Pile Critical Depth

I am confused on how to determine the critical depth of piles. I see that is something I need calculate, if I want to find Q tip and Q friction. I have read in some of my notes that I need to take 50% of the pile length in other notes 10D, 20D etc. I am not really sure which one is it. Also I see the beta, alpha and lambda method to solve for friction, but do I need them all or they should give me the same answer?

If anybody has any information appreciated.

Thanks

How are you applying the concept?

Generally, the Dc = crtiical pile depth is calculated as follows:

Dc = critical depth for piles in loose silts or sands.

Dc = 10B, for loose silts and sands

Dc = 15B, for medium dense silts and sands

Dc = 20B, for dense silts and sands

I am assuming though that you are evaluating the capacity of the pile - more information is needed to answer the question.

JR