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Origin and Function of Soils

1.1 Introduction
We begin by establishing what is a soil. In the most general sense, a soil is
a collection or accumulation of disintegrated rock fragments whose particle
sizes can range from boulders measuring up to a few metres in dimension to
much lesser sizes generally called soil, with particle sizes ranging somewhere
close to 100 mm to sizes that cannot be seen by the naked eyeless than
0.0001 mm (in the micrometre range). Disintegration of rock produces fragments commonly referred to by the public as boulders, stones, gravel, sand,
and claymost often distinguished or characterized by particle (fragment)
sizes, with the largest ones being stones and the smallest ones being clay.
The forces and agents responsible for rock disintegration include mechanical, chemical, biologically mediated, and hydraulic.
Practical experience and reports in the popular literature have often
shown differences in ones perception of what constitutes a soil, the reasons
for which most generally lie in ones use and understanding of the soil material. Strictly speaking, soil materials could also include decomposed organic
matter and other constituents such as evaporates. We use the term soil material in recognition of the fact that soil is a material consisting of various soil
fractions identified as sand, silt, clay, organic matter, carbonates, oxides, and
so forth. The term soil is now commonly used in place of soil material to mean
the same thing. The nature and characteristics of the soil fractions constituting a soil will be discussed in greater detail in the next chapter.
1.1.1 Why Are We Concerned with Environmental Issues?
The very same forces and agents, which we will call environmental forces/
agents, responsible for rock disintegration will persist after rock fragmentation. They will continue to act on the fragmented material (i.e., soil material),
and in many instances, they will act in combination with forces resulting from actions associated with anthropogenic activities. One can expect
that the nature of the various soil fractions constituting a soil material will
change over time.

Environmental Soil Properties and Behaviour

Changes in the nature of soil will be reflected in associated changes in the

properties and characteristics of the affected soil. Because of this, it becomes
important to recognize that if, for example, one designs and constructs an
engineering facility that relies on various predetermined soil properties such
as soil strength and hydraulic conductivity for its long-term survivability,
any deterioration or degradation of these properties would threaten its survival. This means that unless changes in the measured soil properties used
for design of an engineering facility are anticipated and factored into design
considerations, the survival of the constructed facility could be jeopardized
if negative changes in these properties occur. A very pertinent example of
this is the engineered clay barrier used in containment of waste products.
For secure long-term containment, one relies on the hydraulic transmission
and chemical buffering properties to maintain their design capabilities. It
goes without saying that deterioration of any of these properties will allow
transport contaminants into the surrounding regions, thereby impacting the
health of the various vegetative species and biotic receptors.
One needs therefore to (a) focus attention on the kinds of changes that will
occur in soil due to the impact of environmental forces/agents and actions
associated with anthropogenic activities, (b) provide an appreciation of what
the environmental agents and anthropogenic activities are, and what kinds
of forces/stresses are associated with these agents and activities, (c) discuss
the impacts and various processes involved with these forces/stresses, and
(d) establish the likely changes in the various soil properties and show how
these impact on performance of the affected soils.

1.2 Soil Origin and Formation

There are several factors that control or influence the processes that are
involved in producing the type of soil from parent rocks. Most obviously, the
type of rock (composition and texture) leads the group of factors in terms of
importance. This is closely followed by site conditions such as availability
of water, climate, topography, and so forth. These factors have considerable
influence on the kind of processes involved in breaking down rock to its various fragments, and also in the production of soil types from the broken rock
1.2.1 Parent Material
Rocks that when fragmented will eventually form soils are called parent
material. They can also be referred to source rocks. These kinds of rocks fall
into three general classes: igneous, sedimentary, and metamorphic. Igneous
rocks are the product of magma. When they are the result of extruded

