Self weight of piles

[darragh91]

I have been doing some research into piles and I read you can usually ignore the self weight of the pile except in the following situations:


The weight of the pile itself is generally small in relation to the applied load (i.e. Qt >> W) and is usually ignored.

May need to check this assumption for:
(i) friction piles in soft clay,
(ii) offshore piling where a considerable proportion of the pile extends above the sea-bed.

Can someone tell me why in these above cases that it cannot be ignored?

Many thanks

Darragh

 

[dgillette]

Ordinarily, the weight of the pile is not terribly different from the weight of the soil it replaces (e.g., 145 lb./c.f. concrete replacing soil with total unit weight of 125 lb/c.f.). If the pile extends well above the sea bed, it doesn't replace soil, just water weighing 62.4 (fresh), or ~64 (sea water).

 

[jackkingsford32]

Very well said, dgillette.
In behalf of darragh91, I thank you for the explanation. It makes sense to me.
dgillette is right. I have read something like that, too.
Wow, dgillette, you should be a teacher or something. (This is a compliment)
But what about the friction piles in soft clay? I’m afraid you weren’t able to answer that..

 

[dgillette]

Hmm. Not sure about that, but perhaps because soft clay is quite a bit lighter than concrete, intermediate between denser soil and water. If water content is 50 percent (not unusual in CH), gamma-total is about 105 pcf, which is 40 less than 145 for concrete. In a medium-density sand, assuming gamma-dry = 110 pcf, gamma-total is about 131, which is only 14 pcf lighter than concrete.

 

[geotechnicalengine]

In behalf of darragh91, I thank you for the explanation. It makes sense to me.

 

[stacyparks]

But what about the friction piles in soft clay? I’m afraid you weren’t able to answer that..

 

[dgillette]

OK, stacyparks, if it's not the lower unit weight of the soft clay compared to the pile that displaces it, what is the correct answer?

 

[asp1230]

Replacement of soil makes sense for drilled shafts but not for driven piles. For driven piles you should add pile weight, particularly if you are using lower factor of safety.

 

[SlideRuleEra]

IMHO, the self weight of piles is ignored because on history. Piling have been used successfully for centuries... long before anyone had scientifically based ideas on how to calculate their load capacity. Also, as darragh91 stated, the self weight is anywhere from a small fraction to a trivial fraction of the load capacity. Part of that long history as that until the 19th century all piling were wood (which, of course usually floats - possibly even causing a small uplift in some conditions).

Consider that soil properties can be calculated with modest accuracy... and this does not take into account what happens to in situ soils during pile installation. Safety factors are also based on estimates and historically successful applications. The bottom line is that self weight of the pile is nothing more than "noise" when compared to the other factors and that due to the long history the assumption to ignore pile self weight has been "grandfathered".

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net