structural engineering question - I need piles

bazaarboy

bazaarboy

bazaarboy

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Not those piles lol, piled foundations...

we're building a garage and the construction company is suggesting we need 9 piles (CFE), to a depth of 20 metres

to me it seems like an exceptionally over-engineered set of foundations for a double garage!!

I'm getting a second quote of course, but I know there are lots of engineers on this forum, i wondered if I can have an opinion about this?

:cool:

edit: sorry for placing the thread in the wrong forum, I forgot there was a homes & garden section!!
 
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That sounds utterly ridiculous.

I'd simply ask building control to pop out and talk to their man? Presumably you have an open application in with them anyway so this is part of the service. I recently built a fairly large extension to my house and used building control as the designers for aspects like this.

Edit: For reference i had to go down 1m*600mm wide for my foundations which are suitable for 2 storey in solid (extremely dense) bradstone walls, even these seemed massively OTT to me and are approx double that which my main house sit on. This varies with ground conditions though.
 
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Building control would normally dictate what foundations you need.

A double garage is a small bungalow and I've built bungalows on piled foundations yes.

Totally depends on ground conditions, without this information it's a bit vague. Is there large trees nearby? Is it on a flood plain? Is it on clay? Is it on made ground? Is it in an area that was mined? Is is on sift sandy soil? etc etc etc

If you had to excavate down to 2m (which is usual if you are near a large tree) then piling can be = cost
 
The ground conditions are not ideal, I grant you.

There is about 3 metres of topsoil - and the soil report showed variable material in this top 3 metres.
We had a CPT analysis which checked to a depth of 20 metres and between 3-20 metres it's almost completely 100% chalk.
The water table is at 2.5-3 metres.
 
You need to just get building control round and speak to their man - included in their fees. The building inspector has no agenda to increase your outlay and should give you a straight opinion and guidance.
 
bazaarboy said:
The ground conditions are not ideal, I grant you.

There is about 3 metres of topsoil - and the soil report showed variable material in this top 3 metres.
We had a CPT analysis which checked to a depth of 20 metres and between 3-20 metres it's almost completely 100% chalk.
The water table is at 2.5-3 metres.
Click to expand...

Sounds like you need the piles. BC won't let you bear on topsoil (although topsoil effectively dies if this deep). If good bearing strata i.e. the chalk is 3m down then I'd put my 15 years as a QS on piling being the least expensive. Anything over 2m needs so much earth work support, cart away and new materials to fill it back up, a piling rig for a day becomes a less expensive alternative.
 
no doubt the piles are definitely needed - but 20 metres, that's the height of a 6 storey building!
I thought maybe 5-6 metres deep

I just spoke to building control, they said the plan needs to be approved for building regs, but they wouldn't be able to recommend less deep piling, if an engineer has already recommended 20m.
 
Can you find out what piles they have suggested, 20m sounds excessive however if it's their design then they probably have allowed for the worst case and their might be a credit for under measure which is normal. (the builder might not be telling you this)

If it's a driven pile they will drive down till refusal
If it's cfa they will bore till they hit chalk but if it's cfa they won't be able to withdraw the auger without pumping concrete down which might mean they need their own pump and agitator on site or several concrete wagon visits with large part load charges
 
Yep the piles are CFA, 350mm diameter. i should mention we're also building a house, which also needs 16 piles (same spec) of its own.

it's a complex build as the house has a 120 sqm basement, and the whole thing - house and garage is made from reinforced concrete, prefabricated and erected onsite with a 90 tonne crane...

i think the house is estimated to weigh about 500 tonnes :O
 
Sounds like a nice build.

It'll be even better value if they pile the garage with the main house, a great deal of piling cost is transport to and from site especially if it requires a 'convoy exceptional'. I'd say then it'd definitely be less expensive than traditional foundations.

If they are putting through a cost for the piling don't let them divide the piling cost by 16 then x 9 for the garage as the original 16 would have had a large amount of set up and demob costs.

Move and set up on each position £35 each
Bore & construct CFA 350mm dia £29.49 m
Testing £8 each
 
maccapacca said:
Sounds like a nice build.

It'll be even better value if they pile the garage with the main house, a great deal of piling cost is transport to and from site especially if it requires a 'convoy exceptional'. I'd say then it'd definitely be less expensive than traditional foundations.

If they are putting through a cost for the piling don't let them divide the piling cost by 16 then x 9 for the garage as the original 16 would have had a large amount of set up and demob costs.

Move and set up on each position £35 each
Bore & construct CFA 350mm dia £29.49 m
Testing £8 each
Click to expand...


They have quoted me for the whole lot as one job. The costs are:

Rig mobilisation 3600
setting up for each pile 900 (for all 25)
bore and pour all 25 piles, 16260
reinforcement for all 25, 1250
test drill, some extra soil checks 1700

not cheap, but I really dont want the house to shift or sink... :eek:
 
Sounds reasonable.

Ask about a credit for bore lengths, looks like they have allowed 20m for each pile, you should be able to get a credit back for any shorter bores really, it's very standard.

The pile logs will tell you how much they have actually bored.

What factor of safety have they allowed for. Anything under 3 will need a static load test, so I assume (as most pilers do) they have upped the FOS to 3 then they don't need a load test for Building Control.
 
Not sure how the credit thing works - they quoted me £3 credit per metre. That doesn't seem much?
In any case, what determines the final depth of a pile, eg why would they stop at 15m and not go to 20?
thanks
 
bazaarboy said:
Not sure how the credit thing works - they quoted me £3 credit per metre. That doesn't seem much?
In any case, what determines the final depth of a pile, eg why would they stop at 15m and not go to 20?
thanks
Click to expand...

I assume they go down till they meet a specific resistance. This could be less than 20m
 
bazaarboy said:
Not sure how the credit thing works - they quoted me £3 credit per metre. That doesn't seem much?
In any case, what determines the final depth of a pile, eg why would they stop at 15m and not go to 20?
thanks
Click to expand...

If they hit bedrock earlier than expected then they won't bore any deeper.
 
bazaarboy said:
Not sure how the credit thing works - they quoted me £3 credit per metre. That doesn't seem much?
In any case, what determines the final depth of a pile, eg why would they stop at 15m and not go to 20?
thanks
Click to expand...

Credit seems very low, it's usually circa half.

TtNYt57.png


They set the torque on the piling rig and it'll bore down to refusal or if they just have to say bore down to chalk they will bore till chalk is brought up by the auger.

For driven they just set a driven force and drive down till the pile refuses to go any more.
 
thank you - beginning to make sense now. I will still get a second quote for the piling work, but I suspect I shall go ahead with the current proposal.
 
If you're hitting chalk at 3m deep, it might be worth looking into using concrete manhole rings to act as formwork for some pad foundations, then spanning ground beams between to support the superstructure.
 
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