Landsliding as a result of ground shaking is a common
phenomena. Landsliding across a tunnel can result
in concentrated shearing displacements and collapse
of the cross section. Landslide potential is greatest
when a pre-existing landslide mass intersects the
tunnel. The hazard of landsliding is greatest in shallower
parts of a tunnel alignment and at tunnel portals.
At tunnel portals, the primary failure mode tends to
be slope failures. Particular caution must be taken if
the portal also acts as a retaining wall ŽSt. John and
Zahrah, 1987.. During the September 21, 1999 Chi Chi
earthquake in Taiwan slope instability at tunnel portals
was very common