Civil Engineering Reference
In-Depth Information
Figure 9.5
Undrained strength of soil.
the strength of the soil in the ground at its current water content. It will also represent
the strength of soil near a foundation or excavated slope provided there is no change
of water content during construction. This is the essence of the undrained strength: it
is the strength of soil tested or loaded without any change in water content.
The undrained strength
s
u
is given by
τ
f
=
s
u
(9.4)
which is the same as the cohesion failure criterion given by Eq. (3.9). From Eqs. (9.2)
and (9.3) and noting that
τ
f
=
τ
f
=
s
u
we have
log
s
u
tan
e
−
e
=
(9.5)
φ
c
C
c
Equation 9.5 gives a linear relationship between the logarithm of undrained strength
and voids ratio (or water content) as illustrated on Figs. 9.5(d) and (e).
Since undrained strength changes with changing voids ratio it is not a material
parameter but, instead, it is a state-dependent parameter. Later we will discover some
more state dependent parameters whose value depends on the current state of the soil.
Although undrained strength
s
u
is a state dependent parameter it is related to voids
ratio through Eq. (9.5) which contains the material parameters
φ
c
,
e
and
C
c
.
In many topics you will find the undrained strength given by the Mohr-Coulomb
criterion in terms of total stress as
τ
f
=
c
u
+
σ
tan
φ
(9.6)
u
