Pile base carrying capacity

[longisland]

Hello,
Most geotechnical references stated that the allowable base capacity of a pile is the ultimate capacity divided by a safety factor of 2.5 to 3. In the case of cohesionless soil, it correlates with SPT valus & phi angle depending on method of computation adopted. Vesic or Meyerof. My coworker suggested that in the case of bedrock, e.g, sandstone, as long as the pile has penetrated into the bedrock or piled to refusal, no wedging effect will happen & thus we may just adopt the allowable structural capacity of the pile as the pile carrying capacity.i.e. short column. That is assuming that no bearing failure will occur? Do we need a point load test or CPT tip resistance value to verify such method?
Most SI don't do compression tests on rocks? How to get the phi angle? e.g. gravel or shale? Without phi, Meyerof method seems to be the easiest way to compute pile base bearing.

 

[ayfarm]

Run a weap analysis if values are known. Piles are designed to support compression, tension and lateral loads. You may find that you can not drive pile past its stress limit and beyond hammer's capacity during installation, such that you will have to redesign the pile anyway.

 

[Focht3]

There are several good rock mechanics books that cover this topic. Your "handle" suggests that you are from New York; several good papers have been written by faculty and grad students at Cornell (if you have contacts there, that's where I'd start.) Also check your local university library (if they have a CE program) and look for rock mechanics references.

Is the foundation driven or bored/drilled? ayfarm's comments appropriate for driven elements; drilled piers/caissons are a different matter.

In "very hard" rock, you will probably reach maximum foundation deflection before you fail the rock. But wedge failures could occur if the pile/pier does not fail before the critical load is reached.

That said, have you really looked at skin friction? If the foundation is drilled in some manner and concreted into place, skin friction may carry all of the design load. Too often, many geotechnical engineers ignore skin friction, or ignore end bearing, without a good technical reason.

Factors of safety for end bearing are really intended to limit foundation settlement. Look hard at all of your assumptions...

 

[longisland]

Hi,
Thanks for the response;it's a driven pile. Point taken from Focht3 on taking bearing & skin friction into account. That's where my confusion began; if I were to consider skin friction & bedrock bearing, that would be a geotechnical approach. My colleague is suggesting me to treat this as a structural problem as long as my pile is socketed deep enough into the bedrock. i.e, if the rock compression strength is higher than
the pressure imposed from the pile, then treat this as a short column. So is piling to refusal sufficient?

 

[Focht3]

Do you know what the local practice is?

It sounds like these piles will have a fairly high capacity; I'd be concerned about tip damage during driving.

How deep will the piles go? Load per pile? What will you be driving through?

In theory, limiting the bearing to the rock's unconfined compressive strength should be okay. But rubble, misalignment, etc. could cause problems.

Why not use both the structural and geotechnical approaches? That way you will have some additional confidence in the piles (because the geotechnical approach should result in higher capacity) should problems occur during construction. And you can made a better, faster decision on how to deal with each construction issue.

 

[longisland]

Hi,
The average depth would be 15m based on borelogs recorded. The soil is predominantly stiff clay. I'm going to use 600mm or 2 ft diameter spun piles. The bedrock compression strength is about 150 N/mm2. We did 2 rock core of 3m depth & the samples were consistant.