Critical depth for end bearing piles 1

[PSlem]

The COE's Design of Pile Foundations lists a critical depth, 10 to 20 pile diameters, beyond which end bearing does not increase. I cannot understand why this would be applied to end bearing.

Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.

 

[SlideRuleEra]

I have found reference to the 10 to 20 pile diameter critical depth and constant SKIN FRICTION from there down, in cohesionless soil (Paragraph 4.3 on page 4-11 of the .pdf download). However, have not found discussion of the critical depth as applied to END BEARING. What page are your refering to?

 

[PSlem]

1. Cohesionless Soils
page 19
b. End Bearing
For design purposes the pile end bearing capacity can be assumed to increase linearly to a critical depth (Dc) and then remains constant. The same critical depth relationship used for skin friction can be used for end bearing.
Dc=10B for loose sands
Dc=15B for medium sands
Dc=20B for dense sands

3. Piles in Silt
page 20
b. End bearing
The pile tip bearing capacity increases linearly to a critical depth (Dc) and remains constant below that depth. The critical depths are given as follows:
Dc=10B for loose silts
Dc=15B for medium silts
Dc=20B for dense silts


Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.

 

[VoyageofDiscovery]

I have come across this too in AASHTO LRFD. I believe when using CPT to determine pile end bearing capacity, it references "Nottingham and Schmertmann" from my memory.

HTH

VOD

 

[SlideRuleEra]

PSlem, I found the information that you referenced (in the .pdf download the page numbers are are 4-13 and 4-17).

To develope full point bearing capacity, the soil has to remain under the pile point cross section. Until the pile is "deep" enough there is insufficent pressure from the depth of soils (head) above the point to provide confinement. In other words, at shallow depths the soil under the point "squirms out of the way" when loaded - the result being settlement.

Critical depth increases as the sand or silt gets more dense. This seems to be the opposite of common sense. However the more dense the sand or silt, the less that it acts like a "fluid" which can exert the equivalent of a "hydrostatic head" to provide confinement.

 

[PSlem]

I posted this on another forum and received a message from Bengt Fellenius (

http://www.fellenius.net

go to paper 171)that it should be ignored.

Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.

 

[SlideRuleEra]

PSlem - Go with Dr. Fellenius' advice.

You seem to be well aware that engineering, and especially pile driving, are a mix of both "science" and "art". After becoming a PE, I had a six year career as a bridge and heavy construction contractor in the 1970's - we practiced the "art" of pile driving - most of it in the silt and sand in the South Carolina low country. That is when I learned the explanation that I gave you.

Times change, and I will certainly yield to advancements in "science".

Thanks for letting me know, I have downloaded his paper and will read it carefully.

 

[DRC1]

I agree with slidre rule era. I have used Dr. Fellenius's Unified Pile Design concepts for at least 10 years. He is certianly an expert in the field and should not be dismissed. As depth increases bearing should increase. The critcal depth concept is an old concept and is not seen in design very often anymore. I would encourage you to get Dr, Felenius's text on geotechnical design. It is very good.