Civil Engineering Reference
In-Depth Information
3.8
Structural design
3.8.1 General principles
Despite many reservations about the idealisations and the model character of calculation
models in tunnelling, structural safety analyses have to be performed. No tunnel is con-
structed today by just relying on the experience of a practical person. The design work
has to include structural calculations based on geological investigations and forecasts,
mostly combined with parameter studies, and these are useful and required for the de-
sign of the tunnel construction and also for construction planning. The chosen construc-
tion process and the specified construction are checked with measurements during the
construction period and adapted if required. The possible dangers of recalculating and
calculating to a predetermined result can however often be difficult to avoid. Absolute
safety can neither be ensured in the original design nor in recalculation from measured
data.
When selecting a calculation model for a tunnel structure, the person responsible for the
verification of structural safety is faced with the complex problem of including all these
considerations in a realistic model:
- The construction states for the selected construction process with its special excavation
and support phases,
- The interaction of ground and support,
- The spatial and time-dependent alteration of the support with the different load-bearing
functions, particularly at the face and
- The estimation of the loading on the ground and support in the excavation and support
phases.
The recommendations of the DGEG [48] and the project-specific guidelines of the
Deutsche Bahn AG [193] are included in tender documents and suggest the use of a calcu-
lation model, which normally takes account of the following factors in the determination
of the forces and moments in a section:
- Primary stress state and bedding structure of the ground.
- Hydrological conditions in the construction state and completed state.
- The partial relaxation of the ground that occurs before the support measures become
effective, which has the result that the temporary support is only loaded by part of the
full ground pressure.
- Actions that occur later (for example in the completed state: traffic, rising water, frost,
excavation, building).
- Additional loosening of the ground from the effects of deformation, especially in the
crown and in some cases with time-dependent behaviour of the ground. These effects
can normally only be estimated.
The design is performed according to the applicable regulations for reinforced concrete,
steelwork etc. For the special features of tunnelling, the guideline 853 from DB Netz AG
(version 01/06/2002) is quoted here:
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