In recent years, the topic of soil-pile dynamic interaction has received a great deal of attention. Modern codes suggest that it should be taken into account in the seismic design of pile foundations and superstructures. The stiffness and damping characteristics of the soil-pile system during earthquake motion depends on the mechanical properties and geometrical characteristics of soil and piles as well as their mutual interaction. In both research and advanced practice, this problem may be approached with a direct method, modelling the whole dynamic soil-pile system with a 3-D finite element program, or by using theoretical approaches with different degrees of refinement. The results of these approaches are very sensitive to many parameters that define the dynamic characteristics of the soil‑pile system. In this context, experimental results of full- or small-scale in situ and laboratory tests represent an essential instrument to provide parameters to validate numerical and analytical methods and to calibrate 3-D finite element models. However, few full-scale in‑situ tests on pile foundations have been reported in the open literature to date.

This paper presents horizontal impact load tests carried out on a group of three steel pipe piles, vibro‑driven into marine soft clay at the tourist port “Mirabello” in La Spezia, Italy. The site characterization based on laboratory and in-situ tests together with the pile instrumentation system and the relevant chemical and mechanical protections are reported. The instrumentation of the pile consists of an unconventional technique for field tests in marine environment and includes accelerometers at the pile head and strain gauges and pore pressure gauge along the pile. The horizontal impact load tests carried out in two different campaigns, one performed 1 week after pile vibro-driving and the other after 10 weeks, are illustrated. A large amount of the results collected are presented, i.e. time histories of strains and accelerations, which clearly show the dynamic behaviour of the complex soil‑water‑pile system at very small strain. In particular, the response of the loaded single pile, in terms of natural frequencies, damping and mode shapes of the first and second pile bending modes is investigated.