Shaft Adhesion in Bored Piles of Different Diameters.
Shaft Adhesion in Bored Piles of Different Diameters.
(OP)
Has anyone observed a reduction in shaft adhesion for increasing diameters of bored piles in similar subsurface condition?
In a recent project, shaft adhesions were developed from in-situ and laboratory testing. These values were confirmed/modified based on a number of CAPWAP analyses undertaken on 600mm diameter bored test piles.
A collegue suggested that the skin friction values should have been reduced for larger diameter piles (900, 1050 and 1200 mm diameter).
In a recent project, shaft adhesions were developed from in-situ and laboratory testing. These values were confirmed/modified based on a number of CAPWAP analyses undertaken on 600mm diameter bored test piles.
A collegue suggested that the skin friction values should have been reduced for larger diameter piles (900, 1050 and 1200 mm diameter).





RE: Shaft Adhesion in Bored Piles of Different Diameters.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
PSlem, could you elucidate? Do you mean that you noted very little differece for various diameters? This seems counter intuitive.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
Auger rotatation, crowd and concreting can be closely controlled using the rig computer for smaller diameter CFA piles, however greater soil disturbance should be expected for larger diameter piles. Other factors such as the time taken to bore and concrete the pile wil also affect the shaft friction.
For comparison, design of small diameter CFA pies to Eurocode allows us to use an alhpa of 0.7 on shaft within London Clay, if auger rotation & rig records are carefully monitored. We would normally reduce alpha to 0.45 for large diameter piles within the same soils - even if the same rig were used to install the piles.
So yes, it's not uncommon for skin friction to reduce for larger piles.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
I am not sure that "greater soil disturbance should be expected in larger diameters" Why?
I should clarify my original query to add that we were installing bored piles, not CFA. Also we groove the hole after drilling to target depth.
RE: Shaft Adhesion in Bored Piles of Different Diameters.
If you're comparing bored piles of different diameters then I would agree the skin friction value shouldn't change significantly, so I'll table an amendment to my earlier comment!
What sort of soil profile were the piles in?
RE: Shaft Adhesion in Bored Piles of Different Diameters.