With the ever increasing urbanization in the world, the demand for underground space continues to grow. Very many urban areas are located on soft ground. Hence tunnellers need to understand how to create underground spaces in this type of strata. In this context, soft ground is defined as a geomaterial with a low strength – such sand or clay, conglomerate or weak rocks like siltstone or chalk. These geomaterials tend to behave largely as continuum. In harder rock (under low stress), discontinuous behaviour tends to dominate – such as block stability. Hence the construction and design of tunnels in harder rock is distinctly different.

First, this course will look at the over-arching principles of design in the context of soft ground. Topics such as safety, risk management, sustainability and design codes will be covered in outline. The key aspects of geotechnics will be highlighted, on the assumption that the attendees have a basic understanding of this subject already.

To design and construct, safe, economic and sustainable tunnels in soft ground, it is essential to understand the interplay between the geology, the hydrogeology and the underground space being formed. This involves soil-structure interaction.  The key mechanisms acting in these cases will be examined as the first step to identifying the geohazards and then defining risk mitigation measures. The control of water – both in the temporary and permanent cases – is an important subject in its own right. The course will introduce the armoury of tools available and outline their optimum range of application.

The course will introduce the common types of construction used in soft ground – namely, cut and cover, TBM tunnels with segmental linings, Sprayed concrete lined tunnels (also known as SCL or NATM). The logic for choosing which method is best will be explained. All of the construction methods influence the design but this interplay is most pronounced with SCL tunnels. For this reason, one module is dedicated to SCL tunnelling, covering all aspects from the mix design and properties of sprayed concrete, through to construction sequences and modern design approaches (including composite lining theory).

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