Origin and Function of Soils

lava, they are known as extrusive rocks. When the rocks are the result of
unextruded magma, they are known as intrusive or plutonic rocks depending on the depth of the magma. The rate of cooling of the magma has a great
influence on the size of mineral grains formed in the rocks. These minerals
are important participants in the development of soil properties and characteristics. With higher rates of cooling in the extrusive rocks, one would
expect the mineral grains to be fine, that is, very small. Slower rates of cooling will produce larger mineral grains. Since plutonic rocks are found at a
greater depth than intrusive rocks, the mineral grains of the plutonic rocks
will be larger than those found in intrusive rocks. Of the various kinds of
igneous rocks such as andesite, felsites, durite, gabbro, basalt, and others
basalts and granites are by far the greatest proportion of rocks that classify
as igneous.
Sedimentary rocks are formed from the wind and water deposition of the
material derived from chemical and physical breakdown of other source
rocks, compressed under considerable geologic pressure and heat. The
commonly found sedimentary rocks include sandstone, siltstone, dolomite,
shale, and limestone. Metamorphic rocks are formed from metamorphosing of igneous or sedimentary rocks. Since considerable heat and pressure
are required in the process of metamorphic transformation of the igneous
and sedimentary rocks, there will be transformation of the original minerals found in the rocks. The common metamorphic rocks include quartzite,
schist, slate, gneiss, and marble.
1.2.2 Weathering of Rock Natural Processes
The natural processes involved in the transformation of source rocks are
generally referred to as weathering processes. General usage will refer to this
as weathering, with or without inclusion of the term processes. In the context
of soil formation from rocks, weathering is a spontaneous reaction involving
geologic material (rocks) and energy. Since it is a change in the direction of a
decrease in free energy of the system, it is possible to predict the thermodynamic susceptibility of minerals to weathering. The assemblages of minerals
in soils are reflective of the temporary equilibrium stages in the dynamic
process of changes from source rocks to the final end product. The weathering sequence in development of clay minerals from source rocks may be
represented in terms of a reaction series portraying soils as an equilibrium
stage in a continuous progression from, for example, igneous rocks such as
granite and basalt to a final end product such as laterite.
The intensity of weathering is highest where interfaces between the atmosphere, hydrosphere, biosphere, and lithosphere meet and overlap. A good
example of this can be found in the upper soil zone or layer in temperate
humid climate regions, and deeper soil zones in humid tropic regions.

Environmental Soil Properties and Behaviour

Transformation of rock fragments to soil occurs primarily in the regolith: the

region between solid rock and the soil.
The rate of weathering is influenced or controlled by several factors, not the
least of which are the composition and size of the source rock (rock mass) or
rock fragments, size and type of exposed surface areas of rocks, relative solubilities of rock mass, fragments and weathered material, permeability of the
rock, source and availability of water (e.g., position of water table or hydrogeological setting), chemical composition of water, temperature, topography,
oxygen content or availability, organic matter, cyclical freezing and thawing
and wetting-drying cycles, climatic envelope, and others.
The primary types of actions responsible for the forces involved in fragmenting source rocks are physical and chemical in nature. The agents
responsible for these soil-forming forces can be physical, chemical, or biological in nature. Strictly speaking, the actions of biological agents are actually
biologically mediated chemical actions. In that sense, we see references in
the literature to only two types of soil-forming actions or forces: physical and
chemical. Under natural circumstances, that is, without any actions attributable directly or indirectly to anthropogenic input, these types of actions
are often referred to as chemical weathering and physical weathering. In short,
physical weathering and chemical weathering of rock are two of the natural
processes of disintegration of rock that ultimately result in the production of
soil. Figure 1.1 gives a summary illustration of the major weathering processes involved. Physical Weathering
Physical weathering of rocks involves forces and processes that result in
fragmenting of the rocks. Particles of much lesser sizes are obtained as a
result of the breakdown of the various rock fragments or minerals. The
agents producing these forces and processes could be (a) thermal in nature
such as cyclic freezing and thawing, and heating and cooling, since rock
components have different coefficients of expansion; (b) expansion of water
in rock fissures as a result of freezing; (c) physical movement of glaciers atop
the rocks, resulting in application of abrasive forces, and water on/against
rocks and in rock fissures; and (d) physical pressures and loads from superposed glaciers and displacements due to plant growth and activities of animals. Note that in physical weathering, there is generally no alteration of the
chemical composition of the rock material. Chemical Weathering
Chemical weathering of rocks involves processes leading to transformation of the constituents and even loss of some constituents, addition of
protons, and rearrangement of constituents into new materials. It should

Origin and Function of Soils

Parent material
(unaltered source rock)

Weathering envelope
climate, topography,
time, vegetation, etc.

Physical Weathering
Thermalcyclical freeze-thaw,
Pressures and stressesoverburden
loading and load release.
Hydraulicfluid pressures & freezing of
water in fissures.
Abrasionabrasive stresses from movement
of overlying glaciers.

