The state boundary surface in Fig. 11.6 has been drawn as a smooth curve linking
the wet side and the dry side. Later, in Chapter 12, this will be represented by a
simple mathematical expression. Do not forget that the line shown in Fig. 11.6, which
has normalized axes, is really a three-dimensional surface in the set of axes q :p :v.
This surface is rather difficult to draw, which is why it is easier to normalize the results
first. Figure 11.7 illustrates the three-dimensional surface; this shows constant specific
volume sections as full lines and constant stress ratio (i.e. constant η
) sections as broken
lines. Data from soil tests that demonstrate the existence of unique state boundary
surfaces were given by Atkinson and Bransby (1978) and by Muir Wood (1991).
The part of the state boundary surface on the wet side of critical (i.e. between the
normal compression line and the critical state line) is sometimes known as the Roscoe
surface and the part on the dry side corresponding to peak states is sometimes known
as the Hvorslev surface (Atkinson and Bransby, 1978).