Applicability of 1.5 times area of pier for lateral loads

[Terzaghi]

In numerous geotechnical reports that address the lateral capacity of relatively short (5-15 ft) drilled piers I have seen a statement indicating that lateral resistance can be calculated by assuming the passive earth pressure acts against the projected area of the pier times 1.5. Can anyone shed some light on the physics behind this? I can't find any literature that indicates this is acceptable. I don't understand why a cylindrical pier should have some increased lateral capacity over a non-cylindrical pier or pile.

 

[dlew]

Terzaghi,

If the lateral force is applied at or near the top of the drilled pier, the pier is subject to a bending moment and the soil restraint can NOT be calculated as simple as the projected area times the soil passive pressure. For this case you could use the Broms' Method or one of the computer programs for laterally loaded piles.

If the lateral force is applied at the same depth as the soil passive pressure resultant, as it is the case for deadmen or anchors, then the soil restraint is a function of the passive pressure, the projected area of the deadman, and a coefficient larger than 1. This coefficient varies with the spacing and depth of the deadman, and included the additional shear resistance of the soil on each side of the deadman. Many books instead of a coefficient give the equivalent values of Kp. Remember to include a safety factor, since passive pressure is an ultimate capacity.
You could find information on deadman design in very old foundations books, per example "Steel Sheet Piling Design Manual", a publication of United States Steel. "Foundation Analysis and Design", by Bowles also discusses the subject.


Hope this will answer your question.

 

[Terzaghi]

Thanks for the response. I agree with what you are saying, however, I am just trying to find out where this 1.5 times the area of the pier comes from. The senior engineers at my firm are kind of old school. They just say they learned from their mentors that this is the case. I have also read a quite a few geotechnical reports that indicates this "magical factor" may be applied. I just can't find any justification for it and the senior engineers at my office are content in their inability to explain why this is the case.

Perhaps this is just some kind of "geotechnical urban legend" in my part of the country.

 

[DRC1]

Brom's method for lateral load on a pile assumes that the effective passive area is three times the face width. At some point someone probably started using 1.5 to have a factor of safety of 2 on the passive pressure coefficient. This method works well on a braced soldier pile wall, but be careful on a catilevered pile, as the stress distribution along the pile is somewhat more complicated.

 

[BigH]

Check out paper by Golder and Seychuk in Pan Am Geotechnical conference - I think it was 1964 or so. They have provided background to soldier piles for braced excavation of the Toronto subway. Sorry, I am overseas and don't have exact details of paper.

Best regards to all.

 

[wiktor]

Terzaghi:
The physics behind this is the shear developed as the pile shaft moves latterally under the load, both bearing against the soil in front and shearing the soil on parts of the sides.
The factor 1.5 is not universal - for small diameter piles it could be as high as 2 (dia below 450mm), and for piles above 1200mm some authors are recommending 1.25 (J Bowles - Foundation Analysis and Design).

 

[Focht3]

Terzaghi -

You are right to be skeptical - it's only a rule-of-thumb/order-of-magnitude estimate to check calculations from more precise techniques. I think it is based on a paper by Brinch Hansen (1964?) that has an unrealistic passive pressure distribution on the pile. Rather, an unrealistic and dangerous pressure distribution.

Broms techniques are fairly simple, fast and pretty good (I checked them out against COM624 (LPILE's predecessor computer code) in the 1980's. While it is perfectly reasonable and prudent to use a rough estimation technique to review the results of more sophisticated analyses, there's no excuse for using these kinds of rules-of-thumb for final design.