Seismic Design of Tunnels - Parsons Brinckerhoff

Design based on such inappropriate measures may lead to the construction of unsafe
structures or structures that are too conservatively designed.
Although the progress of underground seismic design methodology is lagging, the
earthquake awareness in the country is not. Recent discoveries in seismology, geology
and geotechnical engineering have led to the belief that earthquake hazard is no longer
only a California problem. Many regions throughout the United States, Puerto Rico and the
Virgin Islands are now known to have the potential for tremors of similar or larger
magnitude than that of the Loma Prieta. This situation demands rethinking of the current
seismic design practice for our underground transportation systems.
1.4 General Effects of Earthquakes
In a broad sense, earthquake effects on underground tunnel structures can be
grouped into two categories – ground shaking and ground failure.
Ground Shaking
Ground shaking refers to the vibration of the ground produced by seismic waves
propagating through the earth’s crust. The area experiencing this shaking may cover
hundreds of square miles in the vicinity of the fault rupture. The intensity of the shaking
attenuates with distance from the fault rupture. Ground shaking motions are composed of
two different types of seismic waves, each with two subtypes. Figure 1 shows the ground
response due to the various types of seismic waves:
• Body waves travel within the earth’s material. They may be either longitudinal P waves
or transverse shear S waves and they can travel in any direction in the ground.
• Surface waves travel along the earth’s surface. They may be either Rayleigh waves or
Love waves.
As the ground is deformed by the traveling waves, any tunnel structure in the ground
will also be deformed. If the imposed deformation were the sole effect to be considered,
ductility and flexibility would probably be the only requirements for the design of tunnel
structures (from a structural standpoint). However, tunnel structures also must be
designed to carry other sustained loads and satisfy other functional requirements. A
proper and efficient tunnel structural design, therefore, must consider the structural
members’ capacity in terms of strength as well as ductility and flexibility of the overall
configuration.
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