considered undrained or drained is dependent on rate
which drainage must occur to prevent pore water
pressure changes. One method for estimating whether
a soil will behave in a drained or undrained manner
Dam Slope Stability 101” from the November 2013
newsletter. Alternatively, soils having a coefficient of
permeability greater than approximately 1 x 10-3
centimeters per second (cm/s) can be considered to be
thumb (Duncan and Wright, 2005). Although
conditions can be intermediate between undrained
modeled as either one or the other. In some cases,
undrained or drained, both cases are considered in the
analysis.
Total stresses within a soil mass include both stresses
resulting from forces transmitted through interparticle
contacts and pore water pressures.Effective stresses
within a soil mass include only stresses resulting from
the forces transmitted through interparticle contacts.
At any given location, the effective stress equals the
total stress minus the pore water pressure.
Soil strengths can be defined as a function of either
total stresses or effective stresses. When strengths
defined in terms of total stress are used in stability
analysis, the approach is commonly called the total
stress method, while the term effective stress method
is used when strengths defined in terms of effective
stress are used in stability analysis. Effective stress
choose between total stress methods and effective
stress methods. Total stress methods are used when it
is easier to predict the strength during undrained
case.
Soil strengths are always governed by effective
condition. Total stress strength characterizations are
simply used in those cases where we cannot easily
predict pore water pressure responses and we can
more easily predict the undrained strength. The pore
water pressure is implicit in the selected total stress
strength; the pore pressure is whatever value is
necessary to produce an effective stress state that
results in the predicted strength.
A future article (Part 3 of the series) will provide
guidance on when undrained conditions, drained
conditions, total stress methods, and effective stress
methods are generally used for shear strength
characterization and slope stability analysis. But first,
shear strength parameters and strength testing to
evaluate the parameters will be discussed.
Shear Strength Explained
Shear strength can be defined as the ability of soil to
The shear load is the result of gravity forces from the
depends on:
x Types of soil particles and mineralogy
x Consolidation pressure
x Drainage allowed
x Stress history, including overconsolidation
x Stress paths
The most common way of representing or
characterizing shear strength of soils is the Mohr-
Coulomb failure criterion using the following
equations:
s = c +ı tanࢥ (total stress)
s = c’ +ı’tanࢥ’(effective stress)
Where s = shear strength; c = cohesion;V =
effective or total stress; and I = internal friction
angle.
As represented by the Mohr-Coulomb failure criterion,
the shear strength characterization for a soil consists of
Shear strength of a soil is controlled by
effective stresses, whether failure occurs
under drained or undrained conditions.