Lateral loads on pile groups

[PPalooza]

I am designing a retaining wall with a driven pile foundation and attempting to determine the lateral capacity of the piles due to passive pressure. AASHTO Section 4.6.5.6.1.4 discusses Group Action for drilled shafts and shows a table for the reduction of lateral capacity in the direction parallel to the loading based on the spacing of the piles in that direction. The reduction seems quite large for piles that are closer than 4B and I am trying to determine if this is applicable to driven piles. Section 4.5.6.4 discusses Group Pile Loadings, but I was told that this applies to vertical loading only.
Does anyone have a clear interpretation of these sections? What should I use for my lateral capacity reduction? We are using 12" diameter steel shell piles filled with concrete.

Thanks,
PPalooza

 

[Focht3]

What are you building? Where? Soil/groundwater conditions? Any special conditions or considerations?

I don't have a copy of the AASHTO document you reference, so I can't be sure what the "quite large" reductions are that you have encountered. But putting 12-inch piles on less than 4 foot centers is pretty damn close.

Think about the problem another way: what happens when bolt holes are placed too close together, and the bolts are subjected to shear? The connection fails -

And keep in mind that the reductions in efficiency are based on both theoretical and experimental information. The reductions are real, and failure to incorporate the group effect into your design could have disastrous effects.



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[PPalooza]

This is for a retaining wall with two to four rows of piles. The back row will be vertical and the front rows will battered. No special conditions.
The AASHTO Standard Specs list a ratio of group pile action to single pile action of 0.40 for spacing of 4B and 0.25 for spacing of 3B.
I do recognize the need for reduction due to group action, however by utilizing this method for a second battered pile at 3 feet behind my front pile row, my total passive resistance is actually reduced.
For example, if I have a passive resistance in the pile of 1.0k for a single pile in the front row, when I add a second pile 3 feet behind it (this is the minimum spacing per Illinois DOT), my total resistance becomes
(1.0k + 1.0k) x 0.25 = 0.50k
Intuitively, it seems as though my total resistance should at least be 1.0k, not half of the original resistance.
Should I only reduce the resistance of the additional pile? Should I treat drilled shafts and driven piles the same as they pertain to lateral resistance?
I hope this clears up the question a little bit.

Thanks,
PPalooza

 

[Focht3]

That does seem goofy - I've always seen reductions taken on either the entire group, or on the "trailing" piles. Those seem like awfully punitive reduction factors, though - either way.

And the battered piles are picking up a lot of their resistance in axial force, not lateral bending.

Give me some more details about the soil conditions, loading, etc. and I'll think about it. Don't expect a response until tomorrow.



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[PPalooza]

I have already accounted for the force taken through axial load, but must check for the additional required bending resistance.

This question is not specific to one case but more a practical question that I consider independent of soil conditions since AASHTO provides reductions that do not consider soil. But for arguments sake, I will give you the data for this particular location:

0-5 ft Submerged very loose sand
5-15 ft Medium clay qu = 1.5ksf
15-20 ft Stiff clay qu = 2ksf
20-35 ft Hard clay qu = 8ksf

Thanks,
PPalooza

 

[Dee5]

A number of years ago I was involved in some 1/5 scale field tests of group piles for offshore structures. I believe the offshore industry and engineers designing offshore structures have a fairly good concept of better procedures than AASHTO for group pile "reductions." Bottom line is that I believe that if the clear spacing between piles is more than about 3 pile diameters, there is little or no reduction in maximum lateral resistance. I do not think you should be limited by the guidance in AASHTO as it is very conservative. Also, wall displacements are usually small enough that the initial tangent to the nonlinear soil supports for the piles can be represented by elastic springs, and there are a number of analytical techniques out there to solve the problem.

 

[Focht3]

[blue]Dee5[/blue] is correct; those guidelines are unnecessarily punitive. Can you kindly point me to the appropriate AASHTO section - I'd like to review it. It sounds like a typo, misprint - or someone's work that was taken entirely out of context -



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[VoyageofDiscovery]

Hi Dee5 and Focht3,

This is revolutionary. I, probably like most of us structurals, have been using 75% Kh reduction for 3 pile dia. c-c spacings to no reduction at 8 pile dia. c-c spacings.

Regards

VOD

 

[Focht3]

[blue]VOD[/blue]:

Don't panic - the available research that I've seen generally supports your approach. I suspect that there is something wrong with [blue]PPalooza[/blue]'s setup; I just don't know whether it's with the AASHTO documents or [blue]PPalooza[/blue]'s understanding of the same. And I won't know until I have a chance to read the AASHTO section that [blue]PPalooza[/blue] is referring to -



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[PPalooza]

AASHTO Standard Specifications for Highway Bridges, 17th Edition:
Section 4.6.5.6.1.4

Again, this is a section on drilled shafts and I am working with driven piles. Specifically concrete filled metal shell piles with a 12 in diameter.

AASHTO does not specifically cover lateral reductions for driven piles, as best as I can interpret.

Thanks for the help,

PPalooza

 

[VoyageofDiscovery]

Hi PPalooza,

The way you are approaching the design of battered piles sounds funny.

First, mixing bending and soil passive resistance with battered pile design is unknown to me. Typically, I find the resultant on the footing which accounts for the moment.

Second, I don't use the reduction you mention for this type of design. I use the reduction, as mentioned in my previous post when I am designing a pile group with no batters that are required to resist lateral loads. This is when I also use soil parameters.

For the design of battered foundations, look at a book by Teng, which describes it nicely along with a good example.

Regards

VOD

 

[PPalooza]

VOD:

Check out the bible (Foundation Engineering by Peck, Hanson, Thornburn). ;-) They have an illustrative example in Chapter 26, page 436 that shows a cantilever wall design with piles. They assume 2 kips/pile resisted by moment in piles combined with passive resistance of soil. The balance of the lateral load is taken by battered piles.

Interestingly, now that I have looked at it again, they are not taking a reduction for the proximity of the piles to each other. Perhaps this is the answer. I will find the Teng book and look at it as well.

Thanks,
PPalooza

 

[VoyageofDiscovery]

Thanks PPalooza,

Focht3, what do you mean unnecessarily punitive?

Regards

VOD

 

[Focht3]

This is unnecessarily punitive:

The AASHTO Standard Specs list a ratio of group pile action to single pile action of 0.40 for spacing of 4B and 0.25 for spacing of 3B.



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[VoyageofDiscovery]

Hi Focht3,

According to my math from what you mention research supports, I would apply a 60% reduction to Kh which I believe translates to 0.4 of a single pile.

So why would this be so punitive?

Regards

VOD

 

[Focht3]

I don't know why it is so punitive - and haven't confirmed that AASHTO really requires that much reduction when the piles are so widely spaced. If that is the requirement, I think it needs to be changed. I don't have a copy of the standard and haven't had time to visit the nearby university to read it; could someone kindly scan the relevant section and email it to:

thiswontlast@sbcglobal.net

Thanks!




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