Pile group is a commonly used structure in coastal and ocean engineering. The wave action on pile group structures has always been the focus of scholars' research. Because of the vortex shedding around the piles, small scale piles are different from large scale piles. Except inline force, transverse force of a small scale piles cannot be ignored. In order to explore the interaction between different piles, experimental investigations of the interaction of irregular waves with small scale, vertical bottom-mounted pile group which has 9 piles in side by side arrangement have been carried out. Considering the comprehensive influence of the relative pile diameter and KC1/3 number, a new parameter KCLD1/3 is proposed. The influence of relative spacing on the wave force of the pile group is analyzed. The change of pile group coefficient, inline force and resultant force with KCLD1/3 parameter and relative spacing are discussed.


Pile group structures are widely used in the area of coastal and offshore engineering such as crossing bridge and offshore wind turbine platform. However, there are many uncertain issues in the wave force of such piles. Accurate analysis of wave force is essential for designing pile group-supported marine structures. 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.

So, a lot of laboratory tests had been conducted to study the interference effects of neighboring piles under the action of irregular waves. Chakrabarti (1981, 1982) measured inline forces on instrumented sections of the piles, the inertia and drag coefficients (Cmand Cd) are determined based on experimental data by applying for instance the least square fit. These coefficients are shown as functions of 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. However, any relationship with the Reynolds number could not be established primarily because of the small range of Reynolds number covered by the test. Sundar et al. (1998) found that the variations of Cd and Cm with KC for inclined cylinders are significantly wide. Boccotti et al. (2012, 2013) revealed that the inertia and drag coefficients are given as a function of KC number and Reynolds number Re for KC in (0, 20) and Re in (2*104, 2*105). Calculation of wave force of pile group by the Morison equation depends on inertia coefficient, Cm, and drag coefficient, Cd. However, the inertia and drag coefficients are not easy to be determined.

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