Geology Reference
In-Depth Information
shaking associated with this 8.9 or even 9.0M earthquake (10,000
times as strong, in terms of overall energy release, as the Christchurch
earthquake) and this is testament to the skill and knowledge of the
civil engineer designers. The huge damage and large number of deaths
caused by the Japan earthquake resulted from a 10m high tsunami
wave that came ashore and destroyed whole villages. Regarding
engineered structures, several nuclear power stations had been con-
structed along the shoreline in the impacted region. The structures
apparently performed well in terms of withstanding seismic shaking
but severe damage did occur because of failure of cooling systems.
The initial shaking caused safe shutdown of the reactors, as is the
required procedure for nuclear power stations impacted by a major
earthquake, but the loss of electrical power stopped the
flow of cool-
ing water required to prevent the fuel rods overheating. Backup diesel
generators kicked in and provided the necessary power for an hour or
so but then they failed because of the tsunami. In hindsight, no doubt
the secondary power sources could and should have been designed to
survive inundation, as they are for more modern installations, and the
risk properly identi
ed using an event tree approach.
6.10.4 Tunnels
Tunnels and mines tend to be safer than surface structures during
earthquakes, and this safety increases with increased depth (Power
et al
., 1998). Except where the tunnel passes through particularly
poor ground or intercepts active faults, earthquake resistant design is
generally not a high priority. Of course, where the support in a tunnel
is inadequate or marginal under static loading conditions, then earth-
quake shaking might well trigger failure. This is especially true at
portals of tunnels; landslides and especially rockfalls are very com-
monly triggered by earthquakes, as discussed in the next section.
Failures in some tunnels, and especially the failure of Daikai subway
station during the Kobe earthquake in 1995, have caused a rethink on
seismic stability of underground structures. Hashash
et al
. (2001)
provide a very useful review and examples of aseismic design. A
reinforced concrete lining should have signi
cantly better seismic
resistance characteristics than an unreinforced lining. If the tunnel
intersects a fault that is suspected of being active, then special measures
will be required or, preferably, the fault avoided. Key considerations
are the estimated magnitude of the displacement and the width of the
zone over which displacement is distributed. If large displacements are
concentrated in a narrow zone, then the design strategy may be to
enlarge the tunnel across and beyond the displacement zone. The
tunnel is made wide enough such that the fault displacement will not
close the tunnel and traf
c can be resumed after repairs have been
made. In some cases, an enlarged tunnel is constructed outside the
