Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 139

The hydrostatic pressure in the capillary tube at the level of the free surface of the

water supply is zero, that is, it is equal to the atmospheric pressure. It is known that below the

free water surface hydrostatic pressure u increases linearly with depth :

u = γw. z

where γm = Unit weight of water.

Conversely, hydrostatic pressure measured in the capillary column above the free water

surface is considered to be negative, that is, below atmospheric. This negative pressure, called

capillary tension, is given by :

u = – γw. h

where h is the height measured from the free water surface. The maximum value of the capil-

lary tension uc is :

uc = γw. hc =

4 T

d

s

c

...(Eq. 5.38)

Capillary rise is not limited to tubes. If two vertical glass plates are placed so that they
touch along one end and form a ‘V’ in plan, a wedge of water will rise in the V because of the
phenomenon of capillarity (Fig. 5.14).

Meniscus

Capillary rise

Water level

Pictorial view

Capillary wedge

Glass plates

Plan

Fig. 5.14 Capillary rise in the corner formed by glass plates

The height of such rise is related not only to the attraction between the water and plates

and the physical properties of water, as in tubes, but also to the angle formed by the ‘V’.