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Re: Pole embedment, 3rd Q.

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At 01:53 PM 11/11/99 EST, you wrote:
>Mr. Greenlaw:
>
>Thanks again.  However, I believe I did not state my problem with better 
>clarity.  I have the sign post pole embedded in 5' dia X 12' deep concrete 
>below grade.  For this scenario, can I take advantage of sliding resistance 
>available at the base of the concrete?  In other words, can I subtract the 
>sliding resistance from 'P' to calculate 'A' required?  While it may be 
>conservative to ignore the contribution from sliding resistance, neglecting 
>it may make the solution overly conservative.

        This may come as a surprise, but any sliding resistance acting on
the 5 ft diam bottom surface points in the SAME direction as the applied
load 'P' up in the air, not in the opposing direction. 

        The reason is that the pole anchorage embedment is primarily a
moment-resisting connection in the earth, not a shear-resisting connection.
The shear 'P' typically is only a minor part of the resistance the soil must
offer to the pole. Most of the resistance on the embedded portion takes the
form of a couple that equals the overturning moment that results from P
acting at a net height of [h above the effective grade plus an additional
depth below grade.] 

        The research found that the resisting couple has its UPPER resultant
at 0.34 of the effective depth of embedment. This upper resultant opposes 'P'
as well. The LOWER resultant of the couple was found to act at a depth of
0.90 of the embedment depth and acts opposite to the upper resultant, thus
in the same direction as the applied 'P.' 

        The place where the pole merely rotates in the ground without
lateral displacement or soil resistance was found to be at 0.68 of the
embedment depth.

        You can get a hands-on feel for this by grasping the eraser end of a
long pencil in your fist and pulling sideways on the other end with your
other hand. (The lower, eraser end should be more tightly gripped than where
the long end comes out of your fist, to represent stiffer soil resistance at
depth.) You will feel the resisting couple in your fingers. Without letting
go, now press the eraser down on a table top to obtain some frictional
sliding resistance. See what difference it makes.

        It would seem that any reliable frictional sliding resistance on the
butt end of an embedded pole would replace a small amount of lateral bearing
way down there, and make for a small reduction in depth, perhaps 0.03 of the
total depth. 

>Considering that the lateral resistance and the sliding resistance act on 
>entirely different planes, I am surprised that UBC does not have a different 
>set of formula for cases where sliding resistance can be taken advantage of.

        The original research was interested in much narrower diameters of
foundation than you have, and also was interested in sand backfill in the
hole around the steel or wood pole itself, hence there is no provision in
the resulting formula for a generous bottom surface having substantial
sliding resistance. You have to use some other approach if you want to use
sliding resistance at the bottom end of the foundation. I doubt that the
result would justify the effort. Extra depth helps you faster than does
extra width, in the UBC formula in question. 

Charles O. Greenlaw  SE