Point of virtual fixity and critical length of lateral loading for piles

Some engineers can get confused about the difference between the two terms which are point of virtual fixity and critical length used for piles for resisting lateral loads. Meant for critical length of lateral loading for piles, it denotes to a certain depth from the ground level where piles act as if it were infinitely long. Beyond the critical length, change in lateral response of piles with increase in pile length would be negligible.

Point of virtual fixity denotes to a certain dept below ground surface where piles are fixed without movement under loads. Depth to the point of fixity is helpful in assessing the buckling loads of piles. It is obvious that depth to the point of virtual fixity must be smaller than critical length of piles.