Civil Engineering Reference
In-Depth Information
has been drawn so the deviator stresses
q
y
at the yield points are the same for both
samples but this would not generally be the case.
For undrained loading
p
=
δε
=
0, for isotropic elastic soil
δ
0 from Eq. 11.3 and
v
the paths W
Y
D
are vertical. Figure 11.10(b) shows the deviator stress
q
plotted against the shear strain
→
Y
W
and D
→
s
for both samples and there is very little difference
between them. Before the yield points when the behaviour is elastic the gradients of
the stress-strain curves are 3G
u
ε
3G
. In Chapter 13 we will find that soil stiffness
increases with mean effective stress and with overconsolidation ratio. Sample W has
a large mean effective stress and a small overconsolidation ratio while sample D has
a smaller mean effective stress and a larger overconsolidation ratio, so their elastic
stiffnesses can be nearly the same and they yield at about the same strain
=
y
. Notice
that neither sample has a peak deviator stress even though sample D yields on the peak
failure envelope.
Figure 11.10(c) shows the stress ratio
q
/
p
plotted against the shear strain. Sample D
reaches a peak stress ratio
ε
η
p
at Y
D
but sample W does not have a peak stress ratio.
The yield point is at Y
W
at the same strain
ε
y
as Y
D
but it is not clearly defined
on the stress ratio-strain curve. Figures 11.10(b) and (c) demonstrate how soil test
data can give very different looking curves when they are plotted in different ways.
It is always a good idea to plot test data in different ways to explore soil behaviour
fully. As most soil test data are now captured electronically and stored in spreadsheets
plotting them in different ways is relatively straightforward. Test data should not,
however, be plotted randomly; the axes of graphs should be chosen to investigate
behaviour within a particular theory.
11.6 Stress ratio and dilation
Figure 11.11(a) and (b) shows the variations of stress ratio and volumetric strain with
shear strain for an ideal soil sheared from states initially either on the wet side or on
Figure 11.11
Stress ratio and dilation of soil.
