1693
Technical Committee 204 /
Comité technique 204
Case_0
Case_a_7.0
Case_b_6.5
Case_c
-200 -100 0
-30
-20
-10
0
-100 0
-100 0
Figure 7. Horizontal displacement of the ground
100
200
300
0
10
20
30
Distance from center of tunnel [m]
Vertical earth pressure [kN/m
2
]
0
10
20
30
Case_0
Case_a_7.0
Case_b_6.5
Case_c
Figure 8. Vertical earth pressure distribution (Excavation completed)
Figure 6 shows the surface settlement curves after the tunnel
excavations have been completed. The figure indicates that
surface settlements can be prevented by adopting the pre-ground
improvement method, and that the method becomes more
effective as the improved area becomes larger.
The horizontal displacement occurring in the marked ground
areas (Lines 1, 2 and 3) are shown in Figure 7. The distance
between the center of the tunnel lining and Lines 1~3 are 7.5 m,
10.0 m and 15.0 m, respectively. The ground is displaced
toward the tunnel lining, due to the tunnel excavation, and the
largest horizontal displacements are seen on the ground surface
in all of the cases and all of the examination positions. The
horizontal displacements become smaller as the areas of the
improved ground increase. In particular, there is almost no
displacement seen in Case_b_6.5, for which all cross sections of
the tunnel were improved.
Figure 8 shows the vertical earth pressure distribution on the
marked positions after the tunnel excavations have been
completed. The straight dotted lines show the initial vertical
earth pressure distribution before the excavation. The full black
lines, without markers, show the vertical earth pressure
distribution for Case_0 where the tunnel was excavated without
ground improvement. The figure shows that the vertical earth
pressure, acting on both sides of the tunnel, increases due to the
tunnel excavation, in a certain area. This area is called an
influenced area due to the tunnel excavation. The vertical earth
pressure acting on comparison Lines I and II is concentrated in
the improved area, and the influenced area becomes narrow in
Case_b_6.5. This effect is called an earth pressure redistribution
effect. However, there is almost no change in the influenced
area for Case_a_7.0 and Case_c.
The shear strain distributions for the two analysis stages,
namely, when the top heading has been completed and when all
of the tunnel excavation has been completed, are shown in
Figure 9. In Case_0, for a tunnel excavated in a natural ground,
large shear strain is generated from the foot of the tunnel and
develops to the ground surface. When ground improvement has
been performed, the development of shear strain is intercepted
by the improved area. As a result, the shear strain becomes
smaller due to the improved ground. This effect is called a
Figure 9. Shear strain distribution
Figure 10. Deformation of improved ground
shear reinforcement effect, and it becomes more effective as the
improved area becomes larger. On the other hand, the improved
area is surrounded by relatively large shear strain, as in
Case_a_7.0 and Case_c. The skimpiness of the width of the
improved ground is thought to be the reason for this
phenomenon. The large shear strain is generated over a large
area when the ground has not been improved, and the improved
area is not wide enough to intercept all of the large shear strain
area, as in Case_a_7.0 and Case_c. As a result, the large shear
strain is remaining around the improved area. When all the
cross sections of the tunnel have been improved, as in
Case_b_6.5, the improved ground can intercept the large shear
strain from the foot of the tunnel, despite the width of the
improved ground. However, the pre-ground improvement
method has almost no influence on the shear strain that occurs
from both edges of the invert of the tunnel lining.
3.2
Mechanical behavior of improved ground
Figure 10 shows the deformation of the improved ground. For
easy understanding, the deformation is expanded to 50 times.
The dotted lines represent the original position of the improved
ground. The deformation of the improved ground becomes
smaller when the improved areas become larger. The
deformation of the improved ground shows the same shapes in
Case_a_7.0 and Case_c; the upper part of the improved ground
is compressed and both ends of the improved ground are
moving away from the tunnel. On the other hand, both ends of
the improved ground are moving towards the center of the
tunnel in Case_b_6.5. This deformation shape is the same as the
deformation of the tunnel lining, indicating that the improved
ground can restrain the deformation of the tunnel lining.
4 INFLUENCE OF WIDTH AND HEIGHT OF THE
IMPROVED AREA
The reduced ratios of the settlements of the ground surface and
the tunnel for different widths of the improved area are shown
in Figure 11. The analytical results indicate that the settlement-
preventing effect increases when the width of the improved area
