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homogeneous, isotropic and undisturbed rock mass as a ground pressure, which does or
can correspond to the overburden pressure when, for example, no additional tectonic pres-
sures or other effects are suspected. The primary stress state is influenced by:
- Depth,
- Overburden weight,
- Rock mass type (loose ground, rock),
- Characteristics of the rock mass (tectonics, bedding conditions, water conditions).
Fig. 3-2 (a) shows the primary stress state according to Willmann [259] in a planar model.
Figure 3-3
Principal
stress direction of the
spatial stress state with
assumed tunnel axis.
The directions of the principal stress of the triaxial stress state are mostly defined in rela-
tion to the tunnel axis (Fig. 3-3).
3.2.2.2 Secondary stress state
Fig. 3-2 (b) and (c) show the secondary stress state according to Willmann [259] in a planar
model. In order to forecast the effects of driving the tunnel on the formation of a secondary
spatial stress state depending on the time-dependent complex interaction of rock mass, tun-
nel structure and construction process, model concepts with the aid of idealisations are used
to order and simplify the numerous influential parameters for the load-bearing behaviour, the
loading and the material behaviour of rock mass and tunnel. One aim of this is to make the
influences and the complexity calculable through experience and conceptual models.
3.3
General steps of model formation
The generally formulated phases of operational research methods are also applicable for
the problem being discussed here (Fig. 3-4):
1. The formulation of the problem based on reality.
2. The design of the mathematical model for the system to be investigated.
3. The derivation of a solution from this model.
4. The testing of the transferability of the model to reality.
5. The precautionary monitoring and adaptation of the solution.
6. The practical implementation of the solution.
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