structural engineering question - I need piles

bazaarboy · 24 Aug 2015 at 11:29

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!!

bazaarboy · 24 Aug 2015 at 11:40

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.

bazaarboy · 24 Aug 2015 at 12:02

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.

bazaarboy · 24 Aug 2015 at 12:21

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

bazaarboy · 24 Aug 2015 at 13:34

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

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:

bazaarboy · 24 Aug 2015 at 13:48

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 · 24 Aug 2015 at 14:43

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.

bazaarboy · 24 Aug 2015 at 16:49

nkata said:
It's mainly because it's CFA.

Shaft friction in chalk with CFA is about 0.45.sigma.v' whereas normal rotary bored piles are 0.7.sigma.v'. Due to potential for smear at the pile face and previous experience.

The designer may also have discounted pile end bearing although with 350mm diameter there will be some.

Also 3m of topsoil will produce downdrag in the piles increasing the pile load.

On top of all that unless you are having pile tests, there will be a factor of safety of at least three built in.

CFA are very quick though and you may get 5-7 a day or more if they are on the ball.

sounds like you are very much clued up on piling, can I pick your brain?

the metalwork that goes in is described as follows:

"Fabricate and place 25 No. reinforcement cages formed from 4 No. B16 bars, 6.0 metres long and an R6 helical binding with a 250 mm pitch."

Is that just 6m, does that mean anything beyond 6m won't be reinforced?
How does that affect the strength of the pile?
thanks

bazaarboy · 24 Aug 2015 at 17:02

nkata said:
A quick calculation on capacity and yes he will have used end bearing to get a fos = 3. Your house will impose 300+ kN per pile and i calculate 715kN ultimate skin friction, so >2 fos on shaft alone. I do not think it will settle at all, and I do think the 20m is a reasonable length dependant on achievability, chalk strength etc.

I guess that answers my OP question...
I'm amazed - but relieved I guess, that the structure wont end up in China

funny how the nearby church made of flint, about 15m tall, has stood for 800 years with footings no more than a metre in depth

bazaarboy · 24 Aug 2015 at 17:21

nkata said:
The church probably took about 25 years to build and went up as the ground went down.

I may query the length of the garage piles, they have probably just pro rata'ed the quote?

i think so, I can appreciate the house piles need to be rated at 30 tons each, but the garage, despite made of reinforced concrete, won't weigh more than 100 tonnes

there's 9 piles for the garage - perhaps 10m piles are enough

bazaarboy · 25 Aug 2015 at 08:39

maccapacca said:
I wouldn't be interfering with their design.

God no. I like knowing how stuff works, and I'm not familiar with construction engineering.

bazaarboy · 25 Aug 2015 at 12:06

nkata said:
One comment. CFA Monitoring.

CFA piling rigs are now normally equipped with sensors to read rotation, rate and depth of penetration, rig torque and pump strokes. This is displayed in the cab so that the driver can control the drilling phase and also the concreting phase. The latter will draw a concrete profile on the screen with tramlines to indicate the nominal size and wavy lines to show the actual profile in the ground. These should always be on or outside the tramline to indicate a good pile. Where they are inside the line it indicates necking.

By knowing the volume of concrete per pump stroke and the rate of auger extraction, the section of the pile at any depth is calculated. As the client, you should obtain printouts for each pile from the site managers laptop.

350 diameter piles are skinny and 20m (60 diameters) will be the limit. 400 or 450mm will produce shorter piles but be more expensive in concrete.

Good luck.

it sounds like you're a structural engineer. Would you be able to help me with a design for cantilevered stairs? I'm happy to pay for your time
thanks

bazaarboy · 25 Aug 2015 at 22:56

just a quick update - I received a second quotation for piling.

Company A suggested 350mm CFA piles up to 20m depth, total cost about £25k
Company B suggested 300mm CFA piles up to 18m depth, total cost about £21k

The position, number of piles etc is the same. The engineers requirement is that each pile should be able to take a 30 tonne load.

I appreciate that the first company is proposing bigger piles, but then again there is a £4k difference in the price.
What would you do?
Is a 300mm pile sufficiently strong to meet the engineering requirement?

bazaarboy · 25 Aug 2015 at 23:26

I forwarded the second company's proposal to the construction company's engineer - it's up to them to decide if they are happy with what's being offered.
If they come back to me and say they are satisfied with either piling firm, then I would probably choose company B, not just because of the cost but because they also offered to drill my geothermal boreholes.

The first company don't do that - and the GSHP needs 260m of borehole - as I mentioned earlier, it's a complex build lol

bazaarboy · 26 Aug 2015 at 22:11

nkata said:
Two designers will have different experience in chalk and one may be being more conservative. Chalk is a variable strata from squeezable to strong almost limestone. Designing from your CPT data or from local knowledge can give different piles. Good piling companies usually have a lot of historical data on previous work or work tendered, boreholes etc.

Obvious piling risks in chalk are flint beds and swallow holes. Both may be picked up in a survey or a desk study but are still a risk for piling on site.

Generally the only difference in plant for 300 or 350mm diameter is the auger cutting head. When I was involved with a lot of CFA, we used a 350 head on a 300 string to drill 350mm piles.

In theory a 300mm pile with C32/40 concrete could carry 70T structurally. It is the geotechnical capacity which will dictate the strength. The reinforcement cage for a 300mm pile is obviously narrower and the lateral capacity reduced if a lateral load has been specified.

ok, so the construction company's civil engineer is happy with either piling proposal. might go with the cheaper one because the boreholes need to happen too - adding a whopping £11k, just for another hole in the ground! :eek: