DHARM114 GEOTECHNICAL ENGINEERINGHygroscopic moisture is affected neither by gravity nor by capillary forces and would
not move in the liquid form. It cannot be evaporated ordinarily. However, hygroscopic mois-
ture can be removed by oven-drying at 105° – 110°C. Moisture in this form has properties
which differ considerably from those of liquid water—Hygroscopic moisture has greater den-
sity, higher boiling point, greater viscosity, greater surface tension, and a much lower freezing
point than ordinary water.
Hygroscopic moisture has a pronounced effect on the cohesion and plasticity character-
istics of a clayey soil ; it also affects the test results of grain specific gravity of the soil. This is
because the volume of the displaced water is too low by the amount of hygroscopic moisture,
thus leading to higher values of specific gravity than the correct value. (The error could range
from 4% to 8% depending upon the hygroscopicity).
(ii)Film moisture. Film moisture forms on the soil grains because of the condensation
of aqueous vapour ; this is attached to the surface of the soil particle as a film upon the layer of
the hygroscopic moisture film. This film moisture is also held by molecular forces of high in-
tensity but not as high as in the case of the hygroscopic moisture film. Migration of film mois-
ture can be induced by the application of an external energy potential such as thermal or
electric potential ; the migration will then be from points of higher temperature/higher poten-
tial to points of lower temperature/lower potential. Film moisture does not transmit external
hydrostatic pressure. It migrates rather slowly. The greater the specific surface of the soil, the
more is the film moisture that can be contained. When the film moisture corresponds to the
maximum molecular moisture capacity of the soil, the soil possesses its maximum cohension
and stability.
5.3 NEUTRAL AND EFFECTIVE PRESSURES
As a prerequisite, let us see something about “Geostatic Stresses”.5.3.1 Geostatic Stresses
Stresses within a soil mass are caused by external loads applied to the soil and also by the self-
weight of the soil. The pattern of stresses caused by external loads is usually very compli-
cated ; the pattern of stresses caused by the self-weight of the soil also can be complicated. But,
there is one common situation in which the self-weight of the soil gives rise to a very simple
pattern of stresses—that is, when the ground surface is horizontal and the nature of the soil
does not vary significantly in the horizontal directions. This situation exists frequently in the
case of sedimentary deposits. The stresses in such a situation are referred to as ‘Geostatic
Stresses’.
Further, in this situation, there can be no shear stresses upon vertical and horizontal
planes within the soil mass. Therefore, the vertical geostatic stress may be computed simply
by considering the weight of the soil above that depth.
If the unit weight of the soil is constant with depth,
σv = γ.z ...(Eq. 5.1)
where σv = vertical geostatic stress
γ = unit weight of soil
z = depth under consideration