Seismic Aspects of Underground Structures of Hydropower Plants

The general perception of engineers is that underground structures in rock are not vulnerable to earthquakes as tunnels and caverns are moving together with the foundation rock. Accordingly, in the design of underground structures earthquake action has been disregarded in most parts of the world and many tunneling engineers would still consider any earthquake damage as an acceptable risk. This attitude may have been justified some 30 years ago, but today we know much more about earthquakes and underground structures. As a matter of fact, a significant number of tunnels have experienced some damage during earthquakes. The following aspects have to be considered: (i) tunnels and caverns in rock; and (ii) installations and non-structural elements located in underground structures. Earthquake design aspects of tunnels in soil are not discussed in this paper as tunnels for hydropower plants are mainly in rock. The earthquake safety and seismic design of underground structures in soil is of primary interest for urban transportation systems (subways) and also for highway and railway tunnels, where the whole tunnel or parts of tunnels with shallow overburden are located in soil. Seismic design of tunnels in soft soil has been standard practice in countries like Japan and Taiwan. The vulnerability of underground structures in rock can be classified as follows: 1. Underground structures in zones of high seismicity, which have not been designed against earthquakes;2. Tunnel portals; 3. Shear zones and potentially active faults; 4. Tunnels and caverns in zones with (tectonically) highly stressed rock formations; 5. Large caverns for underground power plants; 6. Intersections of different tunnels and caverns; 7. Geologically weak zones, which need special attention during excavation; 8. Locations where accidents have occurred during construction and operation; 9. Structures in caverns and non-structural elements in tunnels (ventilation system, drainage, ceiling elements etc.); and10. Equipment, installations and secondary structures in underground structures. Tunnels and caverns are built in rock whose behaviour cannot be predicted accurately with limited exploratory information. It has to be assumed that a strong earthquake can trigger failure of unstable rock portions of an underground structure. Therefore, the earthquake resistant design is of particular importance in geologically difficult zones. The type of earthquake damage, which can cause problems in underground structures for hydropower plants may be quite different from that in highway and railway tunnels. In railway tunnels a distortion of the track, a small lateral offset at a rock joint or a piece of rock on the track can have large consequences especially in the case of high speed trains. In highway tunnels the main problem will be falling rock in unlined tunnels or falling lining elements (especially unreinforced linings) and collapsing interior precast tunnel elements (non-structural elements), which can cause accidents and fires. Earthquakes may be the worst-case events as they will also affect all safety systems in tunnels and can make them inoperable, i.e. sprinklers may not function, lighting and communication may fail, emergency doors may be blocked etc.