Lateral Restraint of Piles 1

[Abuh001]

Hi,

As can be seen from the image above have a single pile loaded with service load of 30KN. It is restraint vertically via a ground beam. However, the only horizontal (left to right) restraint is somewhat lacking. Would it be possible to on a sort of cantilever action of the ground beams? I assume one would have work out the restraint force and times it by the lever arm to work out the moment then check the concrete section to see if its adequate.

 

[EZBuilding]

I think the grade beam can be used to restrain your pile in the left to right direction. You should review and determine the amount of lateral load being restrained and the amount of deflection that the grade beam will have. Make sure that this level of deflection does not impact the capacity of the assembly to resist your vertical loads or create any serviceability issues. Also not that the grade beam needs to be able to resist the result of the pile being offset from the center of the vertical load. Where I practice, we provide for a 3 inch tolerance. Torsion of the grade beam is your only available mechanism to provide this restraint.

 

[jayrod12]

What type of piles? What diameter? How much do they stick above the grade? What sort of grade? Liquifaction an issue?

I ask these questions because often where I practice, we would consider the ground that surrounds the pile as capable of providing lateral restraint when designing the pile for axial load.

 

[EZBuilding]

That is a good point from jayrod, I had assumed these piles were helical piles with small lateral capacities...

 

[phamENG]

I wouldn't count on the lateral capacity of the pile. Around here, the "listed" lateral capacity of a pile is 1/2 the load that produces 1" of deflection at the ground surface. That little stub of a grade beam is what? 4 feet long? Imagine the size of the cracks required in that grade beam to mobilize the lateral resistance of the soil...

 

[Abuh001]

Hi all and thank you for your responses. I wish we had a U.K equivalent of this website, it is extremely useful.

I am just doing an initial scheme, so haven't designed this ground beams or piles yet. The project is based in Wallingford, UK. The soil investigation results indicate gault clay with a high volume change potential. The liquid limit varies from 70 to 85%, and the plastic index is at 45 to 50%. Personally, I would not rely on the soil.

Haha Pham, you have a good eye, yes it extends by 1.1m or just a bit over 3 1/2 feet. If we have a factored load on the pile of about 15 x 1.35 (Dead) + 10 x 1.5 (live) = 35.3KN, would it be fair to say that a horizontal force of 2.5% (roughly 1KN) should provide a sufficient restraint. Or have I vastly underestimated the difficulty of the problem.

Kind regards.

 

[jayrod12]

Fair points.

When we design it around here, we assume a 3m (10 foot) cantilever length of the pile for design. So there's some inherent give in the pile.

Also, 30 kN (7 kips) is bugger all for most piles. Where I am, we'd be providing a 400 diameter x 7500 long pile (16" x 25 ft) in that instance as a minimum to negate frost action. Gobs of capacity.

Also, this is the UK version of this website, it's also the Canadian, American, Australian, New Zealand, etc. We're international.

 

[phamENG]

If this is the initial scheme and you're using relatively small piles (8-10" diameter), put in a P-2 there (pile cap with 2 piles). It'll solve all of these problems without a thought. You can come back and shave it down later when you detail the grade beam. Depending on the size and material of the pile, on a large pile job the design and detailing effort to not install that one pile would be just about as expensive as installing the pile.

 

[Lomarandil]

I agree with throwing a pile cap in there -- not necessarily from a restraint perspective, but to give some adjustability when the piles are inevitably located at- or out-of-tolerance in plan view.

 

[KootK]

I feel that what you've proposed will be able to stabilize the pile laterally. In fact, I think that your approach will do a better job of stabilizing the pile than would a pile pair and addition pile cap.

I also don't feel that pile eccentricity / grade beam torsion is a concern. The lateral stiffness of you grade beam should deal with that successfully.

I'd be inclined to design for 5% lateral to acknowledge likely out of plumb tolerances in the pile install.

 

[Abuh001]

Thanks you all for your responses. I feel most inclined to agree with KootK as the load is pretty small and bending induced in the minor axis to stabilise the pile is relatively small. 5% of the factored vertical load sounds wise and would give a value of 1.5KN, this would result in a minor axis moment of 1.5 x 1.2 = 1.8KNm (assuming a 1.2m long cantilever). For a 450 x 450mm ground beam I don't think this should be an issue.

However, this brings me on the my 2nd question. I come across such scenarios on smaller projects often due to overhanging roofs, domestic balconies or projected upper stories, a single post is sometimes required away from the main building. When a pile and ground beam foundation is being designed, it sometimes strikes me as more economical to rely on the minor axis bending of the ground beam to restrain the pile than having to introduce tie beams. But what would you consider the limit for this reliance? When the ground beam fails in minor axis bending, a deflection that results in a lateral displacement of the pile by a given value or a certain beam projection length that you don't feel comfortable exceeding?

Finally, how would you engineers feel about using a localised pad foundation independent of the pile and ground beams. Though easier to construct I fear differential settlement between the piled foundations and the localised pad. I guess it depends on what is being supported.