Civil Engineering Reference
In-Depth Information
6.8 Volume change and drainage
As soil is loaded or unloaded due to changes of effective stress it will generally change
in volume.
In silts and sands the grains themselves are relatively stiff so they do not change
volume. In soils such as shelly sands where the grains are relatively weak, or where
stresses are very large, the grains may break. In either case volume changes are largely
due to rearrangement of the grains and changes in the volume of the voids. In clays
which have high plasticity the clay particles themselves may also change in volume.
At small effective stress the spacing of the grains may be loose and at high stresses it
will be dense, as shown in Fig. 6.8. If the pore pressure
u
0
remains constant then the
changes of total and effective stresses are the same (
σ
; see Eq. 6.16). If the
volume of the soil grains remains constant then, in Fig. 6.8, the change of volume of
the soil
=
σ
V
w
.
In saturated soil changes in volume must be due to seepage of water through the soil
and so soil compression is rather like squeezing water from a sponge. In a laboratory,
test water will seep to the boundaries of the sample while, in the ground, water will
seep to the surface or to natural drainage layers in the soil. For example, Fig. 6.9
illustrates an embankment built on a bed of clay sandwiched between layers of sand
which act as drains. As the embankment is constructed water will seep from the clay
to the sand layers as indicated.
There must, of course, be sufficient time for the water to seep through the soil to
permit the volume change to occur; otherwise the pore pressure will change. As a result
V
is the same as the volume of water expelled
Figure 6.8
Volume changes in soil.
Figure 6.9
Drainage of clay beneath an embankment.
