TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 689

21.3 Shield Design, Example

21.3.1 Statement of Problem
The loads upon a shield can be subdivided into external loads and operational loads.
The rock mass pressure and the water pressure are examples for external loads. The
most important operational loads are the jacking forces, the forces due to segmental
ring installation and the self-weight of the machine components.

Normally, the stresses resulting from external loads and from operational loads are
separately evaluated and then superimposed. In particular, the earth pressure is often
evaluated with strongly simplifying methods. For TBM tunneling with large diameter in
difficult geological conditions, however, the complex interactions of loads and stresses
need to be considered with more realistic approaches.
In the following the evaluation of the stresses in a shield subjected to external loads
and under consideration of the main operational loads is presented by means of the
example of the Hallandsas tunnel described in the preceding sections. The analyses
were carried out according to the FEM (Wittke-Gattermann & Wittke-Schmitt 2004,
Wittke et al. 2006).

Figure 21.26 Statement of problem, principle (Wittke et al. 2006)

In the northern part of the tunnel over a length of approx. 500 - 600 m the Möllebäcken
Zone (MBZ) is crossed (Fig. 21.14). The overburden height here amounts to approx.
150 m (Figs. 21.14 and 21.26). The groundwater table is also located approx. 150 m
above the tunnel's roof and is not allowed to be lowered, as already mentioned. Defor-
mation modulus and strength of the rock mass are comparatively low (Fig. 21.26).