The established foundation type for most offshore wind turbines is monopile. Due to waves and wind, the monopile of such structures experiences cyclic loading that causes the rearrangement of the soil grains around the pile, altering the damping ratio and stiffness of the soil-pile system. In this study, the impact of changing the loading frequency and soil density on the horizontal displacement, stiffness, and damping ratio on piles under cyclic lateral load has been evaluated. A series of experimental tests have been carried out by applying one-way cyclic lateral loading with three frequencies on PVC piles with different length-to-diameter (L/B) ratios in loose and dense sand. The horizontal displacement, stiffness, and damping ratio values were obtained for each test. Also, the constant coefficients of equations proposed by other researchers (to determine deformation, stiffness, and damping ratios) were evaluated. The results are presented that the change of loading frequency has a considerable influence on the absolute values of displacement, stiffness, and damping ratio, however, it has a negligible effect on constant coefficients.

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