buckling of uncased minipiles

[bjb]

I have been approached with an interesting question. A geotechnical engineer has asked me to determine the buckling load of an uncased minipile that is penetrating a zone of poor soil, where the ability of the soil to laterally brace the minipile is questionable. To limit the transfer of downward drag forces to the pile, they want to use a greased PVC sleeve. Hence, there will be no steel sleeve that I can analyze. Because the construction of a minipile is different from a reinforced column, I don't think the ACI methods for column slenderness are completely applicable.

The question: Does anyone know of an accepted procedure for determining the compressive strength of an unbraced, uncased minipile? I have a book by the FHWA, but they only have a method when a steel casing is present.

 

[BigH]

Interesting question. Why not contact some of installers of minipiles - they might have some ideas on the subject. Even though the ground surrounding the pile is soft, even a small lateral constraint helps immeasurably. I would also suggest that you look up reference by Poulos and Davis' Elastic Solutions for Soil and Rock Mechanics. I believe that they discuss pile buckling in their book and the effects of lateral restraint, and the effects that various surrounding soil stiffnesses offer to the situation.

 

[BigH]

Sorry - it is late - but also check out Poulos and Davis' book on Pile Design (1980). There is an entire chapter (Chapter 14) on Buckling of Slender Piles. They discuss extensively the effects of lateral restraint.

 

[bjb]

Thanks. I told them that there is not an established procedure for determining this load in either ACI of the FHWA book on designing minipiles. They are leaning towards extending the steel sleeve through the questionable material, and adding the drag force into the pile.

 

[DRC1]

The problem will probably generally not be buckling, but crushing or shear at the head of the pile. Dr. Bruce did an interesting study several years ago on the failure of uncased piles and the capacity of cased piles on the same project at Cooney Island in New York. Although it does not compute buckling directly, there is an ACI code (not 318) that deals with cast in place concrete piles, including minipiles, both cased and uncased. One concern that is often overlooked is strain compatability. Remember that stress are a result of strain. The central bar should be sized that under the anticipated strain sufficent load is transfered to the bar so the concrete is not overloaded.
If you think you need the casing, I would use it and either addd piles or add capacity to the piles. See Dr. Fellenius' works on downdrag(

http://www.unisoftltd.com)

 

[PEinc]

Allen Cadden (and Jesus Gomez?) of Schnabel Engineering published a paper on buckling of minipiles (micropiles). I believe the following link will give information about the paper.

http://www.schnabel-eng.com/Vol3-no1B.pdf

 

[BigHarvey]

A clever way to escape the problem would be to eliminate negative skin friction ( drag down force ) by consolidating the poor soil under the envisaged fill load by preloading combined to vertical drains.This technique has already been used for East port Saïd Power plant in Egypt and certainly on other projects.

 

[Qshake]

Another reference to use in this case would be Prakash and Sharma's Pile Foundations in Engineering Practice. There is a chapter dedicated to Buckling Loads of Slender Piles. Additional material can be gained from the chapter references.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BigH]

Ran across another reference:
ADSC: The International Assn of Foundation Drilling

http://www.adsc-iafd.com

adsc@adsc-iafd.com

They have a booklet available for purchase: Buckling of Micropiles: A review of Historic and Recent Experience - costs about $8 for non-members.

 

[PEinc]

The ADSC paper should be the same paper I referred to by Cadden and Gomez.

 

[BigH]