Civil Engineering Reference
In-Depth Information
5.7 Current state
Because soil is both frictional and relatively highly compressible its stiffness, strength
and specific volume all depend on the current stresses and history of loading and
unloading during deposition and erosion. In Fig. 5.5(a) the soil at a shallow depth
z
is
lightly loaded by the small vertical stress
σ
z
due to the weight of soil above and it
is loose. After deposition of a substantial depth of soil
z
1
as in Fig. 5.5(b), the same
soil is heavily loaded and has become dense. After erosion back to the original ground
level, as in Fig. 5.5(c), the same soil is again lightly loaded but remains relatively dense.
Thus the current water content or density of a soil will depend on the current stress
and on the history of loading and unloading.
The current state of a soil can be related to the limiting states. For fine-grained clay
soils the liquidity index (
I
L
) is defined as
w
−
w
P
I
L
=
(5.10)
w
L
−
w
P
where
w
is the current water content and for coarse-grained soils the relative density
(
D
r
) is defined as
−
v
max
v
D
r
=
(5.11)
v
max
−
v
min
where
v
is the current specific volume. These relationships are illustrated in Fig. 5.4.
Notice that a liquidity index of 1.0 (corresponding to the loosest or wettest state)
corresponds to a relative density of zero.
Figure 5.5
Changes of state during deposition and erosion.
