Civil Engineering Reference
In-Depth Information
Ageing gives rise to bonding and to other physical and chemical changes. Bonding
is the way in which soil grains are attached to one another. It can arise from natural
attractions of very fine grains or from deposition of salts from solution in the ground-
water. Weathering weakens bonding and may change the chemical composition of the
grains. There may be continuing deformation due to creep at constant effective stress
and there may be changes in the chemistry of the pore water. All these, and other,
events contribute to features in natural soils which are not all present in reconstituted
samples.
It is very difficult to discover the true behaviour of natural soils. The obvious way is
to recover undisturbed samples from the ground and test them in the laboratory but,
unfortunately, the process of recovering the sample from the ground and installing
it in the test apparatus will probably alter its behaviour. There is no possibility of
recovering and testing a truly undisturbed sample; the best we can do is to take and
test an intact sample with the very minimum of disturbance. If the correct procedures
for sampling and testing are followed the behaviour of an intact sample will be very
close to the behaviour of the soil in the ground, but it is essential to follow the correct
procedures.
This topic deals with the basic, simple theories of soil mechanics relevant to recon-
stituted soils and a detailed discussion of all the effects and consequences of structure
in natural soils is beyond its scope. It is, however, important to note these effects,
which is the purpose of this chapter. The important thing is to consider the behaviour
of your intact samples of natural soils within the basic simple framework developed
for reconstituted soils.
16.2 One-dimensional compression and swelling of
soils in the ground
The behaviour of soils during one-dimensional compression and swelling in laboratory
tests was discussed in Sec. 8.5 and similar behaviour will occur during deposition and
erosion of soil in the ground. Figure 16.1(a) illustrates a soil element below a ground
level which rises and falls due to deposition and erosion and Fig. 16.1(b) shows the
resulting changes of effective stress and water content. So far I have considered volume
and volume changes in terms of the specific volume
v
or the voids ratio
e
, but in this
chapter I shall consider water content
w
, as this is a commonly measured and often
quoted parameter. Water content, specific volume and voids ratio are simply related
(see Sec. 5.5 and, for saturated soil,
e
wG
s
. At points A and B the soil is normally
consolidated and at C it is overconsolidated. Notice that although the vertical stresses
at A and C are similar the water contents are very different. Figure 16.1(c) illustrates
the changes of vertical and horizontal effective stresses during deposition and erosion.
These can be related by a coefficient of earth pressure at rest,
K
0
, given as
=
K
0
=
σ
h
σ
z
(16.1)
σ
h
<σ
z
and
K
0
<
For normally consolidated and lightly overconsolidated soils
1,
σ
h
>σ
z
and
K
0
while for heavily overconsolidated soils
>
1. An approximation often
