Environmental Engineering Reference
In-Depth Information
stresses are high. Broms (1965) suggested the use of a mod-
ified bearing capacity approach for the design of highway
structures.
Train loads are transmitted through the rail-tie system and
the ballast to the subgrade. The rail-tie system is a com-
plex loading system. The stresses applied to the subgrade are
of primary interest. Hanna and Meyerhof (1980) analyzed a
layered system similar to a railway structure and suggested
simplifications that could be applied to a conventional bearing
capacity analysis in order to accommodate the rail system.
The shear strength of the subgrade of a highway, rail-
way, or airport structure can be expressed in terms of the
proposed equation for the shear strength of an unsaturated
soil. An important aspect when describing unsaturated soil
shear strength is the assessment of the design matric suction.
For design purposes, it is necessary to have an indication of
how low the matric suction may go during any year. Even
matric suction values below 100 kPa can produce a substan-
tial increase in the bearing capacity of a soil. Studies by van
der Raadt (1988), Sattler et al., (1989), and Majerison et al.,
(2001) have provided a better understanding of seasonal vari-
ations in matric suction in highway and railway subgrades.
A clearer understanding of the role of negative pore-water
pressures (or matric suctions) in increasing the shear strength
of the soil has been forthcoming in recent years. Reasonable
assumptions can be made with regard to plausible distribu-
tions of negative pore-water pressures that might exist over
a time period, and each situation can be analyzed as part of a
parametric slope stability study. It is quite appropriate to per-
form slope stability analyses that include the shear strength
contribution from the negative pore-water pressures. Com-
parisons can be made of the calculated factors of safety
corresponding to situations where negative pore-water pres-
sures are included and ignored. Slope stability analyses that
include negative pore-water pressures can be formulated as
an extension of conventional limit equilibrium analyses.
Several aspects of a slope stability study remain the same
for soils with positive pore-water pressures (e.g., saturated
soils) and soils with negative pore-water pressures (e.g.,
unsaturated soils). For example, the nature of the site inves-
tigation, the identification of the soil strata, and the measure-
ment of the total unit weight remain the same for saturated
and unsaturated soils. On the other hand, extensions to con-
ventional testing procedures are required for the characteri-
zation of the shear strength properties of the soil. Estimation
procedures can be used to approximate the shear strength
envelope for unsaturated soils. The analytical tools used to
incorporate negative pore-water pressures and calculate the
factor of safety need to be extended to embrace unsaturated
soil behavior.
12.5 SLOPE STABILITY
Slope stability analyses have become a common analyti-
cal tool for assessing the factor of safety of natural and
man-made slopes. Any one of several methods can be used
to analyze the stability of a slope. There are a series of
two-dimensional methods of slices that are commonly used
in engineering practice. These methods are based upon the
principles of statics (i.e., static equilibriums of forces and/or
moments) without giving consideration to the amount of
movement in the soil mass.
Several basic assumptions and principles are used in for-
mulating the limit equilibrium methods of analysis. These
methods of analysis can readily be extended to embrace
the behavior of unsaturated soil. There are also more recent
methods of analysis that involve the calculation of the stress
state throughout the overall soil mass through the process of
“switching on” the gravity forces. The calculation of gravity
forces methods are discussed following the presentation of
the method of slices for unsaturated soils.
Effective shear strength parameters (i.e., c and φ )are
generally used when performing slope stability analyses on
saturated soils. The shear strength contribution from the neg-
ative pore-water pressures above the groundwater table are
often ignored by setting negative pore-water pressures to
zero. It may be a reasonable assumption to ignore negative
pore-water pressures for situations where the major portion
of the slip surface is below the groundwater table. However,
for situations where the groundwater table is deep or where
the concern is over the possibility of a shallow failure sur-
face, negative pore-water pressures should not be ignored.
12.5.1 Free-Body Diagram for Slope Stability Analysis
The computation of the factor of safety of a slope must
start with specifying geometric details related to the free-
body diagram. The need for a free-body diagram should go
without saying; however, the free-body diagram is the place
where a slope stability analysis encounters its first difficulty.
The boundaries of the sliding mass are not known at the start
of the method-of-slices analysis. The shape and location of
the most critical slip surface are unknowns.
The ground surface and the stratigraphic divisions
between soil layers may be known for the two-dimensional
cross section under consideration. However, the shape and
location of the most critical slip surface that can be passed
through the soil mass are not known, and as a result the
limits of the free-body diagram are not known.
An assumption must be made regarding the shape of the
slip surface. The slip surface may be planar, circular, or com-
posite in shape. The selection of the shape of the potential
slip surface is mainly dependent on whether the slope con-
sisted of a purely frictional or cohesive material. The effect
of geological considerations also leads to consideration
of slip surfaces that were noncircular
in shape (i.e.,
composite).
The second feature of the free-body diagram that needs
to be determined is the location of the critical slip surface.
Early slope stability studies sought to find the critical slip
 
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