Civil Engineering Reference
In-Depth Information
FIGURE 3.37
Collapse of a span of the Nishihomiya Bridge, apparently due to lateral retaining wall move-
ment during the Kobe earthquake on January 17, 1995. (
Photograph from the Kobe Geotechnical Collection,
EERC, University of California, Berkeley.
)
the location of the zone of material that was believed to have liquefied during the earth-
quake. Once liquefied, the upstream portion of the dam was subjected to a flow slide. The
upper part of Fig. 3.38 indicates the portion of the dam and the slip surface along which the
flow slide is believed to have initially developed. The lower part of Fig. 3.38 depicts the
final condition of the dam after the flow slide. The flow slide caused the upstream toe of
the dam to move about 150 ft (46 m) into the reservoir.
Figures 3.39 and 3.40 show two views of the damage to the Lower San Fernando Dam.
A description of the damage is presented below:
Figure 3.39 is a view to the east and shows the condition of the dam after the earthquake.
While nearly the entire length of the upstream portion of the earthen dam slumped down-
ward, the main flow failure is located at the eastern end of the dam. This is the location of
the cross section shown in Fig. 3.38.
Note in Fig. 3.39 that the water in the reservoir almost breached the top of the failed por-
tion of the dam. If the water had breached the top of the dam, it would have quickly cut
through the earthen dam and the subsequent torrent of water would have caused thousands
of deaths in the residential area immediately below the dam.
Figure 3.40 is a view to the west and shows the condition of the dam after the reservoir
has been partly emptied. The flow failure is clearly visible in this photograph. The concrete
liner shown in Fig. 3.40 was constructed on the upstream dam face in order to protect
against wave-induced erosion. Although initially linear and at about the same elevation
across the entire length of the upstream face of the dam, the concrete liner detached from
the dam and moved out into the reservoir along with the flowing ground.
As indicated in the upper part of Fig. 3.38, flow failures develop when the driving forces
exceed the shear strength along the slip surface and the factor of safety is 1.0 or less. The
engineering analyses used to determine whether a site is susceptible to liquefaction and a
subsequent flow slide failure are presented in Chaps. 6 and 9.
