Civil Engineering Reference
In-Depth Information
Figure 9.3
Residual strength of clay at very large displacements.
For soils that have a peak state it is not easy to decide whether the strength of the
soil - the maximum shear stress it can sustain - should be the peak state that can be
sustained only for relatively small strains or the critical state. I will leave this question
for the time being and, for the present, I will discuss the conditions at the peak state
and the conditions at the critical state separately.
There is another aspect of soil shearing that must be considered here and that
is the development of residual strength at very large displacements on slip planes
(Skempton, 1964). Figure 9.3 illustrates the behaviour of a sand and a plastic clay
soil over large displacements; note the logarithmic scale, which allows the diagram
to represent displacements exceeding 1 m. (Tests of this kind can be carried out in a
direct shear box by moving the box backwards and forwards or in a special ring shear
apparatus in which an annulus of soil can be sheared continuously.) The behaviour
illustrated is for tests in which the effective stresses and the initial states were chosen
so that the peak and ultimate states of the clay and the sand soil happened to be the
same. At the critical state, at displacements of about 10 mm corresponding to shear
strains of about 10% as shown in Fig. 9.1, the movements of grains are essentially
turbulent, involving relative movements and rotations of both clay and sand grains.
At larger displacements, however, the strains become localized into distinct zones of
intense shearing and the shear stresses applied to the clay soil decrease.
The lowest shear stress reached after very large displacements is called the residual
strength. It is associated with laminar flow of flat clay grains which have become ori-
ented parallel to a very thin shearing zone, as illustrated in Fig. 9.3. In sands and other
soils with rotund (i.e. not flat) grains there is no opportunity for laminar flow and the
residual strength is the same as the critical state strength. In clays the residual strength
may be as little as 50% of the critical state strength and it is important for design of
works on old landslides. Choices of soil strength for design of slopes, foundations and
retaining walls are discussed in Chapter 18.
9.3 Critical states
We now come to the essence of soil mechanics, which is the critical state. The idealized
behaviour described in this chapter is based on experimental data given by Atkinson
