**2.2 Simulation of wave spectra**

In the experiment, Johnswap spectrum is chosen as the target spectrum to simulate the physical spectrum, and two significant wave heights, *Hs*=3.0cm and 4.0cm, and three peak frequency periods of waves, *Tp*=0.85s, 1.1s and 1.4s are considered. As examples, two groups of wave conditions, i.e. *Hs*=3.0cm, *Tp*=1.4s and *Hs*=4.0cm, *Tp*=1.1s, are taken to present the

Experimental Investigation on Motions of



that the wave-frequency motion is dominant.

θ (°)

η (cm)

Immersing Tunnel Element under Irregular Wave Actions 203

0 10 20 30 40 50 60 70 80

0 10 20 30 40 50 60 70 80

Fig. 5. Time series of the tunnel element motion responses (*d*=10cm, *Hs*=3.0cm, *Tp*=0.85s)

Fig. 6 gives the results of the frequency spectra of the tunnel element motion responses in the wave conditions *Hs*=4.0cm and *Tp*=1.1s for different immersing depths of the tunnel element. From the peak values of the frequency spectra curves, it is obvious that the motion responses of the tunnel element are comparatively large for the comparatively small immersing depth. Comparing the motions of the tunnel element of sway, heave and roll, the area under the heave motion response spectrum is larger than that under the sway motion response spectrum when the immersing depths are 10cm and 30cm, as indicates that the motion of the tunnel element in the vertical direction is predominant. In addition, it can be observed that there are two peaks on the curves of the sway and heave motion responses spectra. This illuminates that the low-frequency motions occur in the tunnel element besides the wave-frequency motions. The low-frequency motions are caused by the actions of cables. For the sway, the low-frequency motion is dominant, while the wave-frequency motion is relatively small. From the figure, it can be seen that the low-frequency motion is always larger than the wave-frequency motion for the sway as the tunnel element is in the different immersing depths. It reveals that the low-frequency motion is the main of the tunnel element movement in the horizontal direction. This can also be obviously observed from the curve of time series of the sway in Fig. 5. However, for the heave, as the immersing depth increases, the motion turns gradually from that the low-frequency motion is dominant into

**3.1.2 Motion responses of the tunnel element in the different immersing depth** 

t(s)

roll

t(s)

heave

simulation of the physical wave spectra. Fig. 4 shows the results of the comparison between the target spectrum and physical spectrum. It is seen that they agree very well.

Fig. 4. Measured and target spectrum
