Shaft Adhesion in Bored Piles of Different Diameters.

[SmokeyBear]

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).

 

[BigH]

I've not seen anything like that before. If you look at any of the "reduction" procedures (whether Tomlinson, Focht, etc) none have an additional reduction due to shaft scale. Interesting question - I'll be watching to see if any other colleagues out there have seen such a reduction.

 

[PSlem]

I noticed it when i did augercast tiebacks. The difference in pullout was barely noticeable, not proportional to surface area as you would expect.

 

[SmokeyBear]

Big H and PSlem, Thanks for the comments. I await with bated breathe further posts. My collegue mumbled something about Poisson's ratio, but I have my doubts.
PSlem, could you elucidate? Do you mean that you noted very little differece for various diameters? This seems counter intuitive.

 

[PSlem]

We had a crane hung lead with hollow center auger with cables and a lost bit. Auger diameters varied from 10" to 12" to 14" depending on what was available in yard. I saw almost no difference and recommended to the office they only purchase the smaller as it used half the grout of the larger. The grout was an 11 bag ready mix. My thought at the time was why grout needed a strength so much more than soil. Littlejohn, I think, said it had to do with relative stiffness of a rigid inclusion versus the soil. I think it had to be 5 or 6 times stiffer and a larger bore didn't really offer more. We went to high pressure grouting shortly after this and totally eliminated any failures. 30 year old memories so can't add much more.

 

[Rockmynci]

It is normal practice within the UK to limit shaft adhesion on larger diameter piles, typically those greater than about 900mm diameter.

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.

 

[BigH]

Thanks for the information - but this seems to be "rule of thumb" - it would be nice to see some study that shows such reductions. Where is the cut-off from 0.7 to 0.45? I'm not discounting but I've just never seen anything for this - and the large diameter piles they install offshore - what Focht and Srinivasin (?) developed - the "lambda" coefficient - nothing on size . . .

 

[SmokeyBear]

Rockmynci; thanks for the information. I tend to agree with BigH. However, one should certainly consider time to drill and time to pour. Over many years, I have found that for bored piles, 600 and 750mm diameter holes are the quickest to drill, 9oo next, then 600 and less; with 1200 diameter being the slowest (lots of spoil to move, large concrete volumes etc). This may seem counter-intuitive; but with small diameters the flights may only extend 200mm or less past the post and many digs required to get the hole drilled.
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.

 

[Rockmynci]

Typically, if the piles being installed using a CFA technique, the higher alpha values will be used. When a rotary/bored technique is used, the lower values would be used. This mainly comes back to the monitoring information available on the rigs and greater control on drilling with CFA.

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?

 

[SmokeyBear]

Rockmynci: the piles were drilled in very stiff to hard residual clays of medium plasticity, grading to weathered rock at depth. The rock is a slightly metamorphosed sandstone/siltstone, varying from extremely low to low strength. Typically all piles were founded above the water table.