Page 217 - Numerical Analysis and Modelling in Geomechanics

198 ANNAMARIA CIVIDINI AND GIANCARLO GIODA

Figure 6.20 Experimental convergence vs time curves (black squares) at different tunnel
sections and corresponding numerical results (solid lines) based on the back calculated
mechanical parameters (α=1.5).

in the first part of the tunnel, before section 6, is slightly poorer than that
encountered after section 6.
Finally, the comparison between calculated and measured displacement vs.
time data is shown for all sections in Figure 6.20. Here the curves corresponding
to the numerical results were obtained by adopting the optimal values of the
mechanical parameters of the rock previously evaluated for each section.

Conclusions
Some procedures have been illustrated for the solution of back analysis problems
in geotechnical engineering, considering both their deterministic and
probabilistic aspects.
These procedures represent a practical tool for reducing the uncertainties that
often affect the parameters to be used in the design of complex geotechnical
works, in particular when the calculations are based on suitable numerical
models.
Once the numerical model able to describe the behaviour of the rock mass has
been calibrated, the model itself can be used to foresee the “response” of the soil/
rock mass to changes in the construction/excavation procedure, leading to a
possible refinement of the design. An example of this “observational” design
procedure has been discussed with reference to the excavation of a shallow
railroad tunnel.
Nowadays a marked increase is observed in the use of back analysis
procedures in geotechnical engineering practice and in the research for new,
more efficient, back analysis techniques. This effort concerns, in particular, the use