Geotechnical Engineering

DHARM

140 GEOTECHNICAL ENGINEERING

5.9.2 Capillary Rise in Soil

The rise of water in soils above the ground water table is analogous to the rise of water into

capillary tubes placed in a source of water. However, the void spaces in a soil are irregular in

shape and size, as they interconnect in all directions. Thus, the equations derived for regular

shaped capillary tubes cannot be, strictly speaking, directly applicable to the capillary phe-

nomenon associated with soil water. However, the features of capillary rise in tubes facilitate

an understanding of factors affecting capillarity and help determine the order of a magnitude

for a capillary rise in the various types of soils.

Equation 5.36 indicates that even relatively large voids will be filled with capillary water

if soil is close to the ground water table. As the height above the water table increases, only the

smaller voids would be expected to be filled with capillary water. The larger voids represent

interference to an upward capillary flow and would not be filled. The soil just above the water

table may become fully saturated with capillary water, but even this is questionable since it is

dependent upon a number of factors. The larger pores may entrap air to some extent while

getting filled with capillary water. Above this zone lies a zone of partial saturation due to

capillarity. In both these zones constituting the capillary fringe, even absorbed water contributes

to the pore water (Fig. 5.15).

Downward percolation

Capillary rise

GWT

Zone of partial saturation

due to capillary rise, downward

percolation, and adsorbed water

Zone of full saturation due to

capillary rise and adsorbed water

Zone of full saturation below water table

Fig. 5.15 Capillary fringe with zones of full and partial saturation

Capillary water in

the wedge formed at

grain to grain contact

Soil

grain

Soil

grain

Fig. 5.16 Wedge of capillary water at the contact of soil grains
In the zone of partial saturation due to the capillary phenomenon, capillary movement
of water may occur even in the wedges of the capillary V formed wherever soil grains come into
contact (similar to the V formed by vertical plates discussed in the preceding sub-section) Fig.
5.16. This is referred to as “Contact Moisture”.

Since void spaces in soil are of the same order of magnitude as the particle sizes, it
follows that the capillary rise would be greater in fine-grained soils than in coarse-grained
soils. Relative values of capillary rise in various soils are given in Table 5.2 for an idea of
orders of magnitude.