Pile group efficiency for circular pattern 2

[jefe2004]

My name is jeff, and a fresh graduate structural engineer.

I'm facing a problem in reviewing a foundation for a fuel tank, the foundation is composed of 13 dia.400 m CSP pile, with the circular patterns as i attach below:

the problem is, i can't find the efficiency for circular patterned pile group, the only formula i know is the converse labbre which states that the formula is limited to rectangular groups.

I find some literature that in sandy layer soil (the layer of my soil dominated by sand), if the spacing of piles larger than 3x dia. of piles than the efficiency should be taken as 1, so the ultimate capacity of the pile group would be : n x single capacity piles.

is there any efficiency formula for circular patterns or is the n x single capacity would be true for my case?

thank you very much before,

regards,

jeff.

 

[Jalthi]

The attachement shows the minimum center-to-center spacing between any two piles is 8.5 times the diameter. In this case there is no (or extrememly minimal) group effect.

 

[jefe2004]

Thank you for replying Jalthi

So basically if i find another case with pile group which has irregular pattern like this i can overlook it if the spacing larger than 3 times the diameter right?

And in my current design i use a statement from a journal which says that the basic formula for group effectivity:

Geometric effectivity = (perimeter of the group)/ (sum of the perimeter of pile)

so, in my case its:

(8 x 3.015)/(13 x 0.25 x 3.14 x 0.4^2) = 1.5 ( taken as 1)

which 3.015 is the distance between the outer pile..

Can this formula be applied for any irregular structures?

 

[Jalthi]

The answer to your first question is YES. However, note that 3 times diameter is a rule of thumb and applicable for axial loading only. For lateral loading there are different critria.

I am not familiar with the term "Geometric Effectivity". But it seems like this is a comparison of piles failing individually vs piles failing as a block. It means not only the perimeter but also the tip area should be compared to see which failure mode governs the design.

 

[FixedEarth]

In your case the crux may be how many piles or pier are in tension and how many are in compression. And are you staying within the geotechnical capacity for allowable axial compression and uplift. Then after that check for lateral load capacity, pile group settlement, block failure etc. Try to get Poulos or F.D.C. Henry's Foundation texts.

 

[jefe2004]

@Jalthi
actually the Geometric effectivity stands for the pile group efficiency (Eg) which depends on the pattern of the pile, since the i can't use the converse-labbre equation, i use that one from the journal i found.

@Fixed earth
thanks for your suggestions, I'm not very knowledgeable in geotechnical problems, and my company doesn't have much literature about soil, we only have bowles 4th edition..do you have any journal about this?

 

[FixedEarth]

If you go to Steeltools.org search for "PILEGRP" Spreadsheet by Tomanovich. It is free and on the lower hand side of the worksheet, there is a method to enter circular pile geometry. This will give you the pile reactions. Then check the soils report and see for example if 100 kN downward axial is below allowable geotechnical capacity. Do the same for piles that are in tension. Then go to FHWA website and look for Publication # FHWA-NHI-10-016. It is also free and you can read on lateral capacity of single pile and lateral capacity of group piles. Then you can check for group settlement, I think Bowles covers most of this topic. Then you can design the pile cap and anchor bolts, etc. Unless your fuel tank is tall, seismic overturning will not govern.

I like the texts more than the journals. Go to Amazon and look for pile foundations books by Tomlinson, Canadian Foundation Manual, F.D.C. Henry, Teng and Poulos.

Your firm should start you off with combined footings or grade beam foundations. This design, you need to pick up the design steps from a senior engineer.

 

[jefe2004]

Thank you very much jalthi and fixed earth for your kind reply, well atm, i am the senior engineer in my firm, most of them left and i'm the only one left with no experience and tools, which makes me only rely on spreadsheet calculations.