Geology Reference
In-Depth Information
main tunnel and the annulus back
lled with weak cellular concrete or
similar. The back
ll has low yield strength to minimise lateral loads on
the inner tunnel liner, but with adequate strength to resist normal
ground pressures and minor seismic loads. If fault movements are
predicted to be small and/or distributed over a relatively wide
zone, it is possible that fault displacement may be accommodated by
providing articulation of the tunnel liner using ductile joints. This
detail allows the tunnel to distort into an S
shape through the fault
zone without rupture, and with repairable damage. This may not be
feasible for fault displacements more than 75
-
100mm. An alternative
approach is to accept that damage will occur and to make contingency
plans to control traf
-
c and to carry out repairs as quickly as possible in
the event of a damaging earthquake.
Landslides are commonly triggered by earthquake shaking, especially
in mountainous areas. The Wenchuan earthquake in Sichuan
Province, China, of 12 May 2008, was very large (M8) and quite
shallow (14 km) and the active faults ran through populated valleys
surrounded by high slopes. Landslides, including rockfall, caused
more than 20,000 deaths, with one individual landslide killing more
than 1,600 people (Yin
et al
., 2008). One of the main consequences
was the damming of streams, which necessitated emergency engineer-
ing works to lower the water levels in the lakes that formed behind the
landslide debris before they were overtopped or burst uncontrollably.
Slopes affected by strong earthquake shaking can be categorised in
three classes, as set out in
Table 6.4.
These are:
1. Stable slopes: these are de
ned as situations where the shaking is
not strong enough to cause permanent displacement in a slope.
This may be because the peak forces are insuf
cient to overcome
the strength of the ground or because different parts of the same
slope are out of phase so that whilst some parts are being driven
towards failure, other parts are being accelerated in the opposing
direction.
2. Permanently displaced slopes: the key aspect of dynamic loading,
whether it is from earthquakes or blasting, is its transient nature.
The waves pass through the ground and induce inertial forces. In
the same way as discussed in
Chapter 5
regarding a laboratory
start to move. The continued positive acceleration above critical
will cause the displacement to increase in velocity. However, after
