Pole foundations on slopes 3

[civilgee]

I am trying to determine the effects of a light pole foundation located on a slope. When looking at the groundline moment applied to the foundation, how do I account for the slope? What is the angle of the slope for which it can be ignored. I cannot find any guide for determining the appropriate resistant earth pressure due to a slope. Any input or direction would be greatly appreciated.

 

[Focht3]

How steep is the slope? Soil type? Pole size & load?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[JAE]

Well - I would think that the slope doesn't affect the discreet design of the pole/pile itself as the piling only "knows" that there is earth directly around it and usually we ignore the top 2 to 5 feet of soil anyway due to poor compaction, moisture infiltration, and freeze/thaw movements.

But on a slope, there is a more global slope stability issue where you might have a failure plane through the pile. This would be a concern in that there would be a shearing force introduced through the pile, and/or the pile would be dislocated laterally due to slope failure movements.

 

[Focht3]

... we ignore the top 2 to 5 feet of soil anyway due to poor compaction, moisture infiltration, and freeze/thaw movements.

Good points. But they may or may not apply to this case, since it's in/on a slope. (I would expect good compaction.) Moisture infiltration and frost penetration are problems in limited areas of the U.S., and a sloping site should have good drainage...

However, your argument that

Well - I would think that the slope doesn't affect the discreet design of the pole/pile itself as the piling only "knows" that there is earth directly around it ...

is incorrect. Soils around laterally loaded piles (including light/power poles) fail by two mechanisms: wedge failure and flow. Wedge failure occurs near the ground surface - typically (but not always) within 4 pile diameters of the ground surface. Soil flows around the pile at deeper depths, where the wedge failure doesn't occur.

On a slope, the wedge will extend to greater depths in the downslope direction, and to shallower depths in the upslope direction. The real art is in evaluating the effect of the slope on the formation of the soil failure wedge -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[civilgee]

The slope is approximately 20 degrees and is a well-compacted sandy clay. My question is really in regards to the embeddment depth required to resist overturning. Due to the slope, there might not be adequate material in the horizontal direction to develop full earth pressure. How much of a reduction in lateral earth pressure is made on the side of the pole nearest the toe of the slope? Thanks for any help.

 

[Focht3]

Generally speaking, you are concerned with the area that is within four pile diameters of the foundation in question. For a 20[°] slope, my view is that the "equivalent level ground surface" is about 4[×]tan(20[°]) [×]d where d is the pile/pier/pole excavation diameter. The math works out to about 1.5[×]d - so pretend that the 'real' ground surface is 1.5[×]d deeper than actual, and run your calculations from there. It isn't perfect, but it is rational for slopes with "shallow" factors of safety of 2 or more.

This approach doesn't work if the "shallow" and/or critical factors of safety for sliding are less than 2. In that case, you will need to do additional work to address the lateral resistance available to the foundation.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"