The conventional concept of an empirical adhesion factor that relates undrained shear strength of a clay soil to ultimate shaft friction can be shown to have a theoretical basis for vertical piles, even though conditions at the interface are fully drained. The adhesion factor approach and the effective stress approach for shaft resistance are in fact consistent. This is because horizontal stress at the interface and undrained shear strength are both influenced by the same factors. The general implication of the conventional approach that the mechanism is undrained and that the mobilized shear resistance between a structure and clay is a fraction of the undrained soil shear strength is, however, incorrect. This concept can lead to significant errors if it is extrapolated to situations, for which the empirical data were not derived, such as beneath gravity structures on clay foundations, where effective stress normal to the interface is independent of soil undrained shear strength. Key words : piles, clay, adhesion, shaft friction.