Civil Engineering Reference
In-Depth Information
A further interesting point is that, if we assume that the weight of the capillary tube is negligible, then
the only vertical forces acting are the downward weight of the water column supported by the surface
tension at the top and the reaction at the base support of the tube. The tube must therefore be in com-
pression. The compressive force acting on the walls of the tube will be constant along the length of the
water column and of magnitude 2
π
T cos
α
(or
π
r
2
h
c
γ
w
).
It may be noted that for pure water in contact with clean glass which it wets, the value of angle
α
is
zero. In this case the radius of the meniscus is equal to the radius of the tube and the derived formulae
can be simplified by removing the term cos
α
.
With the use of the expression for h
c
we can obtain an estimate of the theoretical capillary rise that will
occur in a clay deposit. The average void size in a clay is about 3
μ
m and, taking
α
=
0, the formula gives
h
c
=
5.0 m. This possibly explains why the voids exposed when a sample of a clay deposit is split apart
are often moist. However, capillary rises of this magnitude seldom occur in practice as the upward velocity
of the water flow through a clay in the capillary fringe is extremely small and is often further restricted by
adsorbed water films, which considerably reduce the free diameter of the voids.
2.13.2 Capillary effects in soil
The region within which water is drawn above the water table by capillarity is known as the capillary fringe.
A soil mass, of course, is not a capillary tube system, but a study of theoretical capillarity enables one to
determine a qualitative view of the behaviour of water in the capillary fringe of a soil deposit. Water in
this fringe can be regarded as being in a state of negative pressure, i.e. at pressure values below atmos-
pheric. A diagram of a capillary fringe appears in Fig.
2.14
d.
The minimum height of the fringe, h
cmin
, is governed by the maximum size of the voids within the soil.
Up to this height above the water table the soil will be sufficiently close to full saturation to be considered
as such.
The maximum height of the fringe h
cmax,
is governed by the minimum size of the voids. Within the range
h
cmin
to h
cmax
the soil can be only partially saturated.
Terzaghi and Peck (
1948
) give an approximate relationship between h
cmax
and grain size for a granular
soil:
C
eD
h
cmax
=
mm
10
where C is a constant depending upon the shape of the grains and the surface impurities (varying from
10.0 to 50.0 mm
2
) and D
10
is the effective size expressed in millimetres.
Owing to the irregular nature of the conduits in a soil mass it is not possible, even approximately, to
calculate water content distributions above the water table from the theory of capillarity. This is a problem
of importance in highway engineering and is best approached by the concept of soil suction.
2.13.3 Soil suction
The capacity of a soil above the ground water table to retain water within its structure is related to the
prevailing suction and to the soil properties within the whole matrix of the soil, e.g. void and soil particle
sizes, amount of held water, etc. For this reason it is often referred to as
matrix
or
matric
suction.
It is generally accepted that the amount of matric suction, s, present in an unsaturated soil is the dif-
ference between the values of the air pressure, u
a
, and the water pressure, u
w
.
s
= −
u
u
a
w
If u
a
is constant, then the variation in the suction value of an unsaturated soil depends upon the value of
the pore water pressure within it. This value is itself related to the degree of saturation of the soil.
