Sakishima Immersed-Tube Tunnel, Osaka

The Sakishima Tunnel, a road and rail link first planned in 1985, connects Sakishima, with transportation lines on the mainland. The tunnel is below ground at both shores, with an immersed tube section at the center terminated at both ends by ventilation towers. The total length of the tunnel is 2200 m, with the immersed-tube section 1025 m long. Since the tunnel passes beneath the Osaka Port's main waterway, unobstructed passage for large vessels was an absolute priority. A minimum depth was set at 13 m. The lower construction costs of the immersed-tube tunnel method led to its selection over two other proposals for the crossing, a bridge and a shield tunnel. The section of the tube consists of three boxes, side-by-side. The inner box carries two subway tracks, while the outer two boxes each have two lanes for motor vehicles. Each 30 000 t tunnel element is 35.2 m wide, 8.6 m high and 103 m long.