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The necessity for tunnels and the benefits they bring cannot be overestimated. Tunnels improve connections and shorten lifelines. Moving traffic underground, they improve the quality of life above ground and may have enormous economic impact. The utilisation of underground space for storage, power and water treatment plants, civil defense and other activities is often a must in view of limited space, safe operation, environmental protection and energy saving. Of course, the construction of tunnels is risky and expensive and requires a high level of technical skill.

The significance of installations in tunnels can be easily illustrated when looking at the cost. For the 9.2 km long Plabutsch western tube (road tunnel) in Austria, the following sums have been spent:

The underground is a vast unknown that can hide many unpleasant surprises (e.g. weak zones, water inrushes etc.). Therefore, a detailed site investigation is necessary not only for technical purposes, but also for the contractual regulations of all involved parties.

The heading of a tunnel comprises the following actions: excavation, support of the cavity and removal of the excavated earth (mucking). It is distinguished between conventional (also called incremental or cyclic) heading and continuous heading. This chapter introduces the several methods applied. A rigorous classification is difficult, as these methods are often combined. It has become usual to distinguish between conventional (or incremental) heading on the one hand and continuous (or TBM) heading on the other hand. This is, however, not reasonable: also TBM-heading consists of several steps and is, thus, incremental (cf. Sections 4.2 and 4.4.4).

The lining of a tunnel is never loaded by the stress which initially prevailed in the ground. Luckily, the initial (or primary) stress is reduced by deformation of the ground that occurs during excavation but also after installation of the lining (here ‘lining’ is understood as the shell of shotcrete, which is placed as soon as possible after excavation). Here we shall consider the important phenomenon that deformation of the ground (soil or rock) implies a reduction of the primary stress. This is a manifestation of . Since the deformation of the ground is connected with the deformation of the lining, it follows that the load acting upon the lining depends on its own deformation. This is always the case with soil-structure interaction and constitutes an inherent difficulty for design as the load is not an independent variable. Thus, the question is not ‘which is the pressure acting upon the lining’, but rather ‘which is the relation between pressure and deformation’.

Grouting is the introduction of a hardening fluid or mortar into the ground to improve its stiffness, strength and/or impermeability. There are various patterns of the propagation of the grout within the ground: : The grout propagates into the pores of the soil but leaves the grain skeleton unchanged. The resulting grouted regions are spherical, if the soil is homogeneous and isotropic and if the source can be considered as a point. If the pore fluid, which initially fills the voids, has a higher viscosity than the grout (as is e.g. the case when water is pumped in into a porous rock filled with oil) then the so-called fingering is observed. The resulting boundary of the grouted region is fractal shaped.

The New Austrian Tunnelling Method (NATM, in German: NÖT) emerged in the years 1957 to 1965 and was entitled in this way to be distinguished from the Old Austrian Tunnelling Method. The NATM was developed by Austrian tunnelling specialists (). Its main idea is to head the tunnel conventionally, to apply support (mainly shotcrete) sparingly and to follow the principles of the observational method. The NATM requires the distortion of the ground to be kept to a minimum (in order to avoid softening and thus loss of strength). But at the same time sufficient ground deformations should be allowed in order to mobilise the strength of the ground. Consequently, thick and stiff linings which do not completely abut on the rock, are no longer in use. According to the main principles of the NATM were guesstimates (mainly by ), which could not be applied until the techniques for shotcrete and rock monitoring had been developed. As many of the NATM’s recommendations were already in use, it is not easy to differentiate NATM against other tunnelling methods. This has led to a lengthy controversy, which is still underway. The debate does not refer to the content but rather to the name of the NATM because the lack of an exact definition makes it unclear in which cases this name should be used.

In this chapter particular attention is paid to the groundwater flow within rock, as the percolation of soil is rather well-known from textbooks on soil mechanics.

The idea to apply compressed air to prevent groundwater from entering into excavated spaces goes back to Sir Thomas Cochrane, who obtained a patent in 1830.

The following possibilities are available for the crossing of water ways: These are slow and have a reduced transport capacity when compared with bridges and tunnels (e.g. the Channel tunnel has shortened the drive from Paris to London from 6 hours to 2 hours and 40 minutes). In addition they are dangerous to other ships and are affected by bad weather.