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& Sommer 2011). As a result, each classification system leads to different support rec-
ommendations, and the tunnel would have been not safely designed with any of the
three classification systems (Fig. 12.18).
Figure 12.18
Österfeld tunnel, design recommendations on the basis of the Q, RMR and
RMi systems (Sommer & Wittke 2011)
12.7
Conclusions
The design method based on site investigations, rock mechanical models and adequate
analysis procedures as recommended by the tunneling study group of the German Geo-
technical Society (DGGT 1995) and the European Regional Technical Committee no.
9 for tunneling and underground construction (ERTC9 1997) has proved to produce a
safe and economic design. This design method has been successfully applied for sev-
eral decades by WBI and other designers and consultants and, in addition, is flexible
enough to allow adaptations and modifications during construction, if necessary. This
topic is intended to provide the fundamentals and executed examples of this design
method in which the rock mechanical model, the analysis model, the analysis method
and safety considerations form a unity. When applying this procedure, no rock mass
classification system is needed.
The application of classification systems, as shown by means of examples, involves risks,
since they reveal substantial flaws and deficiencies that can lead to unsafe or uneconom-
ic designs. One reason is that the case histories that the systems are based on are not
representative for all ground conditions, and the correlation between the rating index
and the required support is ambiguous, that is, the same ground conditions may lead to
distinctly different rating indices. In addition, rating indices do not adequately cover all
influencing parameters such as orientations of discontinuities and potential anisotropies
of deformability, strength and permeability of the rock mass (Sommer 2009, Sommer &
Wittke 2011, Wittke & Sommer 2011).
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