Foundation Advice

[Paul-man]

Am worried I may have made a wrong choice when dealing with my Architectural Consultant regarding my foundations.

Firstly the job: Double storey rear extension to rear of a late 80's or 1990 built house. (Tarmac Homes), in place of existing conservatory. Footprint of extension approx 4m x 5m.

Clay soil. Mature Oak to front, 15 is metres from site. Eucalyptus to rear in neighbour's garden. About 8 metres from site.

Sewer (serves us only) runs 500mm below ground level across the site.

Existing house is built on a 250mm engineered slab foundation.

Had 1 set of BR drawigns approved by BC with 1.5 - 2.5 trench foundations, with B&B floor on top.

After changing design and getting plans drawn up by a new designer, they specified standard 1.3m deep. After I reminded them about proximity of Eucalyptus and told them that the existing house was built on an engineered slab, they are now talking about getting a S.E to design a Piled Raft foundation.

They have informed me that foundation types cannot be mixed - ie, as the existing house is piled raft (are all rafts piled? because all I have told them is that there is a raft- they may have spoken to BC though) Is the above point correct? If so, why did BC approve the last set of drawings, when they knew that the existing was a raft type foundation?

What I don't want to do is under-spec things, as we hope to be here a long time. I also don't want any nasty surprises during the build process, so want to budget before I start rather than digging the garden up and realising it is all too costly.

Architectural Consultant has quoted about £325 for the design of a piled raft (by his S.E) and told us it is likely to be about 1.5 times the cost of regular foundations.

He seems very reluctant to resubmit the previously approved design which had the Block and Beam floor- mainly because to allow the 300mm cavity below, we would start encroaching on the drain which is 500mm down.

Any general advice? I have already authorised the SE to design the raft, but just wanted to check I wasn't making a huge mistake- would rather write £325 off now if I will save £'000's.

 

[Static]

They are correct that it is best to stick with the same foundation.. reason being you want your house and extension bearing into the same material.. therefore you dont get settlement between the two areas..

However suggest they go for piles with pile caps and ground beams between.. therefore you can still have a suspended floor..

 

[noseall]

We managed to merge a standard strip foundation with a raft, last winter.

This was only after a fair bit of back and fourth betwixt an SE and building control, along with some petty chunky reinforcing mesh to boot.

The property we were working on was on the peripheries of a peat bog, but only just. A test hole 2.4m deep had to be dug to see if there was any presence of peat, which there was not.

Had we been working on the next street which is closer to the problem area then they would have insisted on a raft.

I don't see how differential movement can be a major issue considering the amount of modern extensions that are bolted onto old Victorian splayed brick footings etc.

 

[Static]

There's a big difference when talking about piled foundations.. you pile because the top material is rubbish, so if you then found an extension in that poor material it will move differently over time compared to the existing founded on material thats 10+meters below ground..

 

[Paul-man]

Hi, thanks for the replies on this so far. This is where I am a month and a bit on:

Designs received for a piled raft, consisting of 6no. 8m open-bore piles with a 225mm cast-in situ raft constructed on stop of Cellcore and tied to the piles.

Have had a soil test to 10m to check suitability for piles and although results arent back, the operatives reckon we should be ok.

Have had a couple of quotes around the £10k mark which was about what I expected.

When I mentioned this to my neighbour he thought I was mad! Apprantly he has deep trench foundations (1.7m +) with a cast in situ slab.

I phoned my structural engineer this morning who told me I'd have to decide which to chose.

Can I still go for the piled solution with trench foundations at my main house? At what depth of trench foundation (if any) would the piling break even or at least be close to being a competetive option cost wise?

 

[Static]

2-2.5m is the rule of thumb..

 

[Paul-man]

Thanks.

As long as Building Control are happy for me to mix the two foundation types, I'm tempted to stick with the piling...

 

[ban-all-sheds]

If you're building over a drain don't you have to go down to below the invert level anyway?

 

[Emeraude119]

Raft foundations are normally used where ground conditions mean that a traditional foundation or even a short bored pile system would be uneconomical. Typically where a suitable bearing strata is just too deep to reach.
The depth of piled foundations can vary, and may be dictated by the size and cost of the above ground part of a building.
For domestic properties deep piles are generally not an economic option. but for large buildings, deep piles may be a more viable proportion of the total build cost.
Raft foundations are intended to spread the weight of a building over the entire ground floor area, so that like on a raft the building `floats' on the surface of the soil on which they are built
(This however does not mean that they are not prone to movement in the subsoil) All buildings undergo a level of settlement, and where this is uniform as it is likely to be on a raft foundation, there is not normally any major problem. The real killer for buildings is where differential settlement.
occurs.

 

[Emeraude119]

P.s as general rule, buildings should not be constructed nearer to a tree than its predicted mature height, but with building land becoming scarcer and more expensive, we are now building on sites that would have been avoided not so long ago because of issues like this.
Even cutting adjacent trees down is not always a solution, because ground waters that tree would have normally taken up and sent to atmosphere can then build in the ground causing heave in some soils.

 

[ban-all-sheds]

The real killer for buildings is where differential settlement.
occurs.

Indeed.

Good luck extending an old house sitting on a concrete slab on clay soil.