Friction on a posthole foundation 1

[youngstructural]

I'm trying to find what the pull out resistance for a post hole foundation would be. We often use 900mm diameter by 1200 to 1800mm deep, round posthole footings. I would like to use skin friction to assist against uplift. How would I develop the effective strength of a posthole due to friction on the side?

I'm trying to do something rational like apply a simple formula based on the ultimate bearing capacity. Say we assume the typical soil effective internal angle of shear to be 30 degrees (typical for the soil in my area) or 20degrees (if backfill), what would be a reasonable approximation of the withdrawl load? Does this depend on granular vs clay soils even at such a shallow depth?

If anyone knows of a design approach covering this it would be greatly appreciated.

Thanks for any help!

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Never mind... I've sorted it out myself.

If anyone is curious, the result for cohesive soils is a simplification of Terzaghi's bearing equation while cohesionless soils depend only on phi (effective internal angle of shear).

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[civilperson]

I would use 2660 Kg for the 1.8 m hole. Any skin friction value could be for excess safety.