Tunnels of Many Shapes
A composite circular shield tunneling machine. (C)Kajima Corporation
Improvements are constantly being made to Japan's shield tunneling machines. The Fukutoshin Line, a new subway line that opened in Tokyo in 2008, was built using a new machine that can dig oval-shaped tunnels, instead of circular ones. The problem with round tunnels is that the space at the top and bottom of the tunnel isn’t really used for anything, since the trains themselves are straight-edged and rectangular in shape. Elliptical (oval-shaped) tunnels are better, but until now the only way to build one was to use two round-cutting machines together.
The new "composite circular" shield tunneling machine is equipped with an adjustable cutter that can bore an elliptical tunnel. Tunneling machines that can dig other shapes, including square and horseshoe-shaped tunnels, are now at work at many sites.
The composite circular shield tunneling machine has six blades. Because blades B, C, E, and F stretch and contract as they turn, the machine can dig tunnels of various shapes. An oval-shaped tunnel means less space is wasted and less concrete is used.