Lal R., Shukla M.K. Principles of Soil Physics
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FIGURE 8.2 A schematic showing
relationship between soil moisture
content, soil volume, shrinkage
behavior, and soil consistency.
Properties related to soil-tillage interaction and other dynamic properties of soil during
and after tillage operation are closely associated with the Atterberg limits. Soil dynamics
refers to the relation between forces applied to the soil and the resultant soil reaction (Gill
and van den Berg, 1967). Soil properties that affect soil dynamics include texture, nature
and the amount of clay content, and antecedent soil moisture content. A principal
dynamic property involved in soil-tillage interaction is the shear strength (comprising soil
cohesion and internal friction). Shear strength is the maximum at a soil moisture content
in vicinity of the lower plastic limit.
Friction between soil and metal is another important factor that develops in three
phases: (i) phase 1 represents true friction between metal and dry soil, (ii) phase 2 is
governed by the forces of adhesion between soil and the metal which increase with
Principles of soil physics 224
increase in soil moisture content, and reaches the maximum value near the upper plastic
limit. Detail description of soil dynamics and physics of low action and other tillage
operations is given by Nichols (1929), Nichols et al. (1958), Baver et al. (1972), and
Horn et al. (1994).
Scouring or the self-cleaning flow of soil over the tillage implements and draft power
are also related to Atterberg limits. Non-scouring, the process in which the soil mass is
pushed away, is due to: (i) the minimum angle that implement makes with the direction
of travel, (ii) high cohesion as in dry soil, (iii) high coefficient of soil-soil friction, (iv)
low coefficient of soil-metal friction, and (v) low adhesion of soil to metal when soil is
below the lower plastic limit or above the sticky point.
The draft power is needed to overcome the forces of cohesion, adhesion, resistance to
compression, shear strength, and soil-metal friction. The power needed is usually the
maximum when the soil wetness if just above the lower plastic limit and the draft power
increases logarithmically with the increase in PI. The draft power is the least when the
soil is at friable consistency. Sohne (1956) attributed power requirements to several types
of work done during the tillage operation to: (i) cut, (ii) overcome cohesion and shear
forces involved in compressing, shearing, and turning the soil, (iii) lift and turn the
furrow slice, and (iv) overcome friction between soil and the tool on all sides. The
relative magnitude of these forces in relation to different implements and soil condition
has been evaluated by Gill and van den Berg (1967) and Soane and Van Overkerk (1994).
Mole Drainage
Knowledge of the plastic behavior can be useful in installing mole drains. Mole drainage
channels are stable if established when the soil moisture content at the mole depth is
above the lower plastic limit. However, soil above the mole channel must be at the friable
consistency. Appropriate soil moisture content most suitable for mole drain establishment
may correspond to a specific PI which may vary among soils.
Soil Strength and Compaction
Soil is generally most susceptible to compaction when its moisture content is in the
vicinity of the lower plastic limit. In contrast, soil is most susceptible to puddling when
soil moisture content exceeds the upper plastic limit. Road and foundation engineers can
determine the moisture content corresponding to the maximum Proctor density from the
lower plastic limit. There is generally a good correlation between PI and various
parameter related to soil strength, e.g., cohesion, angle of internal friction, and shear
strength. All soils may have similar strength when soil moisture content is in the vicinity
of the upper plastic limit.
8.4 SOIL VISCOSITY
As soil moisture content increases, its consistency changes from plastic, to sticky, to
viscous. When viscous, soil flows under stress and the flow is proportional to the force
Soil rheology and plasticity 225
applied. When plastic, a certain amount of force must be applied before any flow is
produced. The flow behavior of a soil is explained by the Bingham equation [Eq. (8.5)].
V=kµ(F−F′)
(8.5)
where V is the volume of flow, µ is the coefficient of mobility, F is the force applied, F′ is
the force necessary to overcome the cohesive forces (also called the yield value), or F′ is
zero and the volume of flow is proportional to the force
The constant of
proportionality k in viscous flow is the coefficient of viscosity of the liquid (Fig. 8.3).
8.5 SOIL SHRINKAGE
Atterberg limits also have an important application to soil shrinkage. Atterberg defined
“shrinkage limit” as the soil moisture content below which the soil ceases to shrink, and
represents the lower moisture limit of the semisolid state or soft-friable consistency. The
process of shrinkage is due to the manifestations of the diffused double layer, and due to
the forces of surface tension at the air-water interface. The magnitude of volume change
depends of soil structure, aggregate shape, porosity and pore size distribution, nature, and
amount of clay. Therefore, the shrinkage process is related to the change in total volume
(V
t
) in relation to the change in volume of water (θ) in the soil (Fig. 8.4).
A schematic of the shrinkage process shown in Fig. 8.5 shows two distinct types of
shrinkage. The normal shrinkage (curve segment labelled AB) refers to the process in
which decrease in total soil volume (V
t
) is proportional to the volume of water (θ)
withdraw from the soil. The slope of the normal line is an important indicator of the kind
of shrinkage. If the angle is 45°, the soil displays a normal shrinkage. If the angle is <45°,
the soil displays less than normal shrinkage. The angle of the line of normal
Principles of soil physics 226
FIGURE 8.3 Viscous versus plastic
flow.
