Civil Engineering Reference
In-Depth Information
Figure 5.3
Volumes and weights of grains and water in saturated soils.
weight might be 18 to 22 kN/m
3
(i.e. about twice that of water:
γ
=
approximately
w
10 kN/m
3
).
Relationships between these and specific volume can be obtained from Fig. 5.3
together with Eqs. (5.1) to (5.5) as
v
=
1
+
wG
s
=
1
+
e
(5.6)
G
s
−
1
v
=
γ
γ
w
(5.7)
−
1
where
G
s
is the specific gravity of the soil grains which, for many soils, is approximately
G
s
=
2.65.
5.6 Limits of consistency
As the water content and specific volume of a soil are increased it will soften and
weaken; this is well known to farmers and football players. If the water content is very
large we just get muddy water and if it is very small we get a material that is hard
and brittle like rock. Obviously there are limits to the water content within which a
soil has the consistency of soil rather than the consistency of a liquid or a brittle rock.
Tests to determine the precise water contents at which soil behaviour becomes liquid
or brittle are the Atterberg limits tests described in Sec. 7.3; these determine the liquid
limit (
w
L
) where the soil starts to flow like a liquid and the plastic limit (
w
P
) where it
ceases to be plastic and becomes brittle.
The Atterberg limits apply to fine-grained soils. (Soils for which it is possible to deter-
mine the Atterberg limits are often called plastic, but this term must not be confused
with the strict meaning of plastic as a type of constitutive relationship, discussed in
Sec. 3.9.) For coarse-grained sands and gravels the appropriate limits are the minimum
density of a very loosely poured sample and the maximum density of a vibrated and
heavily loaded sample (Kolbuszewski, 1948). Thus the minimum density of a sand is
equivalent to the liquid limit of a clay, while the maximum density is equivalent to
the plastic limit. The relationships between the Atterberg limits and the maximum and
minimum densities are illustrated in Fig. 5.4.
