Brunel and Cochrane used a similar method to finish the Thames Tunnel. The front of the tunnel is excavated, be it with explosives or a modern tunnel boring machine. The following geological section shows the new tunneling method adopted in the construction of the Semmering-Tunnel in Austria around 1850. Depending on the stability of the rocks, various methods to secure the tunnel walls are adopted, from thin walls of brick and concrete to thick walls. The diagram shows also a wooden tunneling shield (Fig.5.

Source: Forbes March 25, 2018 18:22 UTC