Civil Engineering Reference
In-Depth Information
Figure 25.4
Stress and pore pressure changes during tunnelling.
the critical state line and the shear stress is then the undrained strength. The distance
of the effective stress from the critical state line is a measure of the factor of safety.
The initial pore pressure immediately after excavation
u
i
is negative because the total
normal stress at
B
is smaller than the effective normal stress at
B
. If there is no further
excavation the total stress remains at
B
but, as time passes, the pore pressures rise,
the effective stress path is
B
C
. The effective stress point
C
moves towards the
critical state line and the factor of safety decreases. The face open will collapse when
C
reaches the critical state line.
The broken line
A
→
→
D
in Figs. 25.4(b) and (c) represents tunnelling in soil which
is drained so the pore pressure remains constant. If the tunnel heading acts as a drain
there will be steady state seepage and the pore pressures will be determined by a flownet.
Notice that the shear stress at
D
for the fully drained case is smaller than that at
B
for
the undrained case so the shield is required to provide more support for the drained
case than for the undrained case. If the soil around the heading is drained the shield
must also support pressures from the groundwater, as discussed in Sec. 25.5.
25.4 Stability of tunnel headings
The permanent lining of a tunnel in soft ground is usually made from cast iron or
concrete segments which fit together to make a ring. You can often see these out of
