Page 210 - Numerical Analysis and Modelling in Geomechanics

BACK ANALYSIS OF GEOTECHNICAL PROBLEMS 191

Figure 6.16 Characteristic lines of a tunnel and of its temporary support.

crown. Consequently, only the convergence along the horizontal diameter of the
tunnel was considered in the calculations.
Figure 6.15 shows the recorded variation with time of the horizontal part of
the vertical diameters of nine different sections of the tunnel [25]. The distances
of these sections from the Italian entrance of the tunnel, and their depths of
cover, are reported in Table 6.1.
Since the poor quality of the rock mass at section 6 did not permit a reliable
monitoring of the convergence, the data of that section were not considered in
the back analyses.
The calibration problem was solved in two subsequent phases. First, the values
of cohesion and friction angle (c and φ) were determined, considering only the
maximum (long-term) values of the measured displacements. Subsequently, the
viscosity coefficient η was back calculated, taking into account the increase of
displacements with time.
The back analysis of the average mechanical parameters of the rock mass was
based on the assumptions of axial symmetry with respect to the tunnel axis, and
of plane strain regime. Hence, the tunnel is equivalent to a circular opening in a
homogeneous and isotropic, weightless medium, subjected to a hydrostatic in
situ stress state p , and the excavation process is simulated by decreasing the
0
pressure p on the opening contour, from its initial value p .
0