The increase in pile bearing capacity with time after installation is known as soil/pile set-up. A mechanism of pile set-up is discussed in detail and a three-phase model is suggested. A serious of low-strain dynamic tests on full scale pile was carried out in mixed soil for monitoring the increase of wave velocity and stiffness. Wave velocity and stiffness have similar increase trend, but the stiffness shows a larger increase rate. An optimized design procedure to incorporate the pile set-up effects is proposed.