The definition of effective pile length can be concentrated on bearing capacity or on pile head settlement. The study of effective pile length is still in progress because of the various affecting factors. The method of controlling the pile head stiffness is used to determine the effective pile length. Based on the Winkler model and load transfer method

the formula of pile head stiffness in two-layer soil is deduced; and then some affecting factors of effective pile length are analyzed The effective pile length increases with the increase of pile diameter; and decreases with the increase of the soil stiffness ratio (defined as the stiffness of the second soil layer to that of the first soil layer). The increase of pile base stiffness doesn’t play an obvious role in effective pile length. The effective pile length will be increased when the thickness of the first layer increase within a certain range.