Stability requirements for drilled shaft/caissons 1

[EZBuilding]

All,

I am familiar with the stability requirements (IBC 1810.2.2) for deep foundation elements based upon how they related to auger-cast pile foundations. I am currently working on a project which is considering the use of drilled shafts or caissons for the foundation system, and was looking for opinions on how that might change the stability requirements. In using a 54" diameter caisson embedded 80' into the soil, I expect that I will have to stabilize the top of the element with grade beams or a mat type foundation as this element would not qualify for the exception 1. "...height does not exceed 12 times the least horizontal dimension." Does anyone have any discerning opinions on this statement?

My comment is brought by having seen drawings which utilize a single drilled shaft below a single column,which does not meet the requirements for the stability exception. There was nominal attachment from the individual pier "cap" to the slab on grade, but at first glance this does not appear rigid enough to stabilize the drilled shaft.

 

[JAE]

I think a slab-on-grade around a single column/pier assembly would provide adequate bracing.

The concern that this section expresses, I think, is that the pier (pile? per IBC definition) would possibly be constructed a bit out of plumb and the column might then impart a P-Delta effect at the top of the pier - laterally pushing the pier sideways through the soil and creating a possibility of long-term instability.

Even though most slabs-on-grade are separated from the column by a wrap-around or diamond shaped expansion joint I would think that there would be adequate "catch" to prevent such a lateral movement of the column base.

Per 1810.12.2 it does say "Methods used to brace deep foundation elements shall be subject to the approval of the building official."

I know over the years I've done many column/piers where the pier was an 18" diameter to perhaps 24" diameter and extended down 40 ft. or so...which is in excess of the 12D limit. None with dirt floors though.

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[EZBuilding]

JAE, thanks for responding and for your thoughts.

That is also my interpretation for this section of the code, an out-of plum foundation element would have an induced moment which needs to be resisted or "braced". With A.C.P. foundations, I typically design for a 3" eccentricity in any direction. This 3" eccentricity multiplied by the axial load being resisted by the foundation system provides the design moment for the bracing elements. Once we received pile as-builts we are able to adjust this design by adding reinforcement based upon actual pile locations, etc.

Applying this same methodology to my slab on grade would yield some drastic results. Putting some numbers to it...

Pu=4000 kips
e= 3 inches
Mu= 1000 kips*ft
As = Mu/4d = 1000kip*ft/4*2inches =127.5 inches squared = lol

Is there an alternate bracing mechanism I am missing here? Is caisson placement tolerance significantly better than augercast piles in that the expected eccentricity significantly less? Is throwing numbers and moment designs into it overdoing and is this more of a prescriptive bracing requirement?

 

[KootK]

I disagree with your SOG analysis. The SOG should be assumed to act simply as a lateral pin support for the top of the pile rather than a stand alone, moment resisting element. That should yield much more favourable results.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

Yes - per KootK - I was implying a lateral only brace at the top of the pier.

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[EZBuilding]

KootK, thanks for joining the discussion - I didn't even have to mention strut and tie or shear friction to do it.

JAE & KootK; Considering the slab on grade as a pin support, I have to locate another point along my cap/pile element to provide a coupling force to resist the moment induced by the possible eccentricity.

See sketch in link below for two options for the resolving couple force:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1487716942/tips/Caisson_Stability_Sketch_sanla8.pdf[/url]

Maintaining the same Axial load and eccentricity as above,and assuming a 6' cap we could theorize new values.

Considering a resulting couple based upon passive earth pressure against my cap would yield a coupling load of 250 kips. This stills seems like too much load to impose on the slab on grade, and above what I could mobilize in passive earth pressure.

Considering a resulting couple based upon earth pressure against the caisson would yield a coupling load of 16 kips. This seems more reasonable but now it requires carrying the moment and shear through the caisson.

Any additional thoughts?

 

[JAE]

See the sketch below - the pier will be forced (by the P x eccentricity) to lean sideways into the dirt around it and into the slab-on-grade.
The slab-on-grade, in conjunction with the upper dirt around the pier will "fight back". The pier will rotate though and the dirt around the pier near the bottom of the pier will push in the opposite directly of the slab - thus giving you a force couple that would be much much longer than the possible eccentricity from the out-of-plumb pier.

So if your pier was 80 ft. deep and your eccentricity was 3", with an axial load of 400 kips, your initiating moment is 400 x 3" = 1200 in-kips.

With an 80 foot deep pier you might have a force couple of perhaps 50 feet? So 1200 in-kips / 50(12) = 2 kips of lateral force - taken by the soil and slab.

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[EZBuilding]

Jae,

That makes sense, you missed a magnitude in the axial load but that still requires a reasonable lateral demand at the top of the pile. I am more comfortable utilize this connection detailing now.

This may be getting into semantics at this point, but it does seems counter to the code provision to utilize soil bearing to stabilize the pile...

 

[JAE]

With a 4000 kips load I might reconsider the idea of using a slab-on-grade to brace the pier.
That is a very large load and a relatively thin (6") slab might not provide what I would feel comfortable with.

