ABSTRACT
This paper presents the initial results of a full-scale field study of single free-head piles embedded in Auckland residual clay. Four hollow steel pipe piles, each with an outside diameter of 273 mm and wall thickness of 9.3 mm were installed at a site in Albany, Auckland (NZ). The piles were excited dynamically with an eccentric mass shaker mounted at the pile head. Dynamic tests ranging from low level excitation (a zero-mass loading) to high dynamically-induced force amplitudes were performed after a wet winter period, so that the soil was saturated to the ground surface. Prior to the large force amplitude excitation, the shaker was run with zero mass installed and the observed natural frequency of the system was found to be about 11.0 Hz. After the high-level shaking, the natural frequency had reduced to 8.2 Hz. The reduction in natural frequency demonstrated the non-linear response of the pile-soil system due to the degradation of soil stiffness and the formation of gaps between the pile shaft and the surrounding soil. After three weeks of rest and some more rain, the gap between the pile and soil had disappeared and a natural frequency of 10.2 Hz was measured.
