There are few pieces of industrial machinery as huge, as complex and as awe-inspiring as the tunnel boring machine, and over the past century both the machines themselves and the construction projects they lead are some of the most complex, ambitious and dangerous in the world.

A lot of work is placed into both innovative construction, surveying and stabilisation methods to make the process of constructing a tunnel as safe, efficient and economical as possible.

This has led to three main tunnel construction methods that are used depending on the circumstances of their construction.

The cut-and-cover approach is the simplest and often the most appropriate construction method.

It involves digging a trench and building a supporting roof on top of this. It is effective, can be undertaken with standard construction and digging equipment, but is highly disruptive on the surface.

By contrast, the tunnel boring machine method uses a dedicated, often custom-made machine, which leads to a fast, efficient construction but with a huge initial startup cost as every part of the machine needs to factor in the various ground and rock conditions.

A third option first devised in 1957 and first used in the 1960s was the New Austrian tunnelling method (NATM), designed to carefully monitor the types of rock surface encountered during the tunnelling process, uses the surrounding rock mass to keep the tunnel stable and employs sprayed concrete to stop deformation.

It consists of seven main elements:

• Taking advantage of the existing rock surrounding the tunnel to provide support for the tunnel.
• Application of a thin layer of shotcrete (sprayed concrete) to avoid rocks from loosening and becoming deformed.
• Careful and consistent monitoring and measurement to install additional supports only when you need them.
• Flexible support structures that provide active support reflective of the surrounding conditions, rather than a thick concrete lining.
• Keeping the bottom invert of the tunnel closed to ensure a strong tunnel in soft earth.
• Construction contracts that are flexible enough to factor in the variable construction environment inherent with NATM.
• Classification of different rock masses from very soft to very hard, which determines the support required for different parts of the tunnel.