Civil Engineering Reference
In-Depth Information
Further analyses of local drainage in slip planes and its influence on measured soil
behaviour are given by Atkinson (2000). The important thing to remember is once slip
planes appear in a soil sample you can no longer trust the results. Undrained tests are
no longer fully undrained due to local drainage and in drained tests specific volumes
in slip planes are larger than the average for the whole sample. Both lead to erroneous
interpretations of the results.
11.8 Summary
1. The initial state of soil, before shearing, is fixed by the appropriate normal com-
pression 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 rela-
tionships 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 shear-
ing beyond the peak, often develop strong slip surfaces where intense shear-
ing 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
Example 11.1: Determination of state path and yielding
A soil has the parameters
M
3.25. A constant volume section of
the state boundary surface is a semi-circle passing through the origin. Samples were
isotropically compressed and swelled in a stress path triaxial cell to different stresses
but the same initial specific volume
v
0
=
0.98,
λ
=
0.20,
κ
=
0.05 and
N
=
=
1.97; the initial stresses were: sample A,
p
0
=
600 kPa, sample B,
p
0
=
400 kPa, sample C,
p
0
=
150 kPa (sample A was nor-
mally consolidated). The samples were tested undrained by increasing
q
with
p
held
constant.
The state paths are shown in Fig. 11.13. When the state is inside the state boundary
surface the behaviour is elastic and shearing and volumetric effects are decoupled;
hence
p
=
0 for undrained loading. The states of the samples after compression and
swelling, at their yield points and at failure at their critical states, shown in Table 11.1,
were found by scaling from the diagram.
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