## 6 Conclusions  
Herein, the prototype was one of the 6.3-m super-large-diameter piles of the Xhiounen Rail-cum-Road Bridge, and an experimental study was reported of the hydrodynamic loads on a super-large-diameter pile under wave-only and wave-current actions. Wide ranges of wave height and current velocity were used to measure the inline and transverse forces on three piles (\(D\) = 4.5 cm, 7.5 cm, and 10.5 cm). The drag, inertia, and lift coefficients of the piles were calculated based on the modified Morison formula, Kutta-Joukowski formula, and linear wave theory, and the influence of the transverse force on the resultant force on the piles was also analyzed. The main conclusions are summarized below. In deep-water conditions, for waves and current propagating in the same direction, the wave height decreases as the current velocity increases. In the experiments, the influence of high velocity on the wave height was very obvious, and at \(U\) = 0.4 m/s, the wave height decreases by more than 0.3 \(H_{0}\). While the wave period remains mostly unaffected by the increase in current velocity. Both current velocity and wave height have a significant impact on the hydrodynamic loads on a super-large-diameter pile, with the inline and transverse forces obviously increasing with increasing current velocity and wave height. In addition, compared with the wave-only-induced inline and transverse forces, the fluctuation amplitude of the wave-current-induced inline force decreases greatly with increasing current velocity, whereas that of the wave-current-induced transverse force increases rapidly. Under strong waves and currents, the peak value of the transverse force can reach tens of times that induced by wave-only action, becoming of the same level as the inline force. The drag coefficient \(C_{D}\), inertia coefficient \(C_{M}\), and lift coefficient \(C_{L}\) are well correlated with \(K\)C* under wave-only and wave-current actions. Corresponding empirical expressions for the hydrodynamic coefficients \(C_{D}\), \(C_{M}\), and \(C_{L}\) of the super-large-diameter pile for \(K\)C* \(\approx\) 2-16 was proposed from fitting the experimental data, and these expressions can be used to quickly estimate the inline and transverse forces acting on a super-large-diameter pile in deep water. Compared with the small-diameter pile with \(D\) = 4.5 cm, the inline and transverse wave-current forces on the super-large-diameter pile with \(D\) = 10.5 cm increase more \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Pile**} & \multirow{2}{*}{\begin{tabular}{c} **Mean** \\ \(F_{H\_max}/F_{x\_max}\) \\ \end{tabular} } & \multirow{2}{*}{**Mean \(\theta_{H}\)[\({}^{\circ}\)]**} & \multicolumn{3}{c}{**Significant Interval**} & \multicolumn{3}{c}{**Insignificant Interval**} \\ \cline{6-9}  & & & \(K\)C* & \
\begin{tabular}{c} **Mean** \\ \(F_{H\_max}/F_{x\_max}\) \\ \end{tabular} & \(\theta_{H}\)[\({}^{\circ}\)] & \(K\)C* &
\begin{tabular}{c} **Mean** \\ \(F_{H\_max}/F_{x\_max}\) \\ \end{tabular} & \(\theta_{H}\)[\({}^{\circ}\)] \\ \hline \(D\) = 10.5 cm & 1.14 & 25 & 8–16 & 1.23 & 36 & 0–8 & 1.02 & 11 \\ \hline \(D\) = 7.5 cm & 1.12 & 24 & 12–23 & 1.22 & 37 & 0–12 & 1.03 & 8 \\ \hline \(D\) = 4.5 cm & 1.10 & 24 & 19–38 & 1.15 & 32 & 0–19 & 1.02 & 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of resultant and inline forces on different piles. significantly with increasing current velocity and wave height. Similarly, under the same wave-current conditions, the peak values of \(C_{D}\) and \(C_{M}\) for the super-large-diameter pile are slightly larger than those for the small-diameter pile, while the peak value of \(C_{L}\) can become more than twice that for the small-diameter pile.