For a normally consolidated soil the state lies on the normal compression line and
Rp=1.0. Figure 8.5 shows two states, R1 and R2, that have the same overconsolidation
ratio. From the geometry of the figure, or from Eq. (8.6),
Soils at points N1 and R2 have the same current stress, and so would be at the
same depth in the ground, but they have very different stiffnesses related to λ and κ
respectively. Similarly, soils at points R2 and N2 have nearly the same specific volume
and water content but, again, they have very different stiffnesses. Soils at pointsN1 and
N2 are both normally consolidated; they will have different stiffnesses for loading and
for unloading. This means that soil stiffness is not directly related either to the water
content or to the current stress (or depth in the ground) and the overconsolidation
ratio is an important factor in determining soil behaviour.
In Fig. 8.5, the state of the sample at R1 where the stress is p
01 can move to R2 only
by loading to the NCL at N1 where it yields at the yield stress p
y1, further compression
along the NCL to N2 where the yield stress is p
y2 and unloading to R2 where the
stress is p
02. The state of a soil can however move directly from R1 to R2 by creep in
fine-grained soils and vibration in coarse-grained soils. Moreover the position of the
NCL can shift as a result of soil structure. These mechanisms will be described further
in Chapter 16.
Figure 8.6 shows the state of a sample of soil initially normally consolidated at
R0 where the stress p
0
= p
m moving directly to R1 where the stress is same, by
creep or vibration. From the definition of overconsolidation ratio in Eq. (8.6) the
overconsolidation ratios at R0 and R1 are both the same and are equal to 1.0 since
the stresses have not changed. This means that the overconsolidation ratio defined in
Eq. (8.6) does not properly describe the current state of a soil.
The state of a soil can be better described by the yield stress ratio
where p
0 is the current stress and p
y is the yield stress which is the stress at the intersection
of the swelling line through R1 with the NCL. Notice that as the state moves from
R1 to R2 in Fig. 8.6, either by loading, yielding and unloading or by creep or vibration
the yield stress ratio increases because the yield stress increases from p
y1 to p
y2.