shrinkage is an important soil characteristic (Mitchell, 1992) and is influenced by
management (Mitchell and Van Genuchten, 1992; Flowers and Lal, 1999) (Fig. 8.5). The
normal shrinkage continues until the point when there is a strong interaction between
particles, and further shrinkage is caused by compression and orientation of particles
rather than due to decrease in V
t
. This shrinkage is called the residual shrinkage (curve
segment labeled BC). At this point, the air enters the soil. There is a change in slope of
the curve from 1 for the normal shrinkage line AB to less than 1 for the segment BC. The
point B signifies the moisture content at which air enters the soil and corresponds to the
shrinkage limit. In practice, the curve ABC is simplified by drawing ABC, and the
shrinkage limit is then defined as the moisture content corresponding to the point B. The
magnitude of shrinkage beyond this point or the residual shrinkage depends on soil
properties. The amount of air that enters the soil during the residual shrinkage (f
a
) can be
calculated by extending the line AB to point D on the y-axis. The segment DC
corresponds to the air-filled porosity during the residual shrinkage. The residual
shrinkage is usually more in well-structured than poorly
Soil rheology and plasticity 227
FIGURE 8.4 Normal (upper solid
line) and residual (lower solid line)
shrinkage curves for a soil bulk density
of 1.1 Mg m
−3
. (Modified from Flower
and Lal, 1999.)
structured or puddled soils and in heavy-textured than light-textured soils. The shrinkage
curve is strongly influenced by soil bulk density (Flowers and Lal, 1999).
8.5.1 Methods for Determining Soil Shrinkage
There are several methods of determining soil shrinkage (Holtz, 1965; Warkentin, 1993).
The choice of methods depends on the objective. Field assessment of shrinkage involves
measurement of the height of soil surface overtime. Assessment of the volume of cracks
in the field is considered as an indication of the horizontal shrinkage. These are, however,
extremely crude measurements and may be highly subjective. Shrinkage of well-
structured or aggregated soils is measured by taking an undisturbed sample (clod) 3–10
cm across, and its volume change is measured as it is dried slowly under high humidity
environment. Another method used for measuring shrinkage of structured soil involves
determining bulk density of a clod at different moisture contents (Grossman et al., 1968).
Shrinkage of remolded samples
Principles of soil physics 228
FIGURE 8.5 A schematic of the soil
shrinkage characteristic curve (SSCC).
is determined by evaluating change in volume of a saturated paste upon progressive
drying (ASTM, 1989) to develop the soil shrinkage characteristic curve or SSCC (Fig.
8.5). Measured shrinkage is usually the maximum for remolded soil samples.
There are numerous indices of expressing the shrinkage behavior of a soil including
the following (Warkentin, 1993):
1. Volume decrease per unit weight of soil in units of m
3
/Mg [∆V/ M
s
, Eq. (8.6)]
(8.6)
2. Decrease in porosity per unit volume of soil [∆f/V
t
, Eq. (8.7)]
(8.7)
3. Coefficient of linear extensibility (COLE)
Soil rheology and plasticity 229
The COLE is widely used to assess the swell–shrink capacity of soils. This index is also
used in soil classification for pedological purposes. The COLE is defined as follows [Eq.
(8.8)]:
(8–
8)
where Lm is length of moist sample, Ld is length of dry sample,
is wet bulk density
(measured on plastic coated clods at 0.3 or 0.1 bar suction) and ρ
b
is dry bulk density
(Grossman et al., 1968; Warkentin, 1993).
8.5.2 Application of Shrinkage
Soil shrinkage is a rapid process compared with swelling which can continue for several
years under confined environments. In agricultural soils, shrinkage is evidenced by
formation of cracks. Soil cracks are large if the soil is cohesive (e.g., Vertisols) and small
but numerous when soil is wellstructured with little cohesion between aggregates. When
soils develop large cracks, there is a considerable damage to plant roots. Roots in a
severely cracked soil are confined to the small and dense soil mass between the cracks,
thereby decreasing water and nutrient use efficiencies. Roots also affect soil shrinkage
(Mitchell and Van Genuchten, 1992). Soil shrinkage can also be used to estimate soil
profile water content and for scheduling irrigation (Yule, 1984; Mitchell, 1991). In
engineering applications, soil shrinkage jeopardizes safety of buildings, roads, and dams.
There is also interest in soil shrinkage with regards to soil subsidence.
PROBLEMS
1. Describe the agronomic significance of the upper and lower plastic limits, and of
the plasticity index.
2. What is the practical significance of the numerical value of liquidity, index, and
activity ratio?
3. What are the engineering applications of Atterberg’s limits?
4. Distinguish between elasticity and plasticity of soils, and what inherent properties
of soil determine these characteristics.
5. How do change in plasticity characteristics influence engineering properties of
soils?
Principles of soil physics 230
6. While using deionized water, plastic properties of a soil are as follows: upper plastic
limit=80%, lower plastic limit=60%. While using a dilute solution, properties change as
follows: upper plastic limit=70%, lower plastic limit= 40%. Describe the reasons for
change in properties, and practical significance of such behavior.
7. Consider the following data in a shrinkage test:
Volume of saturated wet soil=15 cm
3
Weight of saturated wet soil=25 g
Volume of dry soil=6 cm
3
Weight of dry soil=15 g
Calculate shrinkage limit and particle density of soil.
8. The analysis of two soils produced the following data:
Property Soil A Soil B
Upper plastic limit 25% 10%
Lower plastic limit 10% 5%
Field moisture content 18% 12%
Soil particle density 2.7 Mgm
−3
2.65 Mg m
−3
9. Tabulate soil factor’s that affect Atterberg’s limits, and briefly explain reasons for
these effects.
10. How do soil organic matter and clay contents and clay type influence plastic
properties?
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