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 and Bransby (1978) and by Muir Wood (1991). From Figs. 9.1 and 9.3 the critical state
is the state reached after strains of at least 10% and is associated with turbulent flow.
The relationships between the shear stress, the normal effective stress and the voids
ratio of soils at the critical states are illustrated in Fig. 9.4.
Figure 9.4(a) and (b) shows the critical state line (CSL). This shows that, at the
critical state, there is a unique relationship between the shear stress, the normal effective
stress and the voids ratio. Figure 9.4(c) is the same as Fig. 9.4(b) but with the normal
stress on a logarithmic scale. Also shown on Fig. 9.4(c) is the one-dimensional normal
compression line from Fig. 8.10(b).