Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 333

Site investigation and simultaneous testing for determining the rock mechanical param-
eters and the in-situ stress state (Chapters 13, 14, 15, 16 and 19) have to be carried out
at an early stage of project. The investigation program should be flexible and adjusted
to the state of knowledge and the envisaged construction methods.
A rock mechanical model has to be established in due time. This covers structural models
(Section 2.7.2), mechanical models (Chapters 3, 5 and 8) and hydraulic models (Chapter
6). The rock mechanical parameters describing deformability, strength and permeability
(Chapters 3 and 6) as well as the in-situ stress state (Chapter 9) that are required for stability
analyses (Chapter 10) must be derived for the rock mass units encountered in the project area
from the results of site investigations, laboratory and in-situ testing, previous experience as
well as engineering and geological judgement. Then the results of laboratory and in-situ tests
should be reviewed critically in view of their significance for real conditions (DGGT 1995).
On this basis a suitable construction method is specified as part of the preliminary plan-
ning. The means of temporary and permanent support such as the tunnel lining have to
be designed for the selected construction method.
Stability analyses and serviceability proofs are required that cover all the critical stages of
construction and loading. The applied analysis method should allow a realistic considera-
tion of geometric conditions, the rock-structure interaction accounting for suitable stress-
strain relationships and other influences. This is best done by means of the FEM (Chapter
10). Supplementary analyses with regard to the stability of rock wedges (Chapter 11) may
also become necessary. A safety concept which is based on potential failure mechanisms
can contain criteria such as stresses, strains, displacements or strength. Parameter variations
should be undertaken accounting for simplifications and uncertainties in the choice of the
analysis model and the analysis method. On this basis the final design is then carried out.
During construction, documentation of all construction phases has to be carried out. To
verify the design with the aid of measurements and the comparison with analysis results
and design criteria, a monitoring program has to be established from the very beginning
of the construction works. If little or no experience with the encountered rock mass is
available, the vault of a tunnel or cavern or even the entire cross-section may be excavated
in advance over a certain length as test excavation (Sections 13.7 and 13.8), equipped with
monitoring devices (Chapter 17). In addition, geotechnical mapping of exposed rock sur-
faces such as the temporary face should also be carried out (Section 13.9).
If the monitoring results differ significantly from the analysis results then modifications of
the support measures and/or the construction method will be necessary. The analysis model
and the parameters sometimes have to be adjusted to the ground conditions by means of
back analyses of monitoring results. Thus, the rock mechanical model on which the design
is based is checked and, if necessary, it can be adapted or modified during construction.
Not all load cases actually occur during the construction works (e.g. water pressure,
swelling pressure or traffic loads). In such cases long-term measurements should be en-
visaged, that is, monitoring needs to be continued after construction has been completed.
The rock mechanical model, the analysis model, the analysis method and the safety
considerations should be understood as a unity. Inaccuracies and too many simplifica-
tions in one part of the model have a negative influence on the reliability of the entire
model and thus on the safety concept too. Therefore the demands on accuracy should
be well balanced for all parts of the design (DGGT 1995, ERTC9 1997).