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Re: Pole Embedment Formula

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You might also AASHTO specifications.  The have formulas which are written for
discrete vertical elements in bedrock in their retaining wall section.  I
haven't used the bedrock formulas but have used the other formulas for embedded
H piles with treated timber framing horizontally between.

Joseph R. Grill, PE

"Swingle, Mark" wrote:

> On 9/16/99 EDT, you wrote:
> Both of the pole embedment formulas indeed contemplate ordinary soils that
> are not very rigid compared to the pole, over the pole's depth of embedment.
> Embedment into bedrock is quite a change from that.
> The formulas came from work done in the 1940's at Notre Dame and one other
> university for the Outdoor Advertising (billboard) Assn, and involved hole
> diameters of a foot or two, and depths of ten feet or so. The formulas got
> into UBC a long time ago. Several editions later the allowable lateral soil
> loads in the UBC table were considerably reduced, making the result more
> conservative than originally.
> The "Constrained" condition is for when there is a hard resistance to
> lateral movement at grade (hard compared to the soil below) and that acts as
> a pivot point or fulcrum to the horizontal forces acting on the pole, the
> pole being modeled as a rigid free-body.  Max pole moment is at this
> constraint.
> For "unconstrained" conditions (all-dirt embedment) the max pole moment is
> at the point of zero pole shear, a depth that varies according to how low or
> high above grade the lateral force is, compared to the embedment depth. I
> worked up a table of values;
>   M = P (h + .17d) is about as bad as they got for small h compared to d.
> If you are developing all your resistance in bedrock, I submit that neither
> of the UBC formulas applies.
> Why not go back to the soil engineer for an interpretation of what he/she
> had in mind? Keep principles of mechanics in mind at all times; it ain't
> voodoo.
> Charles O. Greenlaw SE   Sacramento CA
> -------------------------------
> Charles, can you elaborate on your reply?
> 1.  If the pole formlae don't apply in rock, then how would one
>     determine the depth of embedment required to resist the
>     lateral force P?
> 2.  Then, how would one determine the point of fixity in rock?
>     How did you come up with fixity at d/6 below the surface?
>     I was "taught" by several engineers that fixity is at d/3,
>     although no justification was given.  Sometimes soils
>     engineers would allow fixity at 2' below the top of rock,
>     but not all are willing to give an answer.
> Thanks
> Mark Swingle, SE
> Oakland, CA