House extension foundations - Piling or raft?

[rattwood]

Hi

I am confused. I have had plans for my extension approved. The architect suggested a raft as there are trees along the boundary and the building will start 1 metre away. A builder came to give me a quote and suggested I go for piling thinking it would be cheaper than the raft. I asked the structural engineer and he agreed. Although the structural engineer also suggested getting a geotechnical engineer involved.

I just want to know will a raft suffice (which another builder is certain will be cheaper) or do I need piling? It seems like the structural engineers won't confirm unless I have the geotechincal report but that is another £1000.

I have had 2 quotes for piling which are coming in at around £30k. Could this really be the cheapest option? 1 of the quotes included 17 piles at 13m each. Is this necessary? The extension is replacing a double garage with a 2 storey extension, the footprint is 10.2m x 5.1m.

Apologies for all the questions but I am confused by it all and reluctant to spend any unnecessary money on reports if the foundations are going to cost £30k

 

[Buck_68]

Is it a case of, if the foundations are going to cost 30k then the extension won’t go ahead?

If so, I can understand your reluctance.

Has the builder not given you a quote for a raft since he has suggested it from a best value position?

If you’re wanting to go ahead then spending £1000 to get an accurate representation of the land and therefore indicating which foundation is best is a small price to pay vs £30,000 for the pile foundation which may not be necessary or to the spec indicated.

The survey should better indicate the level of piling required as well? Both depth and quantity.

This way you can make a rational decision based on facts which I don’t think you have at present.

 

[rattwood]

Thanks for replying.

If the foundations are going to cost £30k then I am going to have to changes plans significantly to make them affordable. I am reluctant to do this I fear I will end up with an extension that doesn't meet all my needs.

I suppose I was expecting that one of the people I have dealt with (architect, builder, structural engineer) would have been able to tell me exactly what I need.

I am coming to the conclusion that you gave, that the £1000 geotechnic survey will need to be done to get a confirmed answer for what I need.

On the piling quotes, do they sound excessive? 13m piles sounds a lot.

 

[Notch7]

I am doing an orangery for a customer. He has an oak tree about 4 metres from the foundation.

Building control told me NHBC tables would put the trench foundation at around 3 metres deep. The customer then did a trial dig and took samples at 1 metre and 1.4m. He took them to a lab that tests soils which proved the ground has a high load capacity and recommended the foundation depth to be only 1.0m which building control accepted.

I would suggest trying to find out what type of soil you have. If its shrinkable clay soil with low load bearing capacity, that would indicate piles are the only option.

If the ground is not shrinkable and has high load capacity a raft may be a possibility. You also need to know if the ground is virgin rather than backfill and how far down the top soil extends. If solid ground, ie good subsoil is found not too far down a raft may be achievable.

The problem with a raft is that it is floating and may move differently to the house. However piles are probably different to tge house founds as well.

I use this company for structural calcs and they do rafts http://www.quick-calcs.net

There are others online as well.

I guess a 10m x 5m extension is going to need 7 piles across the front then 3 rows of 2 goung back to house. Im assuming you will a sleeper wall for the block and beam.

It may be worth getting more quotes for piles, I got about 6 quotes for 7no piles about 8 metres deep for a job. Prices came back between £3.5k to £8k. That was piles only not reinforcing or casting skirt.

Sometimes concrete pads with steel beams in between can be an option. Im not sure if it will help on a 2 storey jobbie though.

I guess a conventional 1metre found and oversite slab would only have been roughly £10k, so piles would be an extra £20k?

 

[^woody^]

I can't see how a raft could be anywhere near the cost of piling.

May be too late now, but the architect has a duty to design economically and to the clients needs and budget, and not just specify any old how ignorant of costs and practicality.

Also, take builders suggestions for major structural deviations from the approved plan with a pinch of salt. Who's to say that this builder does not have a friend with a piling rig and some spare time, and does not have the skill to form a raft?

Get more quotes for the work to the approved plans. When the work starts, builders should stick to the plans.

Btw, it would be normal for your engineer to specify the required design of any pilings, not the piling contractor. For obvious reasons.

 

[Notch7]

the architect has a duty to design economically

I never knew that. I come across quite a few jobs that have planning consent and building regs drawings but the scheme has been abandoned because prices come back from the builders way above the clienrs budget. Sometimes the architect has given the client a guide price way below actual builders quotes.

Theres always a bit of chicken and egg, because builders wont want to quote without b/regs drawings and so clients end paying for both planning and building regs without assurance they can afford to do it.

Its certainly possible a builder may suggest piling if he doesnt have experience of constructing rafts or doesnt know how to price them.

A raft cant be anywhere near as costly as piling. A raft wont need a suspended floor like piling and so it wont need a central sleeper wall either.

Rafts need a fair bit of steel reinforcement, the structural engineers price needs to include a bending schedule. Its quite straightfoeward because there is a British standard for the bends, the engineer just uses the standard notation for each bend.

There are companies that specialise in groundworks and you may find one near you that is into reinforced concrete and formwork. Maybe you could get a builder to do your extension excluding the foundation, and you arrange that part yourself.

Also as Woody says, how did the piling companies price the job without a specification? 13metres seems quite deep. A house built not far from me had 11m piles but that was on alluvial silt -a river bed basically.

Ask around your neighbours that have had work done to get an anecdotal knowledge of local soil type. Goggle searches may help as well. Im pricing a job at tge moment with tree issues. A bit google seaching and I found a complete soil survey for developmemt just a few 100 metres away. Soil type can be very localised, so ut wont prove anything, but does give you a good approximation of what you reasonably expect.