Timber pile design using end bearing and side friction

[Geoluk]

Hello,

I'm doing a deep dive into some pile types. Regarding timber piles, I often hear that they are considered friction piles and their end bearing is ignored in capacity calculations. However, this is always just stated, but never justified. The best I could find was this quote from the 2015 Timber Pile Design and Construction Manual by the Timber Piling Council:

"The equations presented here assume that both the pile toe and shaft have moved a sufficient distance with respect to the adjacent soil to simultaneously mobilize the ultimate shaft and toe resistance. It should be noted that the displacement needed to mobilize the shaft resistance is generally smaller than that required to mobilize the toe resistance."

Unfortunately no further details are given. It seems to me that his is very poor justification for completely ignoring a potentially large contribution to capacity from end bearing. I have a few questions. If you can only answer some that's still great.
1) Does this apply in all soils? The quote does say "generally"...
2) Does this apply to other tapered piles such as tapered steel pipe piles?
3) Presumably, if the friction capacity is exceeded, the pile would slide a microscopic bit and mobilize enough to engage end bearing. At this point, friction isn't completely gone. It's probably still significant. Why not consider this state when designing, especially if the tip is in a dense soil layer capable of providing good end bearing?

Thanks.

 

[SlideRuleEra]

Regarding timber piles, I often hear that they are considered friction piles and their end bearing is ignored in capacity calculations.

That is not a generally true statement.

I expect it could be a reasonable assumption in certain soil conditions (hard driving that can damage pile tips, e.g. "brooming"). Metal points for timber pile are available to minimize chance of damage during hard driving... therefore preserving point bearing.

However, there are other soil types where there is no significant skin friction... ONLY point bearing (soft, wet coastal soils with hard material (limestone "marl" in my area) down deep to provide point bearing).

Here are some examples of pile damage from incorrect hammer selection and overdriving (pile tips on the second, third, and fourth images are "boomed")

Note: It is very easy for a Contractor to overdrive timber piling (destroying structural integrity of the pile), especially if the hammer is too powerful. When a timber pile tip reaches practical refusal, which can happen suddenly, there may be only a very few seconds to terminate driving before pile damage occurs.

 

[geotechguy1]

It's an old wives tale that some geotechs made up to give an additional factor of safety. The trouble is, unpacking the old wives tale is very difficult - the people who made it may have done it because in the area they work piling contractors use piles half the size or half the length as specified or only install half the piles and the 'only use skin friction or end bearing' is a protection for that in their area.

You can have cases where friction is drastically reduced due to protrusions from the piles (e.g. spiral welded piles often have reduced friction capacity, or oversize projections at the toe) though I would have thought a tapered pile driven small end first wouldn't really have that problem

 

[Geoluk]

Thanks. It did seem like a claim with little basis. Otherwise it would also be said of tapered steel piles, which I've never heard.
I think if there's a concern with the the displacement needed to mobilize the shaft resistance vs. the end bearing, you could just design for side resistance using the residual strength rather than peak. And that would only matter in dense sands or OC clays anyway.