Civil Engineering Reference
In-Depth Information
the window of an underground train. Usually the loads on the lining when it is being
built and when the shield is pushed forward from it are considerably larger than the
loads from the ground. As a result, once a circular lining has been built the tunnel is
generally safe and the subsequent movements, which are due to dissipation of excess
pore pressures, are relatively small. The critical stability condition is in the heading
during construction and most of the ground movements caused by the tunnel occur
during construction.
(a) Stability of tunnel headings for undrained loading
Figure 25.5(a) shows a simple tunnel heading. The soil has an undrained strength
s
u
and, because we are considering an ultimate limit state of collapse, this should be
taken as the critical state strength, as discussed in Chapter 18. The total stress inside
the heading is
t
and the length of the heading is
P
. In an earth pressure balance shield
like that shown in Fig. 25.3(b)
σ
σ
t
is provided by the shield but in an open shield like that
shown in Fig. 25.3(a)
t
is generally zero. If the face is supported by slurry pressure or
if the heading is pressurized then
σ
t
becomes the slurry or air pressure. There is a total
stress
q
on the surface: this may arise from buildings, from free water (see Sec. 21.5)
or from very soft ground whose strength can be neglected.
If the ground is a heavy fluid with no strength
σ
σ
t
must equal the total stress in the
ground which is
γ
z
+
q
at the axis level. (If
σ
<γ
z
+
q
the tunnel will collapse inwards
t
but if
q
the ground will heave and the tunnel will burst.) The undrained
strength of the ground will have the effect of reducing the tunnel collapse pressure
σ
>γ
z
+
t
σ
tc
,
which can be written as
σ
=
γ
z
+
q
−
s
u
T
c
(25.1)
tc
where
T
c
is called the tunnel stability number. Notice that Eq. (25.1) which gives the
collapse pressure for a tunnel for undrained loading is very like Eq. (22.6) which
gave the bearing capacity of a foundation for undrained loading. The tunnel sta-
bility number
T
c
in Eq. (25.1) is comparable to the bearing capacity factor
N
c
in
Eq. (22.6).
Figure 25.5
Stability of a tunnel heading for undrained tunnelling.
