# Experimental Study on the Dynamic Response of an Immersed Tunnel Element Moored inside the Harbor
## 4 Discussion  
In this paper, physical model tests of an immersed tunnel element moored inside the harbor were conducted, and the influences of environmental factors (including wind, wave, wind, and wave combination) and tunnel element load conditions (including no load, half load, heavy load, and full load, each of which corresponds to a different freeboard-to-draft ratio) on the dynamic responses were investigated. The mutual interference of multiple elements moored simultaneously inside the harbor was also analyzed. The dynamic response characteristics presented in the current experiments match those observed in earlier studies conducted in open-water situations [1; 13; 14]. Both show that tunnel motion under waves presents obvious characteristics of low frequency (corresponding to the natural period of the mooring system) superimposed with high frequency (corresponding to the wave period); low frequency is dominant in sway and surge, and high frequency is dominant for other motion components. The wind loads acting on the tunnel element are consistent with the theoretical results, especially when the freeboard-to-draft ratio is high. Figure 15 shows the experimental results and theoretical results of the element wind load under different freeboard-to-draft ratios (corresponding to different load conditions). The experimental results of the wind load calculated from the nylon cable side are very close to those from the steel cable side, and the variation trend with the freeboard-to-draft ratio is consistent with that of the theoretical results. The experimental results at a freeboard to draft ratio of 0.03 seem a little strange, but they are reasonable. As a freeboard to draft ratio of 0.03 corresponds to a full load condition, in this situation there are two pontoons and two survey towers located on it, resulting in an increased wind area as well as an increased wind load compared with that of a heavy load condition. Figure 15 also indicates that the difference between the experimental results and the theoretical results decreases with the increase of the freeboard-to-draft ratio; this is also reasonable. The wind not only acts directly on the freeboard but also indirectly on the tunnel element in the form of a wind-generated surface wave. The proportion of wind-generated wave force in the total wind load will decrease with the increase in freeboard resulting in a progressively smaller difference as the freeboard-to-draft ratio increases. It is somewhat surprising that under wave action, the half-load condition is the most unfavorable of the investigated load conditions. The result under no load is close to that under half load, while the results under heavy load and full load are much smaller than those under half load. A possible explanation for this might be that under no-load and half-load conditions, the centers of gravity are relatively high, resulting in relatively poor static and dynamic stability and greater motion responses and tension forces in cables. Previous studies on the dynamics of an immersed tunnel element under waves mainly focused on the situation with a small freeboard; the influence of the freeboard-to-draft ratio on the dynamics did not get enough attention. The current study indicates that the freeboard-to-draft ratio has a significant influence on the dynamic response of tunnel elements, and the most dangerous situation may not be in the small freeboard situation. The findings have extended our knowledge of the dynamic response characteristics of tunnel elements, which are especially useful for the mooring design of the element with a floating pouring prefabrication method.