structural engineering question - I need piles
- Thread starter bazaarboy
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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.
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.
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Now you're talking about driven sheet piling with manhole pump out frames or boxes, lots of muck away and quite a bit of concrete. Very much doubt that would be less expensive. At the very least it will be a months work where a piling rig will do all that in two days and be gone.
https://www.vpgroundforce.com/gb/shoring-equipment/products/box-systems/manhole-boxes/
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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
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.
That is normal, reinforcement is only required for lateral loading due to external forces like wind or pile out of verticality force or other specified lateral load..
This tends to dissipate to virtually zero at 10 diameters into the ground and he has also allowed for the depth of topsoil / fill/ /made ground.
That is normal, reinforcement is only required for lateral loading due to external forces like wind or pile out of verticality force or other specified lateral load..
This tends to dissipate to virtually zero at 10 diameters into the ground and he has also allowed for the depth of topsoil / fill/ /made ground.
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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
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
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.
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.
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I wouldn't be interfering with their design.
God no. I like knowing how stuff works, and I'm not familiar with construction engineering.
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
Will offer to review the SI report for you, though it seems that there is no alternatives to piles.
Unfortunately, as a structural engineer, sometimes I have had no choice than to design small outbuildings on piles just because of poor ground conditions. I remember a particular project Hampshire County Council hadn't expected so, and the cost was so high they didn't proceed with it (at the time at least).
Unfortunately, as a structural engineer, sometimes I have had no choice than to design small outbuildings on piles just because of poor ground conditions. I remember a particular project Hampshire County Council hadn't expected so, and the cost was so high they didn't proceed with it (at the time at least).
Sorry, no. I am a civil design engineer I have a history in foundation design and temporary and permanent works going back 30+ years. However very little on the building side
I would probably not be cost effective or even architectural enough. However thanks for the offer.
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?
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?
Impossible to tell without knowing the details. One possibility is that they are testing a sample of piles once installed and can therefore use a smaller factor of safety (2.5 instead of 3.0 I believe). THis would allow them to use smaller and shallower piles.
TO be honest, whilst you make the decision, your structural/civil engineer who is doing the basement/substructure should be advising you what the difference is and making a recommendation. There may well be a few subtle details that are leading to a smaller quote. I want to offer to castmy eye over the quotes but I am going on holiday on Thursday and tomorrow will be a busy day for me tying up loose ends.
At the end of the day, whichever you go for, the piling specialist should be providing PI insurance as pile design is their responsibility. Whoever is managing the project should be check the PI level.
TO be honest, whilst you make the decision, your structural/civil engineer who is doing the basement/substructure should be advising you what the difference is and making a recommendation. There may well be a few subtle details that are leading to a smaller quote. I want to offer to castmy eye over the quotes but I am going on holiday on Thursday and tomorrow will be a busy day for me tying up loose ends.
At the end of the day, whichever you go for, the piling specialist should be providing PI insurance as pile design is their responsibility. Whoever is managing the project should be check the PI level.
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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
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
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!
