Page 201 - Numerical Analysis and Modelling in Geomechanics

182 ANNAMARIA CIVIDINI AND GIANCARLO GIODA

Figure 6.5 Finite element mesh for the plane strain back analyses.
observed, however, that the numerical model “tuned” by the elastic back
analyses could not be adopted for the analysis of a different construction
procedure. In fact, the shape and size of the “weak” zone were a priori chosen in
the calculations, and it is quite likely that a change of the method of excavation
would involve, in turn, a change of this zone.

Elasto-plastic calculations
In order to circumvent the above-mentioned drawback it was decided to develop
an elasto-plastic model able to analyse the spreading of the “plastic” zone around
the railroad tunnel. To this purpose, a simple elasto-plastic material model was
considered, which is based on experimental observations from compression and
shear tests on “stiff” soil samples [23].
Among the various features of the mechanical behaviour of these geological
materials only the so-called strain softening was considered, even though other
aspects, anisotropy in particular, could play a non-negligible role in defining the
overall stress-strain response.
Figure 6.8 shows a qualitative representation of the results of a direct shear
test on a stiff soil sample under a constant, average vertical stress σ, in terms of
the diagram between the average shear stress τ and the horizontal displacement
δ.