Immersed tunnels in Japan: recent technological trends

An increasing number of immersed tunnels utilizing steel-concrete composite structures have been constructed in Japan. The full sandwich structure, which is one type of this structure, is a composite structure in which concrete is sandwiched integrally between two steel plates. In the construction of immersed tunnels, tunnel tubes are submerged and connected consecutively. Flexible rubber gasket joints are commonly used to connect tunnel tubes in Japan, but studies are under way to apply new joint structures such as "bellows joints" and "crown seal joints" to provide sufficient dewatering performance when they are located in the depths of the sea and absorb larger displacements that cannot be handled by the conventional rubber gasket joints. However, a gap inevitably remains between the last installed tube and the end section of the. tunnel. The V-block method is a finishing joint work method, which fills this gap with a wedge shaped connecting block utilizing its own weight and water pressure. The key element method uses the last tunnel tube itself as the finishing joint.