Chemical Weathering

Residual soils (remaining in-place)


Transported soils


Soil formation beginning with weathering of unaltered source-rock mass (parent material)
and ending with final resting placethat is, residual or transported.

be noted that chemical weathering will not occur without the presence of
water; that is, water is an essential component in chemical weathering. The
rates of chemical reactions in the weathering process are determined by
temperature, moisture, composition, potential for leaching, and composition
of the leaching solution. Chemical processes involved include hydrolysis,
hydration, carbonation, oxidation, reduction, and solution. Protons in the
leaching solution can be supplied from CO2 in the air or from decaying vegetation or other acids added to the system.
Both the amount of water available and heat are big factors in the chemical
weathering of the parent rock mass. Warm and humid climates are therefore
instrumental in producing correspondingly high weathering of the minerals
in the parent rock mass. The presence and amount of organic matter are also
very important factors in the control and rate of weathering. If organic matter is allowed to remain, as for example in cool environments, high humidity would produce conditions that would successfully promote reactions
between the organic acids and parent rock. However, if the environment is
changed to a hot, humid environment, oxidation of the organic matter would
preclude the formation of organic acids, resulting in little or no reactions
between the organic matter and the parent rock.

Environmental Soil Properties and Behaviour

1.2.3 Processes of Chemical Weathering

The major processes involved in formation of soil by chemical weathering
agents include
r Hydrolysis: Hydrolysis is the reaction between the H+ and OH ions of
water and the elements or ions of a mineral in the source rock or other
type of parent material. It is considered to be an important reaction
for silicates. These are the primary rock-forming minerals. For hydrolysis reactions to cause weathering of rocks, it is necessary for the
weathering products to be removed continuously. This means that
fresh application of water is necessary throughout the weathering
process. A generalized reaction of a silicate is given by Keller (1957) as
MSiAlOn + H+OH M+OH + [Si(OH)0-4]n + Al(OH)3
+ (M,H)AloSiAltOn
where the superscripts o and t refer to the octahedral and tetrahedral
coordinations, respectively (as will be explained later in Chapter 2),
and M and subscript n refer to metal cations and unspecified atomic
ratios, respectively.
r Hydration: Hydration involves the chemical uptake of water. The
minerals in the rocks form hydrous compounds with water uptake,
resulting thereby in an increase in volume. The minerals involved
with water uptake include the silicates, oxides of iron and aluminium, and sulphates. Because of the volume expansion of the rocks in
water uptake, fragmentation of the rocks will occur. Whilst dehydration may occur in arid regions, it is not generally recognized as an
important process in weathering.
r Oxidation: Oxidation occurs when an atom loses an electron e. When
elemental oxygen combines with the weathering substance, elements
are lost to the oxygen, which then becomes ionic. Technically speaking, oxygen need not necessarily be involved. For example, Fe may
oxidize to FeS, FeS2, FeCl3. Oxidation of minerals by gaseous oxygen
occurs by the intermediary action of water. Depending on the partial pressure of gaseous oxygen and acidity of the water, gaseous
oxygen dissolved in water renders the resulting solution an oxidizing potential. Most rocks contain some iron in the form of sulphides
or oxides. With this in mind, it is seen that interaction with this type
of water promotes the process of oxidation, giving rise to weathering
by oxidation of those rocks.
r Carbonation: Because rainfall passing through the atmosphere collects carbon dioxide (CO2), rainwater will contain some amount of
CO2, resulting in the production of carbonic acid:

Origin and Function of Soils

H2O + CO2 H2CO3 H+ + CO3

Carbonation, which is one of the dominant reactions, is the union of
carbonic acid (H2CO3) with bases, the result of which is the formation
of carbonates. This rainwater will attack minerals such as feldspar,
hornblende, and olivine under normal temperatures and pressures.
Carbonation in silicates is accompanied by the liberation of silica.
This silica may remain as quartz or may be removed as colloidal
silica. It is useful to note that present public concern regarding excessive discharge of CO2 due to anthropogenic activities have often not
given this aspect of acidification of rainwater as much attention as it
r Solution: The same carbonic acid responsible for carbonation will
act as a solvent, attacking carbonate-type rocks such as limestones.
1.2.4 Parent Material and Other Influences Parent Material
The composition and texture of the parent rock are important factors in the
initial stages of weathering. Technically speaking, weathering processes are
omnipresent in nature; that is, they are present all the time. The influence of
parent material is relatively short-lived in regions with humid conditions and
high temperatures. In contrast, in arid and arctic regions where the opposite
conditions exist, the influence of parent material remains indefinitely.
Alkali and alkaline cation content of parent rocks are important factors
in determining parent rock weathering products. Rocks containing no
alkalis can only produce kaolinite or lateritic weathering products. Rocks
containing alkalis and alkaline earth cations in addition to alumina, silica,
and so forth, such as igneous rocks, shales, slates, schists, and argillaceous
carbonates, will produce a variety of weathering products. Under different
conditions such as climate, topography, and time, the same parent rock can
produce both kaolinite and montmorillonite. Also, parent rocks with widely
different compositions and texture can produce the same type of soil with a
characteristic clay mineral composition. Climate and Vegetation
Climate and vegetation often go hand in hand since climate affects vegetation and the products produced by decay of the vegetative (organic)
material. The amount and kind of products resulting from the decay of
vegetation are significant factors of weathering. Where vegetation is absent
or rapidly oxidized, production of organic acids is minimal or absent,
meaning that we will not have organic acids from this source as agents for
weathering of parent material. Whilst hot and humid climates encourage

