Civil Engineering Reference
In-Depth Information
Figure 25.6
Stability of a tunnel heading for drained loading.
φ
s
, as described in Sec. 19.5. Alternatively you can apply partial
factors to all the design parameters in Eq. (25.3).
The collapse pressure given by Eq. (25.3) is for a tunnel face in dry soil but many
tunnels will be in ground which is below the water table in saturated soil. In this
case it is necessary to take account of drainage and pore pressures, as discussed in
Sec. 25.5.
safe angle of friction
25.5 Influence of water on tunnels
When a tunnel is driven through soft ground below the water table the groundwater
influences both the loads on the completed lining and the stability of the heading and
the face during construction. Tunnels in soft ground have to have a shield and face
support to prevent collapse during construction and a structural lining to maintain
long term stability. If the lining is fully waterproof then, after the excess pore pres-
sures due to construction have dissipated, the groundwater outside the lining will be
hydrostatic and there will be no seepage. In this case the pore pressures just outside
the lining correspond to the original water table. The loads on the lining are the sum
of the hydrostatic pore pressures and effective stresses. As discussed above the effec-
tive stresses acting on the lining are relatively small and the greatest proportion of
the loading on a waterproof tunnel lining comes from the groundwater. The effective
stresses acting on a supported face below the water table are given by Eq. (25.3) with
(γ
−
γ
φ
c
in the range 30
◦
to 37
◦
values for
K
a
are in the range 1/3 to 1/4 so the loads on the face support from the groundwater are
considerably larger than the loads from the soil.
Most tunnel linings are not completely waterproof and there is often some seep-
age of water through them. Figure 25.7(a) shows a section of a circular tunnel
with a leaking lining. The water table is drawn down above the tunnel which is
acting as a drain. There is part of a flownet for steady state seepage from the
groundwater into the tunnel. Notice that the arrangement of flowlines and equipo-
tentials near the tunnel is similar to that near a slope, with seepage outward
from the ground shown in Figs. 21.8(c) and (d).The effect of the lining leaking is
to reduce the water pressures on it while the stresses from the soil change very
little.
)
instead of
γ
. With typical values for
w
