Permeability and Flow of Water in Soils - Elements of Soil Mechanics - page 54

Fig. 2.13 Example 2.7.
2.13 Capillarity and unsaturated soils
The behaviour of unsaturated soils is a relatively specialised subject area and readers interested in gaining
a good understanding of the topic are referred to the publications by Fredlund, Rahardo and Fredlund
( 2012 ) and Ng and Menzies ( 2007 ). Simple coverage of some of the key aspects involved are offered in
the following sub-sections.
2.13.1 Surface tension
Surface tension is the property of water that permits the surface molecules to carry a tensile force. Water
molecules attract each other and, within a mass of water, these forces balance out. At the surface, however,
the molecules are only attracted inwards and towards each other, which creates surface tension. Surface
tension causes the surface of a body of water to attempt to contract into a minimum area: hence a drop
of water is spherical.
The phenomenon is easily understood if we imagine the surface of water to be covered with a thin
molecular skin capable of carrying tension. Such a skin, of course, cannot exist on the surface of a liquid,
but the analogy can explain surface tension effects without going into the relevant molecular theories.
Surface tension is given the symbol T, and can be defined as the force in Newtons per millimetre length
that the water surface can carry. T varies slightly with temperature, but this variation is small and an average
value usually taken for the surface tension of water is 0.000 075 N/mm (0.075 N/m).
The fact that surface tension exists can be shown in a simple laboratory experiment in which an open-
ended glass capillary tube is placed in a basin of water subjected to atmospheric pressure; the rise of
water within the tube is then observed. It is seen that the water wets the glass and the column of water
within the tube reaches a definite height above the liquid in the basin.
The surface of the column forms a meniscus such that the curved surface of the liquid is at an angle α
to the walls of the tube (Fig. 2.14a ). The arrangement of the apparatus is shown in Fig. 2.14b .
The base of the column is at the same level as the water in the basin and, as the system is open, the
pressure must be atmospheric. The pressure on the top surface of the column is also atmospheric. There
are no externally applied forces that keep the column in position, which shows that there must be a tensile
force acting within the surface film of the water.