Design of Straight Shaft Drilled Pier bearing into Rock

[oengineer]

I am looking for a design example for a Straight Shaft Drilled Pier bearing into Rock. The drilled pier approximate bearing stratum depth is 34 ft below existing grade until it reaches rock. I have skin friction & end bearing pressure after 4 ft penetration into the rock. This is for a moment brace frame for a slab-on-grade foundation. I have applied moment, shear, and gravity vertical force on the pier. I am trying to obtain a good design example of this case.

My goal is to obtain a design example that consists of:
a) The determination of pier size or overall concrete dimensions
b) The design of the concrete pier element itself

I am having issues with considering the applied moment and shear in the soil bearing check.

Suggestions and comments are appreciated.

 

[BridgeSmith]

The typical analysis method for the lateral loading and resistance is a P-y curve method, such as employed by Lpile or Allpile. There are some more sophisticated finite element modeling methods using non-linear soil springs. From what I understand, the results are often different, but no one seems to know whether they are actually any more accurate. In my experience, lateral resistance of soil is always a fairly rough approximation.

The size of the shaft is usually controlled by either the geotechnical axial resistance (skin friction or end bearing capacity) or the structural bending capacity of the shaft. Per most codes, you can have up to 4% or so reinforcement, but to meet the minimum clear spacing requirements (5 time max aggregate diameter or 5" in AASHTO) for drilled shafts, I've found the practical limit to be about 2% for most shafts.

The exception that we run into is foundations for poles, which have a limit on the allowable deflection. The programs I mentioned will output max moments, shears, and deflections. Getting a soil modulus (k) from your geologist or geotech will help considerably. If you have to estimate it, the results can vary widely depending on what you assume.

 

[Lomarandil]

HotRod's take lines up with what I've seen.

If you're in a one-off case where you can't justify the cost of Lpile/Allpile, you can use Brom's method plus the empirical charts from NAVFAC to get an approximation. (As HR mentioned, these are really all approximations, just progressively shinier looking ones).

----
The name is a long story -- just call me Lo.

 

[azcats]

Most geotechs have the capacity to assist with LPile type solutions. How soft is the material above the rock? Will this pier be essentially cantilevering from the socket into the rock?

 

[WARose]

In addition to what the others have said......you may want to check out this thread:

https://www.eng-tips.com/viewthread.cfm?qid=447822

I've done a lot of these over the years....and knowing what the rock socket can take has always been a sticking point.

The design is typically controlled by the moments and shears just above the rock socket.....but sometimes the L-Pile analysis they've given me shows a short spike in (side) bearing pressure/shear within the socket. (Sort of like a stress concentration if you will.) That's something to smooth out (no pun intended) with the geotech. (As i have seen them average it at times.)

 

[oengineer]

@azcats :

The first 34 ft below existing grade is Clayey Soils and Clayey Granular Soils, then the rock layer consist of Severely Weathered Limestone, Weathered Limestone, and Shaly Limestone.

@Lomarandil :

Is this the document (

https://files.engineering.com/getfi...-97da10373cae&file=Step5-Lateral_Capacity.pdf

) you are referring to when you mean using the brom's method? I just found it online.

 

[oengineer]

I was trying to use these formulas to design the pier for my condition. Here is the link for the formulas:

https://files.engineering.com/getfi...0-551f736629e7&file=Drilled_Pier_Formulas.pdf

I know that it is a underreamed pier, but the design principals should still be the same. When I use these formulas, my resisting moment is WAY MUCH LARGER than my overturning moment. Why are these formulas not sufficient for my situation?

Also, since I am using a slab-on-grade foundation, is it possible for me to consider the slab helping to resist some of the shear and moment forces, thus not having it all go into my pier? someone mentioned this to me and I thought I would ask.

 

[BridgeSmith]

The design approach you linked to ignores the contribution of lateral soil resistance along the shaft. It's essentially treating the bell as a spread footing. It may be suitable for shallow shafts with high axial load and small moments, but it would be ridiculously conservative for your situation. If you have any substantial moment, you'd need a massive shaft analyzing it that way.

 

[oengineer]

@HotRod10 :

Thank you for you clarification. When I am using this method, I am getting a very conservative design for my pier.

I myself do not have access to LPile. Is it possible to design the grade beam to resist the moment in the columns? I have been told that the grade beam can resist the moment applied to the pier and that the pier should just be designed for the axial vertical load. If it is possible, is there an example of this calculation?

 

[Lomarandil]

HR, take another look.. I think the method OE linked (but not the one he described in the thread) does account for some contribution from lateral soil resistance....

Mind you, I don't think I agree with the (latter) method shown, because it's treating the lateral pressure as though the shaft is moving laterally as a rigid body, rather than bending/rotating around some point below grade.

OE, your first link is the Broms' method I referred to. I'm not sure on the diameter of shaft you're considering, but if you have 34' to rock, that likely puts you in the "long/cohesive" method rather than the "short/cohesive" shown in that example. edit, check out the steps starting on page 9-86 here:

http://apps.itd.idaho.gov/apps/manuals/Materials/Materials References/FHWA-NHI-05-042-Volume I.pdf

It is possible to design a grade beam to resist the moment in the columns, dependent on relative stiffness. You typically won't be able to get enough contribution from a slab on grade to make that worth the trouble.

----
The name is a long story -- just call me Lo.

 

[oengineer]

@Lomarandil

Thank you for the reference material.

I am looking at using either a 24" or 36" diameter straight shaft pier. I see the reference document that you provided me is from a DOT, but the project that I am working on is for a school. Would this still be applicable?

Furthermore, would you happen to have any reference for designing a grade beam to resist the moment in the columns?

 

[oengineer]

@HotRod10 :

Based on your comments,"The size of the shaft is usually controlled by either the geotechnical axial resistance (skin friction or end bearing capacity) or the structural bending capacity of the shaft. ", would it be possible to just determining the size of the pier based on axial resistance using skin friction or end bearing capacity, then verify that the pier reinforcing can resist the applied moment.

Basically, I am saying I would use Qu = P/A to verify soil bearing capacity then use the applied moment to check that the strength of the pier is adequate. Then either add more reinforcing or increase the size of the pier diameter if needed.

This seems like a simpler way to design this pier.

 

[BridgeSmith]

You can start out either way. Just take your best guess on which will control the size of the shaft.

Be aware that if the top of the shaft can rotate and translate (i.e. it's attached to a column) the max moment in the shaft is higher than what's applied to the top.

OTOH, if it's connected to a grade beam, etc. that doesn't allow it rotate, the moment will be a function of effective stiffness of the shaft itself and the restraint of the surrounding soil, and will be higher than it would be as a column.