Environmetal Soil Properties and Behaviour

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```Water-filled pan
h
2r
Cohesionless soil sample
Capillary tube
concept
T (Surface tension T acts around
circumference of capillary tube)
Circumferential surface
tension = 2pirTcos\u3b1
Force acting at air-water interface
in capillary tube = pir2h\u3b3w g
h = 2Tcos\u3b1r\u3b3w g
Average void diameter in
cohesionless soil assumed as
2r* = 0.01 mm
Calculated h for soil is 300 cm
From summation of forces:
FIguRE 3.1
Height of rise in a capillary tube (left), and application of capillary tube concept to a cohesion-
less soil. The diameter of the capillary tube is 2r, and the diameter of the average voids in the
cohesionless soil is assumed to be 2r* = 0.01 mm. \u3b1 refers to the contact angle, and g refers to
the acceleration due to gravity.
85Soil\u2013Water Systems
3.2.1.1 Wetting and Drying
It is understood that whilst the capillary tube calculation procedure for water
uptake of cohesionless soils is too simplistic, the view of water uptake by cap-
illary activity in such soils is appropriate. Since the void spaces in granular
cohesionless soils exhibit unequal or nonuniform capillary diameters, deter-
mination of water retention should recognize that water retention values are
conditioned by whether measurements are made on the wetting or drying
process of the soil. Consider the left-hand sketches shown in Figure 3.2. The
top sketch shows the uneven capillary in a drying (desorption) process from
a totally saturated condition, including full saturation of the 2r2 diameter
bulb. If P1 < 2\u3c0r1Tcos\u3b1, water will be retained in the top 2r1 diameter portion
of the tube.
Consider the case of water uptake by the cohesionless soil from an initial
dry state, as shown in the bottom left-hand sketch in Figure 3.2. The water
will reach the top of the smaller diameter (2r1) capillary and will not pro-
ceed further. What is lost in the wetting-drying phenomenon is the amount
of water contained in the 2r2 bulb\u2014for the same P1 pressure condition. The
effect of uneven capillary bores (diameters) is best demonstrated by viewing
Wetting
Drying
Variable size capillary tube Fine silt
Water source Lens composed of
medium sand
Stage 1
Stage 2
Stage 3
Stage 4
Uptake of water by
fine silt due to capillary
forces
Continued uptake of
water by fine silt,
BUT no water uptake in
medium sand lens
P1
T
T
Water uptake equilibrium
consistent with capillarity
of fine silt
2r1
2r2
2r1
P1
FIguRE 3.2
Schematic illustration of hysteric performance in water retention as a result of wetting- drying
phenomenon (left), and uptake of water (wetting) in a bulk fine silt soil with an included
medium sand lens.
86 Environmental Soil Properties and Behaviour
the wetting of a mass of very fine silt shown in the right-hand set of sketches
in Figure 3.2. The example shows an included medium sand lens in the fine
silt mass (Stage 1). We assume that the equivalent average capillary diameter
in the fine silt is very much smaller than the equivalent average capillary
diameter of the sand. Stage 2 shows the beginning of water uptake by the
fine silt, continuing onward to Stage 3. Since the equivalent average diameter
of the sand is considerably larger than that of the fine silt, no water uptake
occurs in the medium sand. This is the same phenomenon demonstrated in
the bottom left-hand sketch of the figure. Note that water uptake continues in
the fine silt, to an extent consistent with equilibrium requirements (Stage 4).
3.2.2 Cohesive Soils
Clays constitute the main constituent of cohesive soils. Because the struc-
ture of clays includes both macro- and micropores (Figure  3.3), water
retention is by both capillary-type phenomena and molecular activity,
some of which have been described previously in Section 2.5 of Chapter
2 in relation to the surface properties of the various clay fractions. This
subject is addressed in the next subsection and in greater detail in the lat-
ter part of this chapter when we discuss soil\u2013water interaction and energy
relationships.
Large microstructural unit composed of
smaller microstructural units
Macropore
Micropores in
microstructural unit
FIguRE 3.3
Scanning electron microscope (SEM) picture showing a large microstructural unit composed
of smaller microstructural units. The width of the black band at the bottom (near the centre)
represents 10µ.
87Soil\u2013Water Systems
For the discussion in this section, we can consider the retention of water in
macropores of clays as equivalent to that demonstrated by sand/fine silt cap-
illaries for the macropores, with the exception of the first two to three layers
of water next to the surfaces of clay particles where intermolecular activity
may be dominant. In the case of micropores, water retention can be consid-
ered to range from retention by micro capillarity phenomena at the one end
of the spectrum to retention by intermolecular activity involving the surface
functional groups of clay particles. Some might consider the latter as being
mechanisms of hydration.
3.2.2.1 Hydration of Clays
The forces holding water molecules to clay particle surfaces arise from both
the water and the clay mineral particles. Water is a dipolar molecule with a
separation of centres of the two hydrogen atoms into a V-shape, with the oxy-
gen atom forming the bottom of the V (Figure 3.4). The angle of the V-shape
that separates the two hydrogen atoms is 104.5°, and the length of the arms
of the V at which end the hydrogen atoms reside are 0.0957 nm. The diameter
of a water molecule is nearly equal to 0.3 nm\u2014using van der Waals\u2019 radius.
Water molecules associate in tetrahedral arrangements with each oxygen
surrounded by four others, held by hydrogen bonding.
Cation
Anion
Edge view of clay mineral particle
Clay mineral particle
Plane of centre of first layer of hydrated
cations (Outer Helmholtz plane)
Plane of centre of chemisorbed
anions (Inner Helmholtz plane)
Water molecule
Oxygen
Hydrogen
104.5°
FIguRE 3.4
Schematic representation water attachment to clay mineral particle surface. Anions are specifi-
cally sorbed on the clay mineral particle surface.
88 Environmental Soil Properties and Behaviour
The hydrogen ions of water give rise to hydrogen bonding of water mol-
ecules to the exposed oxygen atoms of clay mineral particle surfaces. The
particle contributes both the negative charge and the oxygen or hydroxyl
surface to attract molecules. Since cations in water are always hydrated, the
exchangeable cations held near the negatively charged particle surface will
hold some of the water at the surface of the particle as water of hydrated ions,
as shown in Figure 3.4. Note that cations are hydrated to different degrees
and that the hydration of the clay mineral particle surface will depend upon
the cation present.
The main bonding force holding the first layer of water to the surface of a
clay mineral particle is the hydrogen bond. The second (next) layer is held to
the first, again by hydrogen bonding, but with a weaker force because of the
distance of orienting influence of the particle surface on the water molecule.
It follows that each successive layer will be held less strongly. Whilst there
is a lack of total agreement as to the thickness of the hydration layer, there
appears to be a three-layer (i.e., three water layers) consensus. Much depends
on the type of clay mineral, the nature of the cations and anions in the water,
and their distribution. There is no distinct demarcation between the water of
hydration and free water.
3.3 Clay\u2013Water Interactions
3.3.1 Electrified Interface and Interactions
The distribution of ions in the porewater of a clay\u2013water system is a function
of the electrostatic and chemical reactions between the ions and the charged
surfaces of```