The behavior of a soil mass depends upon the behavior of the discrete particles composing the mass
and the pattern of particle arrangement. In all these cases water plays an important part. The behavior of the soil mass is profoundly influenced by the inter-particle-water relationships, the
ability of the soil particles to adsorb exchangeable cations and the amount of water present.
The clay particles carry a net negative charge ^n their surface. This is the result of both
isomorphous substitution and of a break in the continuity of the structure at its edges. The intensity
of the charge depends to a considerable extent on tljie mineralogical character of the particle. The
physical and chemical manifestations of the surface charge constitute the surface activity of the
mineral. Minerals are said to have high or low surface activity, depending on the intensity of the
surface charge. As pointed out earlier, the surface activity depends not only on the specific surface
but also on the chemical and mineralogical composition of the solid particle. The surface activity of
sand, therefore, will not acquire all the properties of ^ true clay, even if it is ground to a fine powder.
The presence of water does not alter its propertie
changing its unit weight. However, the behavior ol
of coarser fractions considerably excepting
‘ a saturated soil mass consisting of fine sand
might change under dynamic loadings. This aspect of the problem is not considered here. This
article deals only with clay particle-water relations.
In nature every soil particle is surrounded by \^ater. Since the centers of positive and negative
charges of water molecules do not coincide, the molecules behave like dipoles. The negative charge
on the surface of the soil particle, therefore, attracts the positive (hydrogen) end of the water
molecules. The water molecules are arranged in a definite pattern in the immediate vicinity of the
boundary between solid and water. More than one layer of water molecules sticks on the surface with
considerable force and this attractive force decreases with the increase in the distance of the water
molecule from the surface. The electrically attracted water that surrounds the clay particle is known as
the diffused double-layer of water. The water located within the zone of influence is known as the
adsorbed layer as shown in Fig. 2.7. Within the zone of influence the physical properties of the water
are very different from those of free or normal water at the same temperature. Near the surface of the
particle the water has the property of a solid. At the middle of the layer it resembles a very viscous
liquid and beyond the zone of influence, the propenles of the water become normal. The adsorbed
water affects the behavior of clay particles when subjected to external stresses, since it comes between
the particle surfaces. To drive off the adsorbed water, the clay particle must be heated to more than
200 °C, which would indicate that the bond between the water molecules and the surface is
considerably greater than that between normal water molecules.
The adsorbed film of water on coarse particles is thin in comparison with the diameter of the
particles. In fine grained soils, however, this layer of adsorbed water is relatively much thicker and
might even exceed the size of the grain. The forces associated with the adsorbed layers therefore
play an important part in determining the physical properties of the very fine-grained soils, but have
little effect on the coarser soils.
Soils in which the adsorbed film is thick compared to the grain size have properties quite different
from other soils having the same grain sizes but smaller adsorbed films. The most pronounced
characteristic of the former is their ability to deform plastically without cracking when mixed with
varying amounts of water. This is due to the grains moving across one another supported by the viscous
interlayers of the films. Such soils are called cohesive soils, for they do not disintegrate with pressure but
can be rolled into threads with ease. Here the cohesion is not due to direct molecular interaction between
soil particles at the points of contact but to the shearing strength of the adsorbed layers that separate the
grains at these points.
Electrolytes dissociate when dissolved in water into positively charged cations and negatively
charged anions. Acids break up into cations of hydrogen and anions such as Cl or SO4. Salts and
bases split into metallic cations such as Na, K or Mg, and nonmetallic anions. Even water itself is an
electrolyte, because a very small fraction of its molecules always dissociates into hydrogen ions H+
and hydroxyl ions OH”. These positively charged H+ ions migrate to the surface of the negatively
charged particles and form what is known as the adsorbed layer. These H+ ions can be replaced by
other cations such as Na, K or Mg. These cations enter the adsorbed layers and constitute what is
termed as an adsorption complex. The process of replacing cations of one kind by those of another
in an adsorption complex is known as base exchange. By base exchange is meant the capacity of colloidal particles to change the cations adsorbed on their surface. Thus a hydrogen clay (colloid
with adsorbed H cations) can be changed to sodium clay (colloid with adsorbed Na cations) by a
constant percolation of water containing dissolved Na salts. Such changes can be used to decrease
the permeability of a soil. Not all adsorbed cations are exchangeable. The quantity of exchangeable
cations in a soil is termed exchange capacity.
The base exchange capacity is generally defined in terms of the mass of a cation which may
be held on the surface of 100 gm dry mass of mineral. It is generally more convenient to employ a
definition of base exchange capacity in milli-equivalents (meq) per 100 gm dry soil. One meq is
one milligram of hydrogen or the portion of any ion which will combine with or displace
1 milligram of hydrogen.
The relative exchange capacity of some of the clay minerals is given in Table 2.4.
If one element, such as H, Ca, or Na prevails over the other in the adsorption complex of a
clay, the clay is sometimes given the name of this element, for example H-clay or Ca-clay. The
thickness and the physical properties of the adsorbed film surrounding a given particle, depend to a
large extent on the character of the adsorption complex. These films are relatively thick in the case
of strongly water-adsorbent cations such as Li+ and Na+ cations but very thin for H+. The films of
other cations have intermediate values. Soils with adsorbed Li+ and Na+ cations are relatively more
plastic at low water contents and possess smaller shear strength because the particles are separated
by a thicker viscous film. The cations in Table 2.5 are arranged in the order of decreasing shear
strength of clay.
Sodium clays in nature are a product either of the deposition of clays in sea water or of their
saturation by saltwater flooding or capillary action. Calcium clays are formed essentially by fresh
water sediments. Hydrogen clays are a result of prolonged leaching of a clay by pure or acidic
water, with the resulting removal of all other exchangeable bases.