Civil Engineering Reference
In-Depth Information
the lateral movement and ground surface cracks first develop at the unconfined toe, and
then the slope movement and ground cracks progressively move upslope.
Weakening slope stability analyses are discussed in Secs. 9.4 to 9.6.
9.1.3
Cross Section and Soil Properties
The first step in a slope stability analysis is to develop a cross section through the slope. It
is important that cross sections be developed for the critical slope locations, such as those
areas that are believed to have the lowest factors of safety. The cross section of the slope
and the various soil properties needed for the analysis would be determined during the
screening investigation and quantitative evaluation (see Secs. 5.2 to 5.5). Some of the addi-
tional items that may need to be addressed prior to performing a slope stability analysis are
as follows (adapted from Division of Mines and Geology 1997):
●
Do landslides or slope failures, that are active or inactive, exist on or adjacent (either
uphill or downhill) to the project?
●
Are there geologic formations or other earth materials located on or adjacent to the site
that are known to be susceptible to slope movement or landslides?
●
Do slope areas show surface manifestations of the presence of subsurface water (springs
and seeps), or can potential pathways or sources of concentrated water infiltration be
identified on or upslope of the site?
●
Are susceptible landforms and vulnerable locations present? These include steep slopes,
colluvium-filled swales, cliffs or banks being undercut by stream or water action, areas
that have recently slid, and liquefaction-prone areas.
●
Given the proposed development, could anticipated changes in the surface and subsur-
face hydrology (due to watering of lawns, on-site sewage disposal, concentrated runoff
from impervious surfaces, etc.) increase the potential for future slope movement or land-
slides in some areas?
Other considerations for the development of the cross section to be used in the slope sta-
bility analysis are discussed in Sec. 9.2.6.
9.2 INERTIA SLOPE STABILITY—PSEUDOSTATIC
METHOD
9.2.1 Introduction
As previously mentioned, the inertial slope stability analysis is preferred for those materi-
als that retain their shear strength during the earthquake. The most commonly used inertial
slope stability analysis is the pseudostatic approach. The advantages of this method are that
it is easy to understand and apply and that the method is applicable for both total stress and
effective stress slope stability analyses.
The original application of the pseudostatic method has been credited to Terzaghi
(1950). This method ignores the cyclic nature of the earthquake and treats it as if it applied
an additional static force upon the slope. In particular, the pseudostatic approach is to apply
a lateral force acting through the centroid of the sliding mass, acting in an out-of-slope
direction. The pseudostatic lateral force
F
h
is calculated by using Eq. (6.1), or
