Foundation advice

[acurachris]

Hi

I have been using a building company to clear the site where my extension is going and they have been good at what they have been doing, although I think they may be taking advantage a bit now so a little advice here would be great.

They came and dug a trial hole to about 1.5m last week and said the ground was poor, which was kind of expected as a house down the road had pilings for their extension just two months ago.

The builders indicated that the building inspector (which i have yet to get out) would come out and insist on pilings, although they are saying it won't work as the ground is too tough to pile (brick/stone/rubble/general dumping ground) and it'll be a waste of money to try. They said i'll be best with a raft foundation and they would be at my house when the building inspector arrives to suggest/discuss with him the prospect of a raft.

The extension is a 27m2 footprint (l-shape). I am going to ring for more quotes this week, but they quoted £11,000 for the groundworks to DPC (normal trench foundations to 900mm), £17,000 if it needs pilings to 4m deep (plus extra if they go deeper), and now they are saying it's £2-3k additional to the piles for a raft.

Here are my questions and concerns.

I thought piles could go through pretty much any ground; Is there a chance the piles won't go down and install as required?

The extension isn't massive so £20k for a 27m2 raft, is that a lot?

Anyone had a raft designed and know a rough cost for design from a SE?

Anyone had a raft installed? What did you pay and for what square metre?

Sorry for long post.

 

[^woody^]

You shouldn't be adding a raft to a building on firm foundations as the there is potential for it to be bobbing up and down over the seasons differently to the main building.

You need a competent structural engineer to assess the ground and the loading to determine suitable foundations and not rely on what the builders say or did on their last job.

 

[Notch7]

My guess would be that if piles wont go in, you dont need piles. They will go through rubble, no problem.

Driven piles only 4m deep seems quite shallow to me.

I cant see any point your builders being present when the building inspector is on site. All the building inspector will say is, 'ground is poor, it requires an engineered solution'. Building indpectors might make judgements on trench foundation depths but they wont stipulate a type of solution, all they will say is get an SE to design it and send me the draeings.

Ive done rafts for orangeries, they didnt cost much more than conventional footings. The extra cost is the design, reinforcing bars, possible large amount of type 1 hardcore, site labour, shuttering etc.

 

[Notch7]

As Woody says, rafts are like a floating pontoon, rigid on their own, but differential movement can occur between host building and extension.

Not a problem perhaps for a timber orangery. Definitely a problem for a large brick extension wrapping round a corner.

Do you actually know your ground is poor? Is it mademup ground or maybe silt or shrinkabke clay......

 

[acurachris]

Do you actually know your ground is poor? Is it mademup ground or maybe silt or shrinkabke clay......

I suppose i don't know for sure. Only what i've been told and seen. Two different companies have dug two separate holes (one for soakaway test, one for ground test) and mentioned how tough the ground was to dig out. All kind of stuff in the ground it appears, bricks, rubble, clumps of something resembling black aerated block. Could of been the dumping ground for the houses built before ours! Hit the water table too.

Building officer already got in his mind that the ground is poor as he mentioned over the phone that my area is known for poor ground.

My main worry is the building officer comes out, says pile it, piling company start, fail and still want money for work they've done.

Do you reckon it's best to get the building inspector out first?

If I have an 'engineered solution' designed, is that final, over the building officer or can he still say no? Does the Structural Engineer supersede the building officer?

 

[Notch7]

? Does the Structural Engineer supersede the building officer?

The building inspector will accept a structural engineers report, if it is designed to be compliant. A SE has to underwrite every calculation he makes.

 

[^woody^]

My main worry is the building officer comes out, says pile it

That's not how things work.

The building inspector is an inspector not a designer or a site manager demanding things are done how he wants.

You get the thing designed and all the inspector can do is then either agree your proposal or reject it - but he needs to be brave to reject a properly designed solution from a professional who may well be more qualified than he is.

Where is your plan drawer/designer in all this?

 

[acurachris]

Where is your plan drawer/designer in all this?

He basically drew a strip foundation for the regulations saying to me, we'll not know until ground is broke and the building officer will have the say on whether a strip foundation will be acceptable. Then the builders have said that the building officer will likely suggest to pile it, although they reckon it needs a raft.

Should I get the building officer out first and see what he says about the trial hole that's been dug?

 

[oldbutnotdead]

He......... a raft.

Should I get the building officer out first and see what he says about the trial hole that's been dug?

That'll be the cheapest option up front. You never know, he might let you away with strip (low odds on made ground, which is what you've got there given the amount of bricks and stuff you've pulled out). Did your builders dig by hand or with a machine? 20k does sound a bit rich just to get to DPC but it may be what it is, presumably you've had more than 1 company to quote?

Plan B is to get a structural company on the job (ideally local so you're not paying silly money for them to travel) & get them to assess the ground and design an appropriate solution. Yes it'll cost but (since they won't be building it) they should go for the most appropriate solution rather than the most profitable.

Given your location, it might be worth a trip to the library, see if there's a local history section and find out what was on the ground before your house was built. If you're unlucky it could be an ex industrial site with god knows what buried.....

 

[acurachris]

Did your builders dig by hand or with a machine?

They dug it with a mini-digger, perhaps a 1.5 ton.

Plan B is to get a structural company on the job

Yeah I have requested a couple of quotes today from structural engineers, one has just replied as i was writing this response. I'll be much happier once out of the ground!

Of course, of all the houses and areas, i had to buy here....

 

[oldbutnotdead]

That'd be about right- rock-hard made ground so you have to go deep vs lovely soft clay = 900mm and get on with it. Good luck with it