Why the length:diameter should be more than 3:1 for a caisson

[DavidLiu]

Why the length:diameter should be more than 3:1 for a caisson to be a caisson.

what is the difference of caisson and augered shallow foundation.
Thanks

 

[BigH]

Can you give reference to back up the "caisson" definition? - i.e., where did you see 3:1?? Traditionally, the L/d ratio is taken as 4 to 5 for acting as 'deep foundations' compared to acting as a shallow foundation (see original Terzaghi bearing capacity equations). Subsequent improvements such as Hansen suggests that bearing factors are now "independent" of the L/d ratio. For piles, I still stick with the Berentzev values.