9.12 Summary
1. During shearing soils ultimately reach a critical state where they continue to distort
with no further change of state (i.e. at constant shear stress, constant normal
effective stress and constant volume).
2. Before the critical state there may be a peak state and after large strains clay soils
reach a residual state. The peak state is associated with dilation and the residual
state is associated with laminar flow of flat clay particles.
3. The critical states of soils in shear tests are given by
4. The critical state strength of soil is uniquely related to its voids ratio or water
content so for undrained loading of saturated soil (i.e. at constant water content)
the undrained strength su remains unchanged.
5. If the soil is drained and effective stresses can be determined you can use effective
stress analyses and the critical state strength is given by φ
c
. If the soil is saturated
and undrained you can use total stress analyses and the critical state strength is
given by the undrained strength su.
6. To take account of different normal effective stresses and different voids ratios,
stresses, should be normalized with respect to the critical stress σc or the critical
voids ratio eλ given by
7. The critical states in shear tests are found also in triaxial tests and the critical state line is given by
8. The critical state parameters φ
c
, e and Cc (orM, and λ) are material parameters:
they depend only on the nature of the soil grains.
9. The state of a soil is described by the distance of the voids ratio − effective stress
point from the critical state line and it is given by the either of the state parameters
Sv or Ss.