**1. Introduction**

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An immersed tunnel is a kind of underwater transporting passage crossing a river, a canal, a gulf or a strait. It is built by dredging a trench on the river or sea bottom, transporting prefabricated tunnel elements, immersing the elements one by one to the trench, connecting the elements, backfilling the trench and installing equipments inside it (Gursoy et al., 1993). Compared with a bridge, an immersed tunnel has advantages of being little influenced by big smog and typhoon, stable operation and strong resistance against earthquakes. Due to the special economical and technological advantages of the immersed tunnel, more and more underwater immersed tunnels are built or are being built in the world.

Building an undersea immersed tunnel is generally a super-large and challenging project that involves many key engineering techniques (Ingerslev, 2005; Zhao, 2007), such as transporting and immersing, underwater linking, waterproofing and protecting against earthquakes. Some researches with respect to transportation, *in situ* stability and seismic response of tunnel elements are seen to be carried out (Anastasopoulos et al., 2007; Aono et al., 2003; Ding et al., 2006; Hakkaart, 1996; Kasper et al., 2008). The immersion of tunnel elements was also studied (Zhan et al., 2001a, 2001b; Chen et al., 2009a, 2009b, 2009c).

The immersion of a large-scale tunnel element is one of the most important procedures in the immersed tunnel construction, and its techniques involve barges immersing, pontoons immersing, platform immersing and lift immersing (Chen, 2002). In the sea environment, the motion responses of a tunnel element in the immersion have direct influences on its underwater positioning operation and immersing stability. So a study on the dynamic characteristics of the tunnel element during its interaction with waves in the immersion is desirable. Although, some researches on the immersion of tunnel elements were done in the past years, there is still much work remaining to study further. Also, the study on the immersion of tunnel elements under irregular wave actions is not seen as yet.

The aim of the present study is to investigate experimentally the motion dynamics of the tunnel element in the immersion under irregular wave actions based on barges immersing

Experimental Investigation on Motions of

Immersing Tunnel Element under Irregular Wave Actions 201

springs are chosen. The relations between the elastic force and the spring extension are shown in Fig. 2. There are four strings that join four springs respectively to control the immersed tunnel element in the waves. Two strings are on the offshore side and the other two on the onshore side of the tunnel element. To measure the tensions acting on the strings,

0 0.5 1 1.5 2 2.5

d=10cm d=30cm d=50cm

extension (cm)

The CCD (Charge Coupled Device) camera is utilized to record the motion displacements of the tunnel element during its interaction with waves. Two lights with a certain distance are installed at the front surface of the tunnel element, as shown in Fig. 3. When the tunnel element moves under irregular wave actions, the positions of the two lights are recorded by the CCD camera. Finally, the sway, heave and roll of the tunnel element are obtained from

Fig. 3. Photo view of the tunnel element at the wave flume. (a) wave is propagating over the

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

four tensile force gages are connected to the four strings respectively.

Fig. 2. Relations between the elastic force and the spring extension

the CCD recorded images by the image analysing program.

tunnel element; (b) the tunnel element and CCD

**2.2 Simulation of wave spectra** 

tension (kg)

method. The motion responses of the tunnel element and the tensions acting on the controlling cables are tested.

The time series of the motion responses, i.e. sway, heave and roll of the tunnel element and the cable tensions are presented. The results of frequency spectra of tunnel element motion responses and cable tensions for irregular waves are given. The influences of the significant wave height and the peak frequency period of waves on the motions of the tunnel element and the cable tensions are analyzed. Finally, the relation between the tunnel element motions and the cable tensions is discussed.
