11.5 Soil behaviour during undrained loading
Figure 11.10 shows the behaviour during undrained loading of sample W initially on
the wet side of the critical state and sample D initially on the dry side, both with the
same initial specific volume. The broken line in Fig. 11.10(a) is the yield curve which is
the intersection of the elastic wall with the state boundary surface so the samples yield
at YW and YD where the stress paths meet the yield curve. Thereafter the stress paths
follow the yield curve and reach the critical state line at the same point at Fu because
their specific volumes remain constant, as shown in Fig. 11.10(d). Figure 11.10(a)Figure 11.10(c) shows the stress ratio q/p plotted against the shear strain. Sample D
reaches a peak stress ratio η
p
at YD but sample W does not have a peak stress ratio.
The yield point is at YW at the same strain εy as YD 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.