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Technical Committee 204 /
Comité technique 204
(SAI). A non-linear relation between the SAI and the maximum
serviceable cutting distance for the cutting tools was achieved
and it is displayed on Figure 5. A linear relation was found
between the ratio of the covered cutting distance over the
maximum cutting distance and the ratio of number of tools to be
changed in each intervention over the total number of tools in
the TBM cutter head.
Figure 5. Correlation of the Soil Abrasivity Index (SAI) and the cutting
distances of disc cutters (after Köppl and Thuro).
Sato and Kuwano
in their paper “Effects of buried
structures on the formation of underground cavity” present
several model tests to investigate the consequences of an
unfavourable water flow condition around buried structures.
Such a flow may lead to an underground cavity and eventually a
surface collapse. The observations suggest that the loss of
confinement near the buried structured would increase local
permeability and therefore the water flow around that area.
Xie
et al.
present a thorough analysis of induced settlements
from immersed tube tunnel (IMT) on deep soft subsoil, as stated
by the title of their paper: “Subsoil settlement feature of
immersed tube tunnel in deep soft subsoil with heavy siltation
in open sea”. The study is based on the project of the Hong
Kong-Zhuhai-Macao Link that comprises a bridge that will be
more than 35 km long and a 6 km long IMT (Figure 6).
Figure 6. Location of Hong Kong-Zhuhai-Macao immersed tunnel (after
Huang
et al.
).
Based on on-site CPTU test results and in-situ bored soil
samples numerical modelling was performed to investigate the
stress-path of the subsoil during and after construction of the
IMT. To confirm the results tests were run in a geotechnical
centrifuge to measure and analyse the whole process of
consolidation, excavation, tube location, backfilling, siltation
and re-excavation of channel.
The results indicate that in the process of excavation and
backfill recompression, the stress and settlement distributions
bellow the tunnel cross-section have a saddle-shape, as shown
for the stresses in Figure 7. The measured settlement differences
between the cross section and the longitudinal direction of the
tunnel are small, as expected.
The papers in this topic went through several possible
conditions were the soil behaviour has to be assessed and
predicted for a proper design and construction of an
underground facility. Köppl and Thuro properly discussed the
importance of a proper selection of collected data for an
empirical method. Shahin et al. showed us how the combination
of a powerful constitutive method, with properly assessed
parameters, with a model test can confirm the predictions of
tunnels in special conditions.
Figure 7. Stress distribution of the bottom a foundation trench section
(after Xie
et al.
).
3 MITIGATION MEASURES
Seven papers were classified under this topic, the first three
papers presented are about controlling deformations and
ensuring stability of tunnels in special conditions, the other four
papers are about surface settlements mitigation. The studies
were based on real cases, numerical modelling and full scale
field trials.
Aguilar
et al.
in their paper “Diametric deformations in the
concrete segment lining of a tunnel excavated in soft soils.
Criteria for their evaluation and mitigation actions for their
control” present a graph (Figure 8) divided in five different
zones based on the relation of the percentage of horizontal
diameter increase to the time after the lining installation as a
criterion for the mitigation measures recommended to ensure
stability of the pre-cast reinforced concrete segmented rings that
compose an 8.7 m diameter tunnel.
Each zone has specific recommendations for mitigation
actions. For example, if the measurements reach zone 3 it is
recommended that the ring’s annular space is re-injected. Figure
8 shows the five different zones of intervention and three sets of
measurements of horizontal diameter increase on different
tunnel sections, the numbers at the end of the lines denote the
section number of the tunnel lining. Note that one of the
measurements resembles the measurement presented by Rangel-
Núñez
et al.
There the consolidation caused the deformation,
see Section 2, and reinjection is probably not the solution. The
authors of the GR expect that for the situation of consolidation
it is better to use the metallic frames in an earlier state.
Chang
et al.
in their paper “Application of ductile segments
to tunnels in close proximity” analyse the mechanical behaviour
of twin tunnels and the use of ductile segments to sustain the
unfavourable stress conditions. The case of the Taipei MRT
project, where a distance of 1.5 m occurred between twin
tunnels, is presented. From 2D numerical analyses a vertical and
lateral stress increase, as high as 50%, was calculated for the
first tunnel during the excavation of the second one. This sort of
behaviour was also observed with the automatic monitoring
systems during the tunnel construction. The first tunnel lining
went through a 70% increase in the bending moments and a
30% increase in the axial forces. The ductile segments are
described as lining systems of favourable ductility and
anticorrosion features. The analysis revealed that the ductile
segments were able to resist the final stress conditions, where a
precast reinforced concrete segment would have failed.
