Civil Engineering Reference
In-Depth Information
Behaviour of soil before failure
11.1 Introduction
In laboratory triaxial or shear tests, and in the ground, soil is loaded from some initial
state and it will ultimately reach a critical state. (If the soil is clay it may go on to
a residual state.) In Chapter 8 I described the behaviour of soils during isotropic and
one-dimensional compression and swelling and I showed how these were related. If the
history of isotropic or one-dimensional loading and unloading is known the initial
state described by the current effective stress, specific volume and overconsolidation
ratio are fixed. Notice that the conditions in a shear test when the shear stress is zero
are the same as those in a one-dimensional compression test and the conditions in a
triaxial test when
q
=
0 (i.e. when
σ
=
σ
r
) are the same as those in an isotropic
a
compression test.
In Chapter 9, I described the behaviour of soils in drained shear tests in which
the changes of shear stress
τ
and volumetric strain
ε
γ
are shown
in Fig. 9.1 and I described the critical states in shear tests. I then showed that there
were qualitatively similar peak and critical states in triaxial tests. If either the effective
normal stress or the specific volume at the critical state is known the critical state
strength is fixed.
We can now consider how the state moves from the initial to the critical for different
loadings. In Chapter 9 I suggested that there might be unique states on the wet side
of critical between the normal compression line and the critical state line, as shown in
Figs. 9.7 and 9.10. In Chapter 10 I showed that there were unique peak states on the
dry side of critical.
In this chapter I will consider these intermediate states in more detail. I will mostly
consider the behaviour of soils in triaxial tests because the analyses will lead to devel-
opment of Cam Clay in the next chapter, but of course the behaviour of soils in shear
tests is qualitatively similar to that in triaxial tests.
v
with shear strain
11.2 Wet side and dry side of critical
During drained shearing soil may either compress or dilate, as illustrated in Figs. 9.1
and 10.2, and during undrained shearing pore pressures may either increase or
decrease. What actually happens depends on the position of the initial state with respect
to the critical state line. We can now see the significance of the distinction made in
