Civil Engineering Reference
In-Depth Information
FIGURE 9.32
Normalized liquefied shear strength versus equivalent clean sand (
N
1
)
60
value. (
From
Olson et al. 2000, reproduced with permission of the American Society of Civil Engineers.
)
lateral deformation of the retaining walls. This liquefaction-induced lateral spreading was
usually restricted to the ground surface behind the retaining walls, and thus it is called
localized lateral spreading,
which is discussed in Sec. 10.3.
If the liquefaction-induced lateral spreading causes lateral movement of the ground sur-
face over an extensive distance, then the effect is known as
large-scale lateral spreading.
Such lateral spreads often form adjacent waterways on gently sloping or even flat ground
surfaces that liquefy during the earthquake. This section deals with the analysis of large-
scale lateral spreading of slopes.
The concept of cyclic mobility is used to describe large-scale lateral spreading. Because the
ground is gently sloping or flat, the static driving forces do not exceed the resistance of the soil
along the slip surface, and thus the ground is not subjected to a flow slide (i.e., the factor of
safety is greater than 1.0). Instead, the driving forces only exceed the resisting forces during
those portions of the earthquake that impart net inertial forces in the downslope direction. Each
cycle of net inertial forces in the downslope direction causes the driving forces to exceed the
resisting forces along the slip surface, resulting in progressive and incremental lateral move-
ment. Often the lateral movement and ground surface cracks first develop at the unconfined
toe, and then the slope movement and ground cracks progressively move upslope.
Figure 3.41 shows an example of large-scale lateral spreading caused by liquefaction
during the Loma Prieta earthquake on October 17, 1989. As seen in Fig. 3.41, as the dis-
placed ground breaks up internally, it causes fissures, scarps, and depressions to form at
ground surface. Notice in Fig. 3.41 that the main ground surface cracks tend to develop par-
allel to one another. Some of the cracks filled with water from the adjacent waterway. As
the ground moves laterally, the blocks of soil between the main cracks tend to settle and
break up into even smaller pieces.
Large-scale lateral spreads can damage all types of structures built on top of the lateral
spreading soil. Lateral spreads can pull apart foundations of buildings built in the failure area,
