Chapter 8. Bored tunnels.
Besides the construction methods from surface level and the immersed tunnel method, there is a third
important technique for building tunnels, namely the shield method. This method is suitable for building in soft soil conditions, varying from limestone and clay to sand. The technique was invented by the
French engineer Marc Isambard Brunel. It was first used to tunnel under the Thames in London, in the beginning of the nineteenth century. The shield is actually a cylinder of steel, which bores through the ground. In the front the ground is excavated, then the ground is transported to the back along the already built tunnel part. The actual tunnel is built inside the shield. This usually consists of prefabricated reinforced concrete segments, joined together with bolts. When the tunnel part inside the shield is completed, the shield is pressed forward by screw pumps attached to the previously completed tunnel section.