My other idea is on new construction method of underwater tunnel. There are two present underwater tunnelling methods: bored tunnel and immersion tube. The former is limited in size and shape following those of the boring machine and therefore is generally suitable only for train tunnel. Examples of underwater bored tunnels are Seikan tunnel between Hokkaido and Honshu islands, Japan and Channel tunnel between Britain and France (Figure 1). In the Immersion Tube method water tight and close ended pre-fabricated segments of the tunnel are lowered (or immersed) to the already dredged trench. We have more freedom to decide the size and shape of the tunnel's cross-section, taking into account the underwater depth of the tunnel and the number of traffic lanes. After they are connected with water-tight seal the water trapped between two segments is pumped out and the end walls are removed. The tunnel is then covered with backfill to provide protection. In this method connecting the segments is done underwater and to reduce the number of connections the length of one segment is over 100 m. We need special and heavy equipment to lift and to place those heavy segments (Figure 2) and this may affect the traffic in the river or strait. Furthermore the lowering and connecting process requires high-degree of precision to prevent damages if the lowered segment hits the already laid one and to ensure the quality of the water-tight seal. Examples of underwater tunnels constructed using the Immersion Tube Method is available by clicking here.
Fig. 1 Channel
Immersion Tube Method of underwater tunneling
The proposed method is meant to overcome the above problems of the Immersion Tube Method. Thus to solve the problem of underwater segment connection we connect them on land. The segments can therefore be made shorter and lighter and this reduces the internal stresses during handling as well as the need to have heavy lifting equipment. Instead of towing to the site and lowering (or immersing) them into the water, we slide the already connected segments on railed tracks. For this purpose the segment is equipped with wheels. Figure 3 shows typical cross-section of the tunnel segment with two separate tubes for traffic in both directions and two ventilation tubes at the sides. The cross-section is selected to produce negative buoyancy. The exact number and the dimension of the wheels are to be determined from more detailed analysis.
Figure 3 Typical tunnel cross-section
The construction starts with placing the rails on the already dredged trench (Figure 4) stretching across the river or strait. They are supported at a number of places as the design determines. The pre-fabricated segments of the tunnels are placed on the rail track and connected on land. The first segment has its front side sealed and the next one is connected at its other end with water-tight seal. The watertight seal should be able to deform without being damaged following the tunnel track’s curvature during the sliding process. Once the segments are connected we can slide them along the rails (Figure 5) making use of the gravity, as the track is sloped following the design slope of the tunnel. Then the subsequent segment is connected and the sliding process is repeated until the first segment resurfaces at the other side of the river or strait. Then the already installed segments are locked in their place and the remaining (above water) segments at the other side of the river/strait are placed using the same method or using the conventional cut and cover method. During the sliding process the traffic in the river or strait is little affected. When all connected segments are already on their final position, sand is sprayed underneath and around them to give permanent base (Figure 6) and the tunnel is further protected (Figure 7) by covering it with stones/gravels.
Figure 4 Placing the rails on seabed or river bed
Figure 5 Sliding the tunnel segments on the rail
Figure 6 Spraying sand to provide permanent base for the tunnel
Figure 7 Placing protecting layer on tunnel
Whilst further study is required the proposed Segment Sliding method might offer some advantages as compared to the Immersion Tube method. The segments are shorter and lighter leading to smaller internal forces during their handling and heavy lifting equipment is not required. The sliding process in the construction work much less affects the traffic in the river or strait. Whilst more watertight seals connecting the segments are required their quality and precision are more ensured as the connection is done on railed track and on land. Finally the proposed method might offer faster construction period.