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Shafts are vertical or inclined tunnels. They are either driven downwards or upwards. In the former case, muck and inrushing groundwater has to be conveyed upwards, i.e. against gravity. This can be done, for example, with the help of air lift. If the shaft is driven towards a pre-existing tunnel (so-called bottom access), then muck and water can be conveyed downwards through a pilot borehole. Reaming of the pilot borehole occurs either downwards (‘downreaming’) or upwards (‘upreaming’ or ‘raise boring’, see fig. 11.1). For downreaming with machines the following two methods apply: A vertical TBM with conical cutting wheel is braced with grippers against the shaft wall and pushes downwards (Wirth). Alternatively the downwards thrust is provided by the own weight of the TBM (weight stack downreaming of Robbins, Fig. 11.2). For large diameters upreaming is risky because of possible instability of the face and the wall, difficult exchange of the disk rolls, limited thrust against the face and water inrush. Note that countermeasures can only be taken after the completion of the shaft.

In early days of tunnelling the toll in accidents was very high. It could, however, be gradually reduced, as inferred from the following table:

Tunnels are driven both in soil and in rock. The transition between soil and rock is not sharp, and there are many kinds of rock, which may be considered either as soft rock or as soil. If one refrains from the separation by arrays of joints, the strength of rock is usually modelled in the same way as the strength of soil. The differences are then quantitative, but not qualitative. On the other hand, if the size of the individual rock blocks is comparable with the tunnel diameter, then continuum-mechanical considerations are inappropriate, and one must regard the individual blocks. This requirement proves often to be rather academic, since we hardly know in advance the position of the individual joints. Thus, in most cases jointed rock is considered as a homogeneous medium with whatsoever assumed mechanical properties.

The analytic representation of stress- and deformation fields in the ground surrounding a tunnel succeeds only in some extremely simplified special cases, which are rather academic. Nevertheless, analytical solutions offer the following benefits: In this section, some solutions are introduced which are based on ’s law, the simplest material law for solids. The underground is regarded here as linear-elastic, isotropic semi-infinite space, which is bound by a horizontal surface, the ground surface. The tunnel is idealised as a tubular cavity with circular cross section. Before its construction, the so-called primary stress state prevails. This stress state prevails also after the construction of the tunnel in a sufficiently large distance (so-called far field).

Anchors or rockbolts are reinforcements (usually made of steel) which are inserted into the ground to increase its stiffness and strength. There are various sorts of reinforcement actions and the corresponding terminology is not uniform. The following terminology is used in soil mechanics:

In shallow tunnels the neglection of vertical stress increase with depth due to gravity is not justified. Thus, the solutions presented so far based on hydrostatic primary stress are not applicable. In this section some approximate solutions for shallow tunnels are presented.

In weak rock, the excavation face must be supported. It is important to estimate the necessary support pressure, in particular for slurry and EPB shields, where the pressure must be set by the operator. Several methods can be consulted for the estimation:

Experience shows that underground structures, especially deep ones, are far less vulnerable to earthquakes than superficial ones. The latter are endangered by earthquakes due to the fact that the motion of the ground can be amplified by the response of the structure to such an extent that the induced strains damage the structure. The earthquake waves can also be amplified within soft superficial strata. In addition, loose water-saturated soil may loose its strength (so-called liquefaction), and this can lead to landslides or failure of foundations and retaining walls. In contrast, deep buried structures, especially flexible ones, are not expected to oscillate independently of the surrounding ground, i.e. amplification of the ground motion can be excluded. This is manifested by the relatively low earthquake damage of tunnels. Of course, the portals may be damaged by earthquake-induced landslides. Very revealing on earthquake effects is the report of what happened to the driving of a 7 m diameter tunnel in the underground of Los Angeles during the San Fernando M 6.7 earthquake in 1971:

Apart from the assessment of stability, the determination of settlements at the surface is very important in tunnelling. However, in geotechnical engineering, deformations can be forecast with less accuracy than stability. This is mainly because the ground has a nonlinear stress-strain-relationship, so that one hardly knows the distribution of the stiffnesses. We consider here some rough estimations of the settlement of the ground surface due to the excavation of a tunnel. One should be aware of their limited accuracy.

Leaving aside a precise definition of mechanical stability we only need to mention that a loss of stability occurs if in the course of a loading process a mechanical system becomes suddenly (often the term ‘spontaneously’ is used) softer, so that large deformations appear. These can cause serious damage. The most widespread known stability problem is the buckling of a rod. Here we consider the buckling of a tunnel lining or, equivalently, a pipe.