Frequency- and intensity-dependent integrated impedance function of laterally loaded caisson foundation in cohesionless soil

This paper experimentally investigates the frequency- and intensity-dependent dynamic characteristics of a soil–caisson system. Lateral dynamic loading of varying amplitudes is applied over a wide frequency range at a certain height from the foundation’s head, inducing both lateral and rotational movements in the system. The integrated response of the scaled caisson model embedded in dry sand is evaluated through the impedance function (IF). Results reveal that both loading intensity and frequency significantly influence the real (dynamic stiffness) and imaginary (dynamic damping) parts of the impedance function. At lower frequencies, the impedance function exhibits strong amplitude-dependent behaviour, with dynamic stiffness decreasing as the amplitude increases, while at higher frequencies, the sand mass and resonance effects dominate. The dynamic damping follows a similar amplitude-dependent trend within the lower frequency range, governed by hysteretic damping primarily due to local nonlinearities (partial separation between foundation and sand), whereas the dominance of radiation damping is observed at higher frequencies. Additionally, upon correlating impedance function results with the foundation’s depth of rotation, it is revealed that the depth increases with amplitude because of a drop in dynamic stiffness and the existence of soil and local nonlinearities, which are further confirmed through near-field pressure measurements. Furthermore, the results of this study also show that the dynamic stiffness tends to diverge from the secant static stiffness as the amplitude of dynamic loading increases. These findings highlight the importance of considering the system’s nonlinearities while investigating the dynamic behaviour under increasing loading intensity.