Civil Engineering Reference
In-Depth Information
normal compression line is a boundary to all possible states for isotropic compression:
later we will see that this state boundary line forms part of a state boundary surface.
This is also a yield surface like that illustrated in Fig. 3.16 and the NCL is a yield curve
like that shown in Fig. 3.12.
At any state such as B inside the boundary surface the soil is overconsolidated and
its overconsolidation ratio is
p
m
p
0
R
p
=
(8.6)
where
p
0
is the current stress and
p
m
is the stress at the point C which is the maximum
stress which the sample at B has experienced in the past. Notice that any isotropic state
can be described by only two of the parameters
p
,
v
and
R
p
.
For a normally consolidated soil the state lies on the normal compression line and
R
p
1.0. Figure 8.5 shows two states, R
1
and R
2
, that have the same overconsolidation
ratio. From the geometry of the figure, or from Eq. (8.6),
=
ln
p
y1
−
ln
p
01
ln
p
y2
−
ln
p
02
ln
R
p
=
=
(8.7)
so that the line through R
1
and R
2
, where the overconsolidation ratio is the same, is
parallel to the normal compression line.
Soils at points N
1
and R
2
have the same current stress, and so would be at the
same depth in the ground, but they have very different stiffnesses related to
κ
respectively. Similarly, soils at points R
2
and N
2
have nearly the same specific volume
and water content but, again, they have very different stiffnesses. Soils at points N
1
and
N
2
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 R
1
where the stress is
p
01
can move to R
2
only
by loading to the NCL at N
1
where it yields at the yield stress
p
y1
, further compression
λ
and
Figure 8.5
Overconsolidation ratio.
