Ignore pile capacity for embedment < 3D

[Ophirian]

I am working on pile design and I was advised to ignore the capacity of the edge of the pile if the embedment to the soil layer is less than 3xpile diameter. Please refer to the attachment for clarity.

I have not used such assumption in the past and I am wondering if anyone came across with it?

Cheers!

 

[Ron]

I assume you are referring to ignoring skin friction for stratified layers of less than 3 pile diameters. This is not unreasonable considering it would be very low anyway and it takes relatively thick layers of some soils to develop and mobilize skin friction.

 

[Ophirian]

Thanks, Ron. Is it something that is code defined or more of an engineering practice?

 

[EireChch]

I think its complete nonsense. If you had a pile diameter of 1.2m, and your soil profile was layered in 3.5m layers, would you ignore contribution from skin friction? Definitely not.

There may be other reasons to ignore skin friction (i.e. compressible layers below stiff/dense layers, desication etc.) but not just if the layer is less than 3D.

 

[Okiryu]

Agree with EireChch...

 

[Ron]

@EireChch and Okiryu....I should have clarified that my consideration was for driven piles which are typically 12-24 inches in diameter. A 1.2m "pile" is more likely to be a drilled pier or shaft, thus skin friction has more impact.

 

[BridgeSmith]

I think most drilled shaft design guides say to ignore skin friction within 3 diameters of the base. I believe it's supposed to account for the deformation of the soil under the end producing downdrag of sorts in the soil around the base. The soil around the side of the base relies on the soil below for support; if the soil below it is already pushed down due to end bearing pressure, the support for the soil around the side is compromised.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jayrod12]

We consistently neglect the top 3*d(roughly, usually it's just 1.5m) to account for dessication of the soil near the top of pile causing shrinkage of that soil.

 

[EireChch]

Not sure on that BridgeSmith, as you are aware, end bearing requires larger deformation to achieve any resistance. Skin friction 3D above the base would be fully mobilized before end bearing is not even 1/5 mobilized. (Assuming 10% deformation to mobilize end bearing and 2% to mobilize skin).

Jayrod - yes I adopt that too.

 

[Retrograde]

I think most drilled shaft design guides say to ignore skin friction within 3 diameters of the base.

I have never seen a design guide that recommends this.

 

[Settingsun]

The question doesn't match the sketch. The sketch says to use different parameters (those of the overlying layer) rather than ignore completely. I've seen many reports that advise that the design parameters assume a certain embedment in the founding layer. This could be the way this particular engineer words the same advice.

I also vaguely recall something from a long time ago about the potential for lowered skin friction from poor soil adhering to the pile and being dragged down into lower layers. I don't remember the context and won't be able to find the document now.