Shallow Pier Foundations

[angledesign]

Looking for a design resource for shallow pier foundations for very light loadings. In the range of 4' to 6' deep and appx 12" in diameter. Something to econimically support small cabins and barns. On shallow piers can you count on any skin friction or just end bearing. Every resource I come across is for much larger loads and diameters.

 

[JAE]

If you are in a cold/winter region, the frost action of the top layers of soil will negate any skin friction...usually the top 3 to 5 feet or so is ignored anyway.

I would think the 4 to 6 ft. piers would be end bearing only.

 

[geodan]

I would use end bearing only. Skin friction increases with depth and as such, there is very little friction in the top 5 to 10 feet. Also as they are not (I assume) driven piles, the increase in soil stress along the pier would be minimal. I would neglect skin friction for 4 to 6 foot deep piers.

For end bearing, I would probably use Meyerhoffs method (it is a good 'average' or 'middle of the road' approach/result)

Sorry, I don't have any links to an online website for Meyerhof's method, but it is Qpoint = Area of point x effective overburden x Meyerhof's Nq* factor for piles

and the limit is: qlimit(kN/m2) = 50 x Nq* x tan (phi)

use whichever is lower.


Also as a side note, for dia = 1' and L = 4' your results will be close to results from typical bearing capacity formulas for shallow foundations.

 

[emmgjld]

As geodan notes, With piers so short, you are essentially dealing with a shallow foundation.