Civil Engineering Reference
In-Depth Information
These types of plastic soil are often characterized as strain-softening soils, because there
is a substantial reduction in shear strength once the peak shear strength is exceeded. An
example of the stress-strain curve for a strain-softening soil is presented in Fig. 8.12.
During the earthquake, often failure first occurs at the toe of the slope, and then the ground
cracks and displacement of the slope progresses upslope. Blocks of soil are often observed
to have moved laterally during the earthquake, such as illustrated in Fig. 3.46.
It is very difficult to evaluate the amount of lateral movement of slopes containing
strain-softening soil. The most important factors are the level of static shear stress versus
the peak shear stress of the soil and the amount of additional shear stress that will be
induced into the soil by the earthquake. If the existing static shear stress is close to the peak
shear stress, then only a small additional earthquake-induced shear stress will be needed to
exceed the peak shear strength. Once this happens, the shear strength will significantly
decrease with strain, resulting in substantial lateral movement of the slope. If it is antici-
pated that this will occur during the design earthquake, then one approach is to use the ulti-
mate (i.e., softened) shear strength of the soil. For example, in Fig. 8.12, the ultimate shear
strength of the soil is about 25 lb/in
2
(170 kPa). Suppose a slope was composed of this clay
and a shear strength of 25 lb/in
2
was used in the slope stability analysis. If the factor of
safety is greater than 1.0, then it could be concluded that a massive shear failure of the slope
during the earthquake is unlikely.
9.7
MITIGATION OF SLOPE HAZARDS
To evaluate the effect of the earthquake-induced slope movement upon the structure, the
first step is to estimate the amount of lateral movement. The prior sections present differ-
ent types of analyses based on differing soil and slope movement conditions. Once the
amount of earthquake-induced lateral movement has been estimated, then it can be com-
pared with the allowable lateral movement for the proposed structure. If the anticipated
earthquake-induced lateral movement exceeds the allowable lateral movement, then slope
stabilization options will be required.
9.7.1
Allowable Lateral Movement
To evaluate the lateral movement of buildings, a useful parameter is the
horizontal strain
h
,
defined as the change in length divided by the original length of the foundation. Figure 9.33
shows a correlation between horizontal strain
h
and severity of damage (Boone 1996, orig-
inally from Boscardin and Cording 1989). Assuming a 6-m- (20-ft-) wide zone of the foun-
dation subjected to lateral movement, Fig. 9.33 indicates that a building can be damaged by
as little as 3 mm (0.1 in) of lateral movement. Figure 9.33 also indicates that a lateral move-
ment of 25 mm (1 in) would cause “severe” to “very severe” building damage.
The ability of a facility to resist lateral movement depends on its tensile strength. Those
facilities that cannot resist the tensile forces imposed by lateral movement will be the most
severely damaged. For example, Figs. 9.34 and 9.35 show roadway damage caused by
earthquake-induced slope movement. Asphalt pavements have low tensile strength, and
hence they will be simply pulled apart during the earthquake-induced slope movement,
such as shown in Figs. 9.34 and 9.35.
The ability of buildings to resist lateral movement also depends on the tensile strength
of the foundation. For example, Fig. 9.36 shows severe damage to a building caused by
landslide movement. The foundation is too weak, and the amount of slope movement is too
large for the building shown in Fig. 9.36 to be able to resist the lateral deformation. Those
