need serious thought.

We did one that was a little over 1.5m in clay. We ended up making ply shutter boxes that were dropped into the trench every time we went back far enough to accommodate one.

It increased the cost of the original (1m dig) by several thousand pounds.

We have dug deeper but for only a short distance and we increased the trench width as we got nearer to the surface.

Any broken services at that depth will be a nightmare to repair so be careful!

 

[coxsox]

thanks guys, yes this may indeed end up being a nightmare, will know more in a couple of hours.

 

[coxsox]

ok, well SE arrived and carried out some soil tests. Turns out the clay is too dry so a few options were discussed for the foundations, from digging really deep to using pad stones, which would still require digging to same levels.

Of all the options suggested, the most viable one seemed to be using a raft as this would only involve digging down approx 600mm, but also using a timber frame construction. This would provide a lighter load to the raft and would be easier construction. The extension would also be separate from the house with expansion joints allowing movement.

Next step is to run this by BC and work out all the costs involved, but it will save cost of block and beam floor and blockwork etc. Now just need to research raft construction methods and timber frame buildings....