Abstract: The vertical cyclic loading device designed in-house was used to study the bearing characteristics of monopiles in sand and the deformation mechanism of the soil around the pile by laboratory model tests. According to the test results, the cumulative settlement of the pile can be divided into three regions: the non-developing region, the gradually developing region and the damaging region; the envelope area of the hysteresis loop of the hysteresis curve shows a trend of gradual decrease with the increase of the number of cycles, the hysteresis curve develops from non-closed to closed curve, and the deformation of soil around the pile gradually changes from elastic-plastic deformation to elastic deformation. The particle image velocimetry (PIV) technique is used to measure the soil deformation around the pile in real-time under the cyclic load, and the complete displacement field and shear strain field of the soil around the pile are obtained. The results show that the cyclic period, amplitude, and compactness are the main factors influencing the soil deformation around the pile. The shear damage zone shows an inward trend close to the soil surface, and the sheer damage surface is nearly parallel to the pile-soil interface as the increase of the cyclic period. The larger the cyclic load amplitude is, the more the surface soil tends to be compacted under the cyclic load, the lateral earth pressure increases, the displacement influence area decreases, the corresponding shear strain field shows an “ear” distribution, and the amplitude is more likely to cause the soil around the pile to sink than the cyclic period. The cumulative settlement of the pile in different densities of sands shows different characteristics with cyclic period. The displacement field of the soil around the pile shows an inverted truncated cone in loose sand, while dense sand shows a cylindrical shape.

Key words: vertical cyclic load, pile foundation, soil deformation, particle image velocimetry (PIV)