TUNNEL ENGINEERING

20.54 n Section Twenty
deformations in the plane of the cross-section can
occur in tunnels, but not usually in vertical
shafts, and is caused primarily by seismic
waves propagating perpendicular to the tunnel
longitudinal axis. Vertically propagating shear
waves are generally considered the most critical
type of waves for this mode of deformation. Axial
and curvature deformations are induced by
components of seismic waves that propagate along
the longitudinal axis.
The effects of a seismic event on a tunnel as a
whole can be integrated to give an effective
acceleration at the tunnel location, expressed as a
seismic coefficient times the acceleration due to
gravity. Three seismic coefficients are usually
TUNNEL ENGINEERING
Fig. 20.29 TARP Calumet Shaft.
obtained, for longitudinal, lateral and vertical
effects. Internal elements, not in contact with the
soil and with a natural frequency approaching that
of the seismic waves, may need to be designed to
substantially larger seismic coefficients. The vertical
seismic coefficient can be reasonably assumed
to be two-thirds of the design peak horizontal
acceleration divided by the gravity.
Special precautions are needed for tunnels in
soils that might liquefy or slip, especially so if
crossing active faults. Liquefaction may cause
tunnels to float up. Since it is virtually impossible
to design against these conditions, the best policy
is either to improve or replace the soil in question
or to avoid it. Faults are best avoided, but if that
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