Experimental And Numerical Investigation On The Immersion Motion Of Tunnel Element Under Wave Actions

The immersion of large-scale tunnel elements is one of the most important procedures of immersed tunnel construction under the ocean environment conditions.The winds,waves and currents in the ocean will have an unfavorable influence on the immersion of tunnel elements. Studying the dynamic characteristics of tunnel elements in the immersion and the behaviour of cable tensions will be helpful for the immersion stability and construction safety of the tunnel elements.Up to now,there are still a very few researches about the motion dynamics of tunnel elements in the immersion under the ocean environment conditions.On the base of this consideration,the investigation on the motion characteristics of a tunnel element under wave actions during its immersion is carried out in this thesis,including the experimental and numerical study.The experimental study is conducted on the immersion of the tunnel element under regular wave actions in the finite water depth.The emphasis of the investigation is on the characteristics of the tunnel element motion responses and the cable tensions.The influences of the immersing depth and the relevant wave factors on the motion responses of the tunnel element in the immersion and the cable tensions are analyzed.The relations of phase among the waves,the motions of the tunnel element and the cable tensions are discussed under the right-angled wave condition.The experimental results indicate that the wave period and immersing depth have large effects on the motions of the tunnel element.Especially,as the immersing depth is relatively small,the motions of the tunnel element show strong nonlinearity.In the case of relatively small immersing depth,the effects of the wave height on the non-dimensional motion displacements of the tunnel element are obvious,and there occurs slack state periodically in the cables during the movement of the tunnel element.The investigations on the cable tensions and motion characteristics of the tunnel element under combined wave-current actions are also carried out.For the case of the joint actions of the right-angled waves and current,the current has large effect on the motions of the tunnel element.Under the current actions,the excursions occur to the tunnel element in the horizontal and vertical direction,as well as in the tunnel element roll motion direction.The magnitude of the excursions depends on that of the current velocity.And the cable tensions increase with the increase of wave height and current velocity.The experimental investigation on the immersion of the tunnel element under irregular wave actions is also conducted.The frequency spectrum characteristics of the motion responses of the tunnel element and the influences of the immersing depth,the significant wave height and the peak frequency period on the motion responses and cable tensions are analyzed.According to the experimental results,under the irregular wave actions the tunnel element has the wave frequency motions corresponding to the wave peak frequency period, and has the low frequency motions in addition.For the sway motion of the tunnel element,the low frequency motion is dominant in different immersing depths.But for the heave motion, that the low frequency motion is dominant gradually turns to that the wave frequency motion is dominant with the increase of the immersing depth.The larger are the significant wave height and peak frequency period,the larger are the motion responses of the tunnel element. Corresponding to the motions of the tunnel element,the frequency spectra of the cable tensions have similarly wave frequency and low frequency peak,and the cable tensions are comparatively large as the immersing depth is relatively small and the significant wave height and peak frequency period are relatively large.The frequency-domain and time-domain numerical models are developed respectively in the thesis.The frequency-domain numerical model is presented based on the 3-dimensional source distribution method,and with the model the wave loads and frequency responses of the tunnel element under wave actions in different immersing depths are calculated.The frequency analysis for the reflection to the immersion motion of the tunnel element in a linear system is a simple and convenient method.Based on the time-domain Green function that satisfies the free water surface condition in the finite water depth,the integral equation is established,and solved by the boundary element method.By adopting Newton second's law, the time-domain motion equations of the tunnel element are proposed,and solved by the fourth order Runge-Kutta method.On the basis of the theories above,the time-domain numerical model is developed.The model can reflect the characteristics of the nonlinear motions of the tunnel element and the nonlinear variation of the cable tensions,and can give the variation information of the motion responses of the tunnel element and the cable tensions with the time.Compared with the results of the experiments,the time-domain numerical model proves to be correct and effective.By using the model,the motion responses of the tunnel element and the cable tensions are calculated under different immersing depth, different negative buoyancy and different incident wave direction conditions,etc.The computational results further show the agreement with the experimental results.