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Chapter 1
Introduction
1.1Philosophy of tunnelling
Tunnels are unlike any other civil engineering structures. In buildings or bridges, the building materials have defined properties and they can be tested to ensure they meet these, whereas this is not the case in tunnelling. Table 1.1 illustrates some of the issues associated with tunnel design when compared to above ground construction projects.
Table 1.1Comparison between tunnels and above ground construction projects
| Above ground construction | Tunnel construction |
| Construction material | The defined properties of the construction materials are guaranteed by the quality control procedures during the production process, including control testing | The ground, with all its uncertainty, and the general inability to influence its properties (notwithstanding ground improvement techniques) is the construction material |
| Loads | The loads for which the structural analysis is carried out are mostly known | Only by making assumptions is it possible to estimate the loads, which means that the magnitude of the load is based on assumption and is thus basically unknown |
| Safety | Since the properties of the construction materials and the loads are known, the safety factor relative to failure can be determined | Owing to the number of uncertainties related to the loads and material properties, it is not possible to calculate a quantitative factor regarding the safety of the tunnel construction |
Although a tunnel structure often needs support systems made up of concrete and steel, it is the ground that is the major part of the structure, and this can have both a supporting and a loading role. The key to successful tunnel construction is therefore to understand this material, in particular its strength and stability characteristics. No matter how much of the ground we test in preliminary site investigations and how many borehole cores we take for testing in the laboratory, we can only ever test a small fraction of the total ground to be affected by the tunnel construction. Therefore, it is up to the engineer to determine the relevant ground conditions and its associated properties. However, since only a small fraction of the material can be tested and with limited knowledge of, for example, the effects of layering, fissures and discontinuities, much of this assessment is based on judgement and experience. One might even suggest that emotions are involved. So how can this then be used as the basis for tunnel design? It is up to engineering judgement to interpret the site investigation report and suggest suitable design and construction techniques.
Often, the assumption is that the ground acts as a continuum and allows three-dimensional (3D) stress redistribution around the tunnel void, thus taking some of the load, so that the full overburden does not act as the load on the tunnel. However, how can anyone determine the percentage of this load-bearing capacity? Again, this comes down to engineering judgement. If a tunnel is lined using sprayed concrete, how can the residual stress-intensity-index be determined for the lining? If the displacement of discrete points is measured, how do we know that the maximum displacement has not been exceeded and the tunnel is not in danger of collapse? When is a crack in the tunnel lining significant and a sign of worse to come? Often, it simply comes down to engineering judgement and experience. Many of these questions do not have a single answer, but depend on the individual case. No new tunnel construction is the same as a previous one. During the construction of a tunnel, it is important to listen to the miners who have worked in many tunnels and use their experience to respond to different behaviour of the ground when excavating the tunnel. The key is to understand that tunnelling is not a discrete science with definite answers. There are many unknowns and the answer to most of the above questions is ‘it depends’.
Experience and engineering judgement help to make a considered and informed decision, but continuous measurements during construction are essential to compare actual behaviour with those predicted. This book does not propose to give the reader all the answers related to any tunnel construction. Rather, its aim is to provide the reader with background information so that he or she can either make an informed decision and/or consult more specialist references on a specific topic. It will hopefully give the reader the tools needed to critically assess tunnel constructions and to realise that not all can be learnt from textbooks but that, to become a tunnelling expert, many years of experience are required. At the same time, this book hopes to demonstrate to the reader the excitement associated with tunnelling and to make it clear that there are many unknowns that require engineering judgement. Solving these issues is the challenge the civil engineer faces. If the reader takes away one message from this book, it should be that the answer to a lot of questions regarding tunnelling design and construction is ‘it depends’ and sometimes using emotions is essential to overcome the challenges posed by tunnelling.
1.2Scope of this book
Tunnelling is an extensive topic and so the objective of this book is to provide a general knowledge base and guidance for further reading. It not only concentrates on different tunnel construction techniques but also brings in associated relevant topics such as site investigation, which have a large impact on the final tunnel design and its subsequent construction. It is important to note that tunnels in the context of this book include all types of tunnels, not just the larger-scale metro, road and rail tunnels, but also utility tunnels for water, sewerage and cables.
This textbook aims to provide a comprehensive introduction to tunnel construction. It is aimed at undergraduate and postgraduate students with little or no previous experience and knowledge of tunnel construction, as well as recently graduated engineers who find themselves working in this exciting field of civil engineering.
1.3Historical context
There has been considerable development in tunnel construction techniques in the past 200 years, especially since Marc Brunel’s famous first use of a tunnelling shield when constructing the first tunnel under the River Thames in London in 1825. Nevertheless, if Marc or his son Isambard Kingdom Brunel were to look at today’s tunnelling methods, they would see certain similarities with the techniques used in their day, particularly drill and blast and even tunnel boring machines (TBMs). The primary purpose of a TBM is to provide stability to the face and the surrounding ground, thus improving health and safety for the tunnell...
