Pile design info 1

[Okiryu]

Hi, I am just wondering how structural engineers use the information from geotechnical engineers for pile foundations. For example, most of the times, I provide axial and lateral capacities without any load information, so I provide pile capacities (axial and lateral) based on single pile analysis. I provide axial capacities for different sizes of piles. Also, along with lateral capacities, I provide the displacement and maximum moments for each specific capacity.

However, most of pile foundations are composed of pile groups so I would like to know how my information based on a single pile is used by structural engineers.

 

[oldestguy]

There are formulas that a designer can use to all for the grouping behavior of piles close together. However, should you know the approximate loads to be carried, your report should address the grouping effects. the more cooperation there is between the geotech and the structural engineers, the more economical the result.

 

[Okiryu]

OG, thanks for your reply. I was doing some internet searching and found that LPILE has some multipliers for pile groups:

http://www.piledrivers.org/files/db...aterally-loaded-pile-design-pptx-caliendo.pdf

In this link, I found interest what it is indicated in page 25: "The lateral deflection of a pile group is typically 2 to 3 times larger than the deflection of a single pile"... I was thinking the opposite.

Agree that cooperation between structural and geotechnical is needed. However, our investigations are done in the preliminary stage of the project where sometimes few design information is available. Thanks again.

 

[Okiryu]

Oh! Also, I have not have the chance to look at my Tomlinson's book, but I will check it once I get back to the office on Monday...I will post here later if I have any comments or found something interesting.

 

[jayrod12]

Typically the geotechs around here will specify a minimum spacing between piles that would allow them to be considered as single piles. If you need more than one pile you just design an adequate enough pile cap to not violate said spacing.

 

[Okiryu]

Jayrod12, thanks, does it mean that you consider the axial capacity of the pile group as the sum of the capacities of each individual single piles? Also, how the lateral capacities are considered?

 

[jayrod12]

Yes to the axial capacities, same rule applies to the lateral. But that's also assuming the lateral load on the piles is not abnormally large. If we need to be resisting moments at the tops of the piles we generally do a pile cap with multiple piles to transfer it into axial loads on the piles.

 

[Okiryu]

Thanks again jayrod12, this is good to know.

 

[Okiryu]

jayrod12, forgot to ask, what do you think about the above statement from page 25 from the link I posted above?

 

[jayrod12]

That's news to me. I'm no geotech so I can't comment on how that works exactly but I assume it would have to do with overlapping stresses from the pile group.

Again, I feel that has a lot to do with pile spacing. There's another thread in this forum where there's a 3 pile group of 8" square piles spaced at 18" o/c. That spacing seems really tight to me.

 

[Okiryu]

jayrod12, I think you are correct. With proper pile spacing high stresses due to overlapping should not occur. We normally consider the minimum pile spacing as 2.5D or 750 mm. And yes, I saw that thread also and agree that 18" o/c is really tight.

 

[Okiryu]

I meant to say 2D or 750 mm whichever is larger...

 

[dik]

If you're talking pile spacing, it's often 3xD for spacing...

Dik

 

[Ron]

It depends on what stage the geotech is engaged. If done prior to design, then the global approach Okiryu described is appropriate. It gives the SEOR options. If the geotech is engaged as the structural design is being done, the geotechnical function becomes more of a delegated function from the SEOR and the geotech should be given column loads that would help him/her provide a more specific foundation design recommendation, with the decision to use single or grouped piles being that of the geotech.

 

[SlideRuleEra]

Okiryu - I agree with dik, spacing of 2D is too close for lateral loads. Look at page 28 (Table 9-19 Laterally Loaded Pile Group Studies) of the linked document, the closest spacing tested is 3D. Lateral loading efficiency in both sand & clay is severely impacted (ranges from 30% to 80%) at 3D spacing. At 5D spacing in sand lateral loading efficiency is much improved (ranges from 70% to 100%). Spacing is critical for the reason jayrod12 mentioned and this is easy to see in a plan view of a pile group... too much pile and too little soil:

For axial point bearing pile loads, 2D spacing may be considered as the absolute minimum spacing under optimum conditions, but not for any lateral loads. For axial friction pile loads even 3D spacing may be too close.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Okiryu]

Thanks for your input. I think that the spacing will also depend on the pile installation methods. I have to look at it closer but the spacing that I indicated above (the larger of 2D or 750 mm) applies for piles installed by pre-boring methods. This spacing is what is normally used here in Japan for that specific method of installation. For driven piles without pre-boring perhaps spacing requirements are larger.

Also, as pointed out by oldestguy at the begging of this thread, do you know any equations/formulas that relate single and pile groups that are used for design?

Thanks again!

 

[SlideRuleEra]

Okiryu - I agree that a displacement piling group could not be driven successfully in most soils at 2D spacing unless there was preboring for each pile. In that case, the piling would almost certainly have to be point bearing, not friction piles. Cast-in-place bored piles that are spaced at 2D may be ok for axial point bearing loads, too - as noted previously. Lateral loads are another matter.

There are equations for driven pile block efficiency but they do not apply to point bearing piling. The equation I'm familiar with is for driven friction piles. It won't apply for your work with cast-in-place piles, and is not for lateral loads anyway.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Okiryu]

SRE, I agree with you on your above reply. I started this thread because I was talking with some Japanese engineers and they told me that single pile analysis is useless as it may not reflect the actual conditions where pile groups are used. Here in Japan, geotechnical input is very limited as structural engineers deal with geotechnical issues as well. My projects cover US DoD projects so I work with US engineers following US design approach, but I always talk with local engineers to interchange ideas. Thanks again!