Pile group is a commonly used structure in coastal and ocean engineering. The pile group effect is a major problem in wave force calculation of pile group structure. However, there are little systematic researches on the group effects for more cylinders with complex arrangements. In this paper, the experimental investigation of the interaction of irregular waves with the group of eight piles in a double row side-by-side arrangement is carried out. Statistical analysis and spectral analysis are applied in experimental research. The characteristics of the pile group in double rows side-by-side arrangement were analyzed.


Pile group structures are broadly applied in the area of coastal and offshore engineering such as offshore wind turbine platform and crossing bridges. Therefore, how to calculate wave force accurately is a problem that must be solved for designing pile group-supported marine structures. However, there are many uncertain issues in the wave force of such piles. When the distance between the piles in the pile group structures is small, wave force on a single slender pile is significantly affected by the neighboring piles. The formula which is based on the concept of Morison et al. (1950) for calculating the wave force of a single isolated pile is not applicable.

Many experiments had been conducted to study the interference effects of neighboring piles under the action of irregular waves. The inertia and drag coefficients (Cm and Cd) are related to the KC number which is suggested by Keulegan (1958). The total forces on the piles were computed from the mean curves of the inertia and drag coefficients. The correlation between the maximum calculated forces and the corresponding measured maximum forces is good. Chakrabarti (1981, 1982) measured inline wave forces for different pile group arrangements. The coefficients (Cm and Cd) are determined based on experimental data by applying the least square fit. Sundar et al. (1998) found that the variations of Cd and Cm with KC for inclined cylinders are significantly wide. Because of the limitation of laboratory experimental conditions, any relationship with the Reynolds number could not be established. Boccotti et al. (2012, 2013) revealed that the inertia and drag coefficients are given as a function of KC number and Reynolds number Re within the coefficients considered in their studies. Calculation of wave force by the Morison equation depends on the inertia coefficient, Cm, and the drag coefficient, Cd. So the key of calculating wave force of pile groups is to get inertia and drag coefficients accurately. However, they are not easy to be determined if the pile group structure is complex.

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