Environmental Soil Properties and Behaviour

oxidation destruction of vegetative organic material, cool humid climates

allow for active organic acids from decaying organic material to react
with the parent material, thereby contributing to the production of decay
The climatic factors of significant importance are temperature and rainfall. Warm and humid climates are favourable conditions for rapid weathering of the minerals in the source rocks. Continuously or persistent wet
climates allow for percolation to remove decay products, thus allowing for
weathering to proceed. At the other end of the spectrum, dry climates do
not allow for removal of the decay products partly because of the much
smaller amounts of decay material but mainly because of the absence of
rainfall. Topography and Time
We link topography and time together because of the importance of time
of exposure of the parent material or weathering products to weathering
agents. Percolation and infiltration of water through weathering products,
in the natural environment context (i.e., without human intervention), are
determined by topography. This influences the rate of erosion at the surface
and hence the rate at which fresh parent material is brought close to the
surface. In regions of high rainfall, low flat areas that are completely water
saturated will produce conditions that will retard weathering.
The time factor is important also when weathering is moderately severe
and the composition of the parent material permits production of various
alteration products. Longer time periods allow for leaching and other weathering processes to proceed.

1.3 Soil Classification

The purpose of soil classification is to provide a means to convey information of the type of soil under consideration. Soil classification schemes may
be simple descriptive schemes or may be detailed schemes that include
grouping soils into various types or groups depending on certain features
and characteristics. Whether or not the information on soil type would also
convey some idea as to the general characteristics and properties of the soil
depends on the protocols adopted in classifying the soil in question. There
are several ways in which soils are classified, depending on the intended use
of the soil. For example, geological classification of soil, which is mostly in
terms of surficial deposits, is genetic but partly descriptive. This differs from
those used in soil and geotechnical engineering, and from those in agriculture and soil science.

Origin and Function of Soils

1.3.1 Subdivision of Surficial Deposits

The major subdivisions in the geologic classification of surficial deposits include
r Residual soils: Soils formed from weathering of source rock remain
in place or are transported to other locations due to the actions of
transport agents. Those soils that remain in place are called residual soils. Whether or not they retain most of the characteristics of
the constituents in the source rock will depend on several factors
related to further weathering processes of these soils. Those soils
that retain the characteristics of the parent material are called saprolitic soils.
r Transported soils: The major transport agents responsible for transporting soils from one location to another include glaciers, wind,
water, and humans. Discounting human activities, the soils formed
from the actions of these transport agents include
Glacial soilsFrom transport/deposition of rock grinding and erosion products. Soil formations following retreat of the glaciers
are in the form of drifts, moraines, till, drumlins, and so forth.
Lacustrian depositsThese soils are former lake sediments. The
region where soil deposits are of considerable interest is the river
discharge zone, that is, the zone where the river feeding the lake
enters the lake. In a sense, this is a minor version of formation of
deltas, which are characteristic of alluvial deposits.
Fluvial depositsThese soils are the result of deposition of soil material transported by streams and rivers. It is not uncommon for the
modern literature to include fluvial deposits under the umbrella
of alluvial deposit.
Alluvial depositsThese refer to alluvial fans and deltas and other
water-borne deposits not associated with fluvial deposits.
Marine soilsThey are the result of sediments formed from deposition of suspended material in a marine environment, that is, sediment/soils obtained upon retreat of the ocean from a particular
region. Typical kinds of soils are marine clays and silts.
Aeolian depositsFormed from wind transport. Soil materials carried by wind power range from fine particulates to sands, and
soil formations are usually found as dunes, beaches, loess, and
1.3.2 Soil Horizons
The concept of soil horizons, part of the consideration of soil genesis, is one
that is frequently used by pedologists and soil scientists working in agriculture. We define a soil profile as a succession of soil strata generally called


