When is a pile too short?

[sdz]

When designing piles/bored piers for lateral loads what is the limiting depth at which it ceases to behave as a laterally loaded pile and instead behaves as a spread footing? Is there a minimum embedment depth as a ratio of the pile diameter?

 

[Ron]

It depends on soil conditions and the depth of embedment needed to achieve pile fixity to resist the lateral load/moment.

Run L-Pile or do hand calcs using a variety of different soil strata and you'll see.

 

[SlideRuleEra]

For background information, see this 1960's article "How To Determine Lateral Load Capacity of Piles". It is on my website:

http://www.slideruleera.net/PileLateralLoadCapacity.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[sdz]

Pile design methods assume that the lateral resistance comes from bearing on the sides of the pile with lateral displacement of the soil, i.e. the predominant pile movement is sideways. However near the surface the soil will fail in a simple passive failure wedge with much lower pressure. Also the pile will tend to rotate about the base like a pad footing. So my question is really where does the transition from pile behaviour to pad footing behaviour begin?

In my particular case because of design considerations we are using vastly oversized piled and loads are small. We have a 3m x 2.5m plan shelter and we are using 750mm diameter piles. Using the Brinch Hansen method (BHPile) the calculated pile depth is only about 1 diameter. I suspect the method and the software are not appropriate for these conditions but I can't find any reference to say what the validity limits are.

 

[FixedEarth]

We need to determine if you have a circular shallow foundation or a rigid drilled pier. You may have to analyze it both ways. Jumikis's 2nd ed. "Foundation Engineering" book covers circular foundations. The 5th ed. Bowles book and also the '88 Reese & O'Neil manual on drilled shafts both recommend that the deflection at the pier tip be very small. I prefer to increase my drilled pier embedment until I reach a deflection of less than 1 mm at pier tip. Then I check lateral bearing pressure, settlement & axial capacity.

http://www.soilstructure.com/

 

[GeoBo]

Just run some lateral force analysis for the pile with using either LPile or PileLAT 2014. No other quick ways apart from that. This is simply because the minimum pile length depends on the loading, soil types and strength and the pile top boundary conditions. It is up to the design requirements such as maximum allowable deflection at the pile head, maximum bending moment or shear force mobilised in the pile under the lateral force, etc. Having said that, you can use 10 times pile diameter as a good starting point.