Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 137

Capillus literally means hair in Greek, indicating that the size of opening with which
the phenomenon of capillarity is connected or related, is of this order of magnitude.

5.9.1 Rise of Water in Capillary Tubes

The principle of capillary rise in soils can be related to the rise of water in glass capillary tubes
in the laboratory. When the end of a vertical capillary tube is inserted into a source of water,
the water rises in the tube and remains there. This rise is attributed to the attraction between
the water and the glass and to surface tension which develops at the air-water interface at the
top of the water column in the capillary tube.

The surface tension is analogous to a stretched membrane, or a very thin but tough film.
The water is “pulled up” in the capillary tube to a height, dependent upon the diameter of the
tube, the magnitude of surface tension, and the unit weight of water.

The attraction between the water and capillary tube, or the tendency of water to wet the
walls of the tube affects the shape of the air-water interface at the top of the column of water.
For water and glass, the shape is concave as seen from top, that is, the water surface is lower
at the centre of the column than at the walls of the tube. The resulting curved liquid surface is
called the ‘meniscus’. The surface of the liquid meets that of the tube at a definite angle,
known as the ‘contact angle’. This angle, incidentally, is zero for water and glass (Fig. 5.12).

Capillary

rise hhcc

a

Ts Ts

dc

Meni

scus

a: Contact angle

(zero for water and glass)

Glass capillary tube

Free water surface

a

Fig. 5.12 Capillary rise of water in a glass-tube
The column of water in the capillary tube rises, against the pull of gravity, above the
surface of the water source. For equilibrium, the effect of the downward pull of gravity on the
capillary column of water has to be resisted by surface tension of the water film adhering to
the wall of the tube to hold the water column.

If Ts is the surface tension, in force units per unit length, the vertical component of the
force is given by πdc. Ts. cos α where α is the contact angle and dc is the diameter of the
capillary tube. With water and glass, the meniscus is tangent to the wall surface, so that the
contact angle, α, is zero.

Therefore, the weight of a column of water, that is capable of being supported by the

surface tension, is πdc. Ts. But the weight of water column in the capillary tube is πdc

2

4

· hc · γw,