Behaviour of soil before failure - The Mechanics of Soils and Foundations - page 162

Sec. 8.4 between states on the wet side of the critical state (i.e. normally consolidated
or lightly overconsolidated clays or loose sands) and states on the dry side (i.e. heavily
overconsolidated clays or dense sands).
Figures 11.1 and 11.2 illustrate the idealized behaviour of soils initially on the wet
side or on the dry side during undrained or drained triaxial tests. In Fig. 11.1 the initial
total stress is at the point I with q 0 and a total mean stress p . Sample W is normally

consolidated and its initial effective stress state is on the wet side of critical: it has an
initial pore pressure u 0w . Sample D is overconsolidated and its initial effective stress
state is on the dry side of critical: it has an initial pore pressure u 0d . Samples W and D
both have the same initial specific volume v 0 .
The samples are loaded undrained in a test in which the mean total stress p is
constant. (A test in which p is constant can be carried out in a hydraulic triaxial cell
(see Chapter 7) by simultaneously reducing the cell pressure and increasing the axial
stress in the proportion

=

F in Fig. 11.1(a).
During any shearing test the states must move towards, and ultimately reach, the
critical state line. For undrained loading the states must remain at constant volume
and both samples reach the critical state line at F u , where they have the same undrained
strength because they have the same specific volume. You can see from Fig. 11.1 that
for the soil initially on the wet side the pore pressure increases on shearing, while for the
soil initially on the dry side the pore pressure reduces. Notice that the overconsolidated
soil reached a peak stress ratio at P but the deviator stress at F u is greater than that at P.

δσ = 2 δσ r .) The total stress path is I →

a

Figure 11.1 Behaviour of soil during undrained shearing.