How are underground structures constructed?

Some of you have been asking me how underground structures are built, so here’s a simple summary.

There are two methods of excavation, drill-and-blast and using the tunnel boring machine. Drill-and-blast is a cyclical procedure, where the rocks are first drilled, blasted with explosives, ventilation is created, then the overhanging rock is supported by rock bolts or concrete, and the whole process repeats itself (Zhao & Tan, 2012). 

Drill-and-blast process. Picture taken from www.mtr-shatincentrallink.hk/en/construction/construction-methods

Using the tunnel boring machine on the other hand is a continuous procedure (Zhao & Tan, 2012). The tunnel boring machine looks like a ferocious, teeth-baring earthworm enlarged up to a 100 times. I know, earthworms do not have teeth, but just imagine a set of nasty spiralling teeth in them, and you have a tunnel boring machine! The Cutterhead, which is the front part of the elongated machine, contains roller cutters. Pushing the machine in and rotating the Cutterhead helps to crack hard rocks (Zhao & Tan, 2012).

Tunnel boring machine. Picture taken from www.dlr.de

Both methods have their strengths and weaknesses and they are used for different rock types and ground conditions. For example, the tunnel boring machine can work on soft to the very hard rocks, but it fails in drilling rocks of differential geology, where strong rocks are mixed with weak rocks. Drill-and-blast is suitable for differential geology but it cannot cope with groundwater seepage (Zhao & Tan, 2012).

Therefore, before tunnelling works can take place, engineers have to investigate the underground properties, like rock types and the level of groundwater infiltration to choose the appropriate excavating process.

Literature cited:

Zhao, J. & Tan, S. B. (2012). Underground space development in Singapore rocks. PTRC and NCUS Workshop on Underground Space and Rock Cavern Development in Singapore, NTU. 17 January 2012.