11.8 Summary
1. The initial state of soil, before shearing, is fixed by the appropriate normal compression
and swelling lines and the final state is fixed by the critical state line.
The path between the initial and final states is governed by the loading (i.e. drained
or undrained) and by the state boundary surface.
2. There is an important distinction to be made between the behaviour of soils on
the wet side of critical (which compress on drained loading, or where the pore
pressures rise on undrained loading) and soils on the dry side of critical (which
dilate on shearing, or where the pore pressures fall).
3. In the simple idealization the behaviour is taken to be elastic when the state is
inside the state boundary surface. Yielding and plastic straining occur as the state
moves on the state boundary surface.
4. There are relationships between stress ratio and dilation for states on the state
boundary surface on the wet side and on the dry side of the critical state. These relationships
provide a means of determining the critical state of soil from tests in
which the sample did not reach the critical state.
5. Overconsolidated soils, on the dry side of critical, which soften on shearing
beyond the peak, often develop strong slip surfaces where intense shearing
and volume changes are concentrated in a very thin region of material.
In this case measurements made at the boundaries of a test sample become
unreliable.
Worked examples