Civil Engineering Reference
In-Depth Information
9.4 WEAKENING SLOPE STABILITY—FLOW
SLIDES
9.4.1 Introduction
The next three sections discuss the weakening slope stability analysis, which is the preferred
method for those materials that will experience a significant reduction in shear strength dur-
ing the earthquake. This section is devoted to flow slides. As discussed in Sec. 3.4.4 and
Tables 3.2 and 9.2, flow slides develop when the static driving forces exceed the weakened
shear strength of the soil along the slip surface, and thus the factor of safety is less than 1.0.
There are three general types of flow slides:
1.
Mass liquefaction:
This type of flow slide occurs when nearly the entire sloping
mass is susceptible to liquefaction. These types of failures often occur to partially or com-
pletely submerged slopes, such as shoreline embankments. For example, Fig. 9.22 shows
damage to a marine facility at Redondo Beach King Harbor. This damage was caused by
the California Northridge earthquake on January 17, 1994. The 5.5 m (18 ft) of horizontal
displacement was due to the liquefaction of the offshore sloping fill mass that was con-
structed as part of the marine facility.
Another example of a mass liquefaction is shown in Fig. 9.23. This figure shows the
flow slide of the Middle Niteko Dam caused by the Kobe earthquake in Japan on January 17,
1995.
For design conditions, the first step in the analysis is to determine the factor of safety
against liquefaction. If it is determined that the entire sloping mass, or a significant portion
FIGURE 9.22
Damage to a marine facility caused by the California Northridge earthquake on January 17,
1994. (
From Kerwin and Stone 1997; reprinted with permission from the American Society of Civil Engineers.
)
