Environmental Engineering Reference
In-Depth Information
initial stress state. To overcome this difficulty, two
solutions were studied. in the first one, the initial
stress state was determined by the unit weight of
the terrain (and k0) for the whole mesh. Then, the
alluvia were excavated in sequence until the desired
terrain profile was obtained. in the second solu-
tion, the initial stress state was only generated for
the breccia, followed by the staged construction
of an embankment until the desired profile was
reached.
in Figure 14 the relative shear stresses for the
two models are presented and it can be seen that
the excavation procedure leads to higher stress lev-
els and even to some plastification. The embank-
ment model has, in general, lower stress levels,
except near the portal zone where some of the
stress points have reached the failure criterion.
a)
b)
Figure 14. Relative shear stresses (initial stress state):
a) excavation model; b) embankment model.
4.3 Comparison between the two models
even though the two models depart from different
initial stress states, the results obtained after the
excavation of the cavern were quite similar, espe-
cially for deformations.
The calculated vertical displacements for the last
phase of the calculation are shown in Figure 15,
where no significant differences between the two
models can be pointed out. in both models, the
maximum vertical displacement occurs at the top
of the roof arch, in a section 15 m from the end
of the cavern. another important conclusion is
that the excavation of the cavern doesn't seem to
influence the stability of the hillside because no
internal relevant displacements were obtained in
the calculations.
From Figures 15 and 16 it is possible to con-
clude that the vertical displacements are similar
for the two models, not only for the last phase
but also during the complete excavation process.
another conclusion is that the largest percentage
of displacements happens when the pilot tunnel is
widened to the top heading. This effect tends to be
reduced when the points are closer to the surface
(point a0). after the conclusion of the top head-
ing, it can be observed that the evolution of the
displacements is almost constant in depth.
For horizontal displacements ( Fig. 17 ), a dif-
ference, although very small, resulted from the
two calculations. The larger deformations were
obtained when the terrain profile was generated by
excavation.
From the results of the 3D calculations, it can be
concluded that rockbolts are indeed very important
to maintain the stability of the vertical walls of the
cavern. Due to this fact, several parametric studies
were conducted in order to determine the influence
of some parameters (both geotechnical and geo-
metrical) in the rockbolt's mobilized axial force.
a)
b)
Figure 15. Vertical displacements—a) excavation model;
b) embankment model.
0
-1
-2
-3
-4
-5
-6
Stage Construction
3D-Exc. - A0
3D-Emb. - A0
3D-Exc. - C0
3D-Emb. - C0
Figure
16.
evolution
of
vertical
displacements—
profile P4.
a)
b)
Figure 17. Transverse horizontal displacements:
a) excavation model; b) embankment model.
 
 
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