Civil Engineering Reference
In-Depth Information
Figure 25.7
Steady state seepage towards tunnels and headings.
Figure 25.7(b) shows part of a flownet for seepage towards a supported face. The
shield and lining leak so there is radial seepage towards the tunnel like that shown in
Fig. 25.7(a). The face has to be supported to maintain stability but the support is fully
permeable. The seepage is three-dimensional and it is difficult to draw an accurate
flownet in two dimensions; the one shown in Fig. 25.7(b) shows reasonably well the
flowlines and equipotentials near the face. These are similar to those close to a slope
with seepage out of the ground shown in Figs. 21.8(c) and (d). Taking the datum for
potential at the invert the potential there is zero and the total drop in potential is
P
as
shown. The hydraulic gradient close to the face depends on the geometry of the flownet
but there is a hydraulic gradient towards the face and there is the risk of piping and
erosion like that described in Sec. 14.6. To prevent seepage towards the face and to
maintain stability it is necessary to apply a total stress to the face at least equal to the
hydrostatic pore pressures. This may be applied by air pressure inside the tunnel or,
more usually, by use of a closed face or earth pressure balance shield like those shown
in Figs. 25.3(b) and (c).
25.6 Ground movements due to tunnelling
The changes of stress and pore pressure in the ground around headings and tunnels
during and after construction cause ground movements; this is inevitable. Figure 25.8
shows a tunnel excavated with a diameter
D
and the volume of soil excavated (per
unit length along the tunnel) is
V
e
=
D
2
. Due to ground movements which occur
during construction the diameter of the outside of the lining is a little smaller and the
finished volume of the tunnel and lining is
V
t
. The ground loss
v
l
is the difference
expressed as a percentage of the excavated volume
1
4
π
V
e
−
V
t
v
l
=
×
100%
(25.4)
V
e
Values for volume loss in soft ground tunnels vary with the ground conditions and the
method of tunnelling but are often 1% to 2%.
