Civil Engineering Reference
In-Depth Information
11.5.7.4 Support next to the tunnelling machine
If the ground is insufficiently stable, a tunnelling machine with active support and a shield
should be selected. Next to the tunnelling machine, the ground is then supported by the
shield skin, which however also completely obstructs access to the ground. Better passive
support is provided by a short cylindrical shield than by a longer tapered shield.
11.6
Ground and system behaviour
11.6.1 Preliminary remarks
The system behaviour is of essential importance for the selection of a tunnelling ma-
chine, i.e. the behaviour of the overall system consisting of ground and selected tunnelling
process [Austrian standard ÖNORM B 2203-2]. When a tunnelling machine is used, the
ground behaviour criteria are fundamentally different from those in conventional tunnel-
ling.
The geotechnical investigations are generally carried out based on DIN 4020. The deter-
mination of the characteristic values, the display and evaluation of the results of the geo-
technical investigation and the conclusions, recommendations and notes should already be
matched to the (probable) later tunnelling process early in the design phase.
More extensive and meaningful the preliminary investigations provide better precondi-
tions for the selection of a process and a tunnelling machine. In this regard, it is recom-
mended to include consideration of the entire process chain from excavation of the face,
clearance of the muck and the final tipping or recycling of the excavated material in the
planning of the geotechnical investigation.
The essential geotechnical parameters are summarised in Appendix 2 for each process
and can serve as a rule of thumb for the selection of a tunnelling machine. They should
be determined on each project for the relevant ground conditions. It should be noted that
deviations of these ground parameters from the assumed values can result in complex and
inconsistent consequences for the process chain. It is therefore recommended to provide
appropriate provisions in the contract.
It is helpful and practical to display the expected ground conditions in a geotechnical lon-
gitudinal section and assign sections to relevant tunnelling classes.
There now follow basic remarks about the process-oriented analysis of system behaviour.
A summary of the required characteristic values - split into soft ground and hard rock - is
given in Appendices 2.1 and 2.2.
11.6.2 Ground stability and face support
The stability of the ground is the primary criterion for the selection of a type of tunnelling
machine. The basis is the global and local stability of the face.
For an initial approximate evaluation of the stability (Austrian standard ÖNORM B2203,
1994, Tab 1), the following assignments can be defined according to the RMR classifica-
tion:
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