**4. Conclusions**

The motion dynamic characteristics of the tunnel element and the tensions acting on the controlling cables in the immersion of the tunnel element under irregular wave actions are experimentally investigated in this chapter. The irregular waves are considered normal incident and the influences of the immersing depth of the tunnel element, the significant wave height and the peak frequency period of waves on the tunnel element motions and the cable tensions are analyzed. Some conclusions are drawn as follows.

As the immersing depth is comparatively small, the motion responses of the tunnel element are relatively large. Besides the wave-frequency motions, the tunnel element has also the low-frequency motions that result from the actions of cables. For the sway of the tunnel element, for different immersing depth the low-frequency motion is always larger than the wave-frequency motion. While for the heave, with the increase of the immersing depth, the motion turns gradually from that the low-frequency motion is dominant into that the wavefrequency motion is dominant.

For the large significant wave height, the motion responses of the tunnel element are accordingly large. The peak values of the frequency spectra of the motion responses increase rapidly with the increase of the peak frequency period of waves. Especially, for the heave motion of the tunnel element, the peak frequency of the response spectrum corresponding to the low-frequency motion increases with the increasing peak frequency period.

The total force of the cables at the offshore side is larger than that of the cables at the onshore side of the tunnel element. Corresponding to the motion responses of the tunnel element, the cable tensions are relatively large and their variations are more complicated in the case as the immersing depth is small and the significant wave height and the peak frequency period are large comparatively. The changing laws of the tunnel element motions and the cable tensions reflect the interrelation of them.

In this chapter, the immersion of the tunnel element is done from the fixed trestle in the experiment, by ignoring the movements of the barges on the water surface. Actually, when the movements of the barges are relatively large, they have influences on the motions of the tunnel element. The influences of the movements of the barges on the tunnel element motions will be considered in the further researches. The numerical investigation will also be carried out on the motion dynamics of the tunnel element in the immersion under irregular wave actions.
