For non-cohesive soils the skin friction does not increases infinitely with depth as e.g. effective stress, but from a certain so called critical depth it acquires a constant value - see the following figure, where dc is the critical depth, Sc is the skin friction at critical depth and d is the pile diameter. Similar rule holds also for the pile base resistance in non-cohesive soils, where the same values of the critical depth dc are considered for simplicity.
The reference value of the critical depth for soft sand is 10d (d is the pile diameter or its width), for medium compact sand and compact sand the values are 15d and 20d, respectively.
The coefficient of critical depth kdc can be specified in the "Ver. capacity". The critical depth follows from:
critical depth coefficient