Beneath cities are the foundations of large buildings and many pipes carrying water, electricity, gas and telephone cables. Builders have either to tunnel very deeply or to use a technique called “cut-and-cover”, which means that they run the railway under existing roads, so that they simply have to dig a huge trench along the road, build the railway, and cover it up again.
The building method used for many years was a so-called “cut-and-cover” system. It was easier to dig out a large open hole in the road, build the arch of the false tunnel with bricks, and then refill the hole with the dug-out material. As a result, the first underground lines were not very deep, something that tends to be the case with the older underground lines in major cities.
One of the companies of the time, C&SLR, was the first to use electric traction for pulling the trains, as well as a new method for digging circular tunnels, using a technology known as “shield tunneler”, which initially was opearated manually. The front part was used to dig out a circular section and the tunneling machine was called the Greathead Shield. Later on, the shield became mechanical and the machine advanced much more rapidly and could cut through any type of material. Today, these are called TBMs, or Tunnel Boring Machines.
These tunnelling machines made it possible to dig under the city at a greater depth and create new underground lines on another level: they could dig under buildings and keep away from electricity lines, sewers and other infrastructures. They could even dig under the Thames.
Fast forward 155 years to our times. A massive new rail undertaking spanning over 100 km, more than 40 km of which run below the streets of London, connecting with the underground network at some points.
Picture Credit : Google
