Civil Engineering Reference
In-Depth Information
Figure 8.1
Volume changes in soil.
Figure 8.2
Isotropic compression and swelling of soil.
to move into) to dense states (where there is much less opportunity for grains to
rearrange). As shown in Fig. 8.2, the stress-strain line is curved. Thus the mechanisms
of volume change in soils due to rearrangement of the grains largely accounts for the
non-linear bulk stiffness behaviour. For the unloading-reloading loop ABC the soil is
very much stiffer (i.e. the volume changes are less) than for first loading because the
grains will obviously not 'un-rearrange' themselves on unloading. Behaviour similar
to that shown in Fig. 8.2 is also found for soils which have weak grains (such as
carbonate or shelly sands) that fracture on loading. In this case most of the compres-
sion during first loading is associated with grain fracture but obviously the grains do
not 'unfracture' on unloading. Soils which contain a high proportion of plastic clay
may swell significantly on unloading due to volume changes in the clay grains them-
selves. From Eq. (3.12) the instantaneous bulk modulus at any point is the gradient of
the curve for first loading or for unloading or reloading, given by
d
p
d
K
=
(8.1)
ε
v
and the value of
K
is not a soil constant.
