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.