Geotechnical Engineering

DHARM

142 GEOTECHNICAL ENGINEERING

responsible for capillary rise. There would be menisci at both ends of the suspended column,
each meniscus being in tension. The length of such a column would be controlled by the same
factors that effect capillary rise.

5.9.5 Removal of Capillary Water in Soil

The existence of an air-water interface is a prerequisite for the occurrence of capillary rise.
Since capillary water can exist only above the water table, it follows that capillarity will cease
to exist where submergence of a soil zone exists.
Evaporation is another means of removing capillary water. This capillary water is very
mobile as evaporation is continually replaced by capillary water.

5.9.6 Effects of Surface Tension and Capillarity

At the level of the meniscus the surface tension imposes a compressive force onto the soil
grains in contact with the meniscus of magnitude equal to the weight of water in the capillary
column, as indicated in an earlier sub-section. This effect applies to both a meniscus resulting
from capillary rise and for pore water suspended above a capillary zone. The compressive force
imposed on the soil in contact with the held column of water causes compression or shrinkage
of the soil.
When the ground water drops subsequent to the time of formation of a clay deposit,
internal compressive stresses in the clay mass due to the surface tension and capillary forces
make it firm and strong. This is referred to as drying by desiccation and the clays are therefore
called “desiccated clays”. Sometimes, such desiccated clays may overlie soft and weak deeper
clays. However, the strength and thickness of the desiccated zone may be such that roads and
light buildings could be satisfactorily supported by it.

Since the intergranular pressure in the capillary zone is increased by capillary pres-
sures, the procedure for determination of the effective stress when such a zone overlies a
saturated soil mass, gets modified as illustrated below (Fig. 5.17) :

g¢hc

R

Q

Capillary zone

(saturated)

Water-table Q

R

hc

h

ghc

hs

PP

SaturatedSaturated soilsoil

(h+ h)g¢c gsat s

g¢(h+h)sc

Fig. 5.17 Capillary zone-computation of effective stress
Let a saturated soil mass of depth hs be overlain by a capillary zone of height hc assumed
saturated by capillarity.