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[KootK]

KootK, thanks for joining the discussion - I didn't even have to mention strut and tie or shear friction to do it.

Yes, I prefer to be summoned by way of a large, rooftop lamp shone into the night sky with the shadowed outline of a CCT node inside.

This thread has been on my mind for a few days now. Like you, I find the matter a bit confusing with regard to the code intent and what I see in practice. I've developed a working theory that I'd like to share with you. I won't be able to supply any of those precious jewel "references" to back it up though. You'll just have to weigh it's measure as nothing more than another engineer's considered opinion.

First this:

This may be getting into semantics at this point, but it does seems counter to the code provision to utilize soil bearing to stabilize the pile...

Hell no. Consider:

1) No pile could be truly stable without relying on the surrounding soil, even if some restraint is provided at the top of the piles by foundation elements. Most piles utilize lateral soil bearing for stability. And those that don't certainly use friction against the soil at the bottom of the pile for restraint.

2) 1810.2.1 makes it pretty clear that lateral soil bearing is considered a valid means for stabilizing piles below grade.

3) I used to worry about soil stabilization of piles a great deal. Back in 2009, I took a seminar with Fellenius Bengt, one of the gods of modern pile design and research. I mostly took the seminar so that I could corner Mr. Bengt and get him to admit that we're negligent in not checking piles for buckling. Nope. He was absolutely adamant that it wasn't an issue and no pile in history had ever buckled below grade when embedded in anything that could plausibly be called "soil". He was so adamant, in fact, that he barely let me finish my sentences before interrupting me to tell me again how much of a non-issue it was. He even showed me a case study on some deep, driven steel piles that ended up having crazy, 25 degree kinks in them half way down. Still good performance and no buckling.

Okay, so back to the main question. I'll give my answer to that by essentially rewriting the provisions as I think that they were intended and, according to me, as I think that they should be. Commentary in red italics.

K1810.1 All piles shall be provided with a mechanism for ensuring rotational stability of the pile. Such restraint may be provided by one the methods described in K1801.1.1 through K18.01.4 or at the discretion of the building official. A close read of 1810.2.2 (pile caps & 1:12) leads me to believe that the code's concern is pile rotational stability

K1810.1.1 Provide a stiff foundation element at the top of the pile to provide rotational restraint to the pile. In particular, the code's discussion of two pile caps leads me to believe that the concern is rotational restraint rather than transnational restraint. You're only allowed to assume restraint along the axis of the grade beam. Really, for transnational restraint, a two pile cap would be more stable perpendicular to the grade beam.

K1810.1.2 Make the piles fat enough that that the applied axial load would still land over the footprint of the pile at the pile tip. Do this assuming a 1:24 out of plumbness in the pile installation. This makes the pile self righting and essentially provides rotational stability by providing faux rotational fixity at the bottom of the pile via precompression in the joint. If you track track the math through, looking at it this way with 1:24 tilt will lead to the H/12 requirement. 1:24 is also about 2.5% which I've seen used as a limitation in pile specifications on many occasions.

K1810.1.3 Provide lateral pile restraint at both the top and bottom of the pile for 1/24th of the axial load in the pile. This would provide rotational restraint to the pile and is consistent with the 2.5% out of plumb business discussed above. If you wanted to jack it up a bit to cover some additional misplacement eccentricity, so be it. In a way, this is kind of strut and tie and shear friction both. The sloping pile and the SOG are both compression struts joined by a node. And the bottom of the pile is kept from kicking out laterally by a version of shear friction between the pile tip and the bearing medium. There, that's what you get for taunting me.

K1810.1.4 Any deeply embedded pile that can be rationally shown to be rotationally fixed at some depth below grade via the clamping action of the adjacent soil shall be deemed to have met the requirement of section K1810.1.4. Again, the pile is afforded rotational restraint via fixity at the bottom, just as pile caps can provide that same fixity at the top. And demonstrating this shouldn't be too hard. The second paragraph of 1810.2.1 describes a situation where piles cantilevered into the air must have rotational restraint provided by the soil. In conclusion, I feel that, even without a SOG, all deeply embedded piles satisfy the stability requirement. And really, we're talking about a pile here that's almost as wide as I am tall and embedded three first downs into the soil. Are we really worried about the stability of it? Do we really feel that a 4" SOG makes a big difference? I'm not worried about it at all except, perhaps, under seismic/liquefaction action



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

Per FM5-134 (The Army Pile Construction Guide) (section 5-4):

"There is no danger of buckling a fully-embedded, axially loaded, point bearing pile of conventional dimensions because of inadequate lateral support, provided it is surrounded by even the softest soils"

It does make an exception later for slender steel piles embedded in very soft clays in figure 5-3.

It's also addressed in NAVFAC DM7-02 pages 7.2-197-200. This gives a few cases to consider, but also states several times that buckling is a non-issue for most fully-embedded piles

 

[humanengr]

Regarding interpretation of IBC 1810.2.2 (Stability of Deep Foundations);
If the Geotech report provides a lateral capacity for a single pile, does that
lateral capacity not provide the required lateral stability?