Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 6 SLOPE STABILITY
Table 6.4 Guideline to Selection of Method of Slope Stability Analysis (After FHWA, Soils and
Foundation Reference Manual)
Foundation
Soil Type
Cohesive
Granular
Type of
Analysis
Short-term or
end of
construction
Stage
construction
(embankment
s on soft clays
– build
embankment
in stages with
waiting
periods to
take
advantage of
clay strength
gain due to
consolidation
Long-term
(embankment
on soft clays
and clay cut
slopes.
Existing failure
planes
All types
Source of Strength Parameters Remarks (see Note 1)
• UU or field vane shear test
or CU triaxial test.
• Undrained strength
parameters tested at p 0
(ground overburden stress)
• CU triaxial test. Some
samples should be
consolidated to higher than
existing in-situ stress to
determine clay strength gain
due to consolidation under
staged fill heights.
• Use undrained strength
parameters at appropriate p 0
for staged height
• CU triaxial test with pore
water pressure
measurements or CD triaxial
test.
• Use effective strength
parameters.
• Direct shear or direct simple
shear test. Slow strain rate
and large deflection needed.
• Use residual strength
parameters.
• Obtain effective friction
angle from charts of
standard penetration
resistance (SPT) versus
friction angle or from direct
shear tests.
Use Bishop Method. An angle of
internal friction should not be
used to represent an increase of
shear strength with depth.
Use Bishop Method at each
stage of embankment height.
Consider that clay shear
strength will increase with
consolidation under each stage.
Consolidation test data needed
to estimate length of waiting
periods between embankment
stages. Piezometers and
settlement devices should be
used to monitor pore water
pressure dissipation and
consolidation during
construction
Use Bishop Method with
combination of cohesion and
angle of internal friction
(effective strength parameters
from laboratory test).
Use Bishop, Janbu or Spencer
Method to duplicate previous
shear surface.
Use Bishop Method with an
effective stress analysis.
Note 1: Methods recommended represent minimum requirement. More rigorous methods such as
Spencer’s method should be used when a computer program has such capabilities.
6.5.9 Finite Element Methods
The finite element methods can be used to compute stresses and displacements in earth structures
caused by applied loads. The method is particularly useful for soil-structure interaction problems, in
which structural members interact with a soil mass. The stability of a slope cannot be determined
directly from finite element analysis, but the computed stresses in a slope can be used to compute
a factor of safety. Use of the finite element methods for stability problems is a complex and timeconsuming
process.
March 2009 6-11