Environmental Soil Properties and Behaviour

soil horizons. These horizons are basically soil layers or strata constituting a
mature soil profile, and are distinguished on the basis of distinct differences
in texture and physical or chemical properties between neighbouring layers.
In many instances, there will also be colour differences between these layers. These horizons are generally labelled O, A, B, and Cstarting from the
top O horizon and continuing downward to the bottom C horizon. In some
disciplines, for example, soil science, there is also a R horizon that is also
called a D horizon. Figure1.2 shows the horizons and the main distinguishing features. We should emphasize that these horizons are not necessarily
clearly separated; that is, the boundaries separating A from B and B from
C are not well defined. In fact, it would be unusual to find clear separations
between these horizons. The more usual situation would be transition zones
between these horizons.
The degree or level of maturity of a soil profile plays an important role
in how well the three horizons are discerned. This is because leaching is
a major weathering agent in producing the different properties and characteristics that distinguish between the A and B horizons. In the A horizon, one finds soil material that has been altered by natural processes such
as leaching and removal of leached products and other loose material by
water or wind. This horizon is commonly called the leaching zone. The presence of concentrations of greenhouse gases in the atmosphere together with
Top layer of a horizon, which is
composed of decaying vegetation,
is called the O or Ao horizon.
This zone of leaching is typically
populated by loamy soils.

A Horizon


Leaching and precipitated products from

A horizon are accumulated in this zone.
Clays constitute the typical kinds of soils
in this zone, with iron minerals being
responsible for the red-brown
colour of the clays.
In soil engineering,this is the typical
subsoil zone.

B Horizon

Incomplete weathering of the

source rocks (parent material)
in this zone. Typically, there
is a scarcity of organic
material in this zone.

C Horizon


Unaltered source rock

(parent material)

D Horizon


Zone of

Soil horizons typical of a mature soil. Immature soils (soils that have not had much exposure
to weathering and leaching phenomena) will not show distinct layering for zone classification.

Origin and Function of Soils


decaying vegetation on the ground surface will contribute to the acidity of

rainwater and snow, the primary leaching agents for this horizon. Note that
when appropriate, the organic top surface of the A horizon is referred to as
the O horizon, to identify or take into account the thin layer of organic material consisting of decaying vegetation, plant litter, and humus.
The B horizon is characterized by leaching products and other washed material deposited from the A horizon. The common name for this horizon is the
zone of accumulation. Accordingly, one would expect to find various kinds of soil
(clay) minerals, oxides, and organic matter in this stratum. This is probably the
most common soil zone encountered in many soil engineering projects, and is
generally referred to as the subsoil. Below this is the C horizon, normally considered to contain soil that is unaltered by weathering subsequent to deposition
or formation. The top layer of the C horizon will contain partially weathered
material, and the interface layer separating this horizon from unaltered solid
rock is called the regolith. This is also sometimes referred to as the R or D layer.
Because of the slow reaction rates associated with weathering processes,
development of mature soil profiles and soil horizons takes hundreds to
thousands of years. As stated previously, the important factors involved
in this time (development) period are source material, climate, vegetation,
topography, and time. For example, warm and wet regions allow for more
rapid chemical weathering of parent material, especially if the leaching
agents contain organic acids. One could obtain a lesser time requirement
for development of a mature soil profile with significant proportions of aluminium and iron in the zone of accumulation. Weathering of dark-coloured
coarse-grained and fine-grained mafic rocks containing large amounts
of iron and magnesium will produce soil profiles that will contain darkcoloured soils that would have high clay content. On the other hand, weathering of felsic rocks such as granites containing high proportions of iron
and silica will produce soils that contain light coloured clays with significant
proportions of quartz sands. The evidence shows that an understanding of
how soil profiles are developed would be useful in the construction of classification schemes for soils.
1.3.3 Classification for Engineering Purposes
It is understood that whereas a full detailing of soil properties and behavioural
patterns determined from measurements obtained from a comprehensive
suite of laboratory tests might provide for an ideal soil classification scheme,
this is neither feasible nor practical. This is because the engineering use of
soils includes a large variety of applications and situations demanding specific knowledge of different types of soil properties and behavioural characteristics. The purpose of soil classification is to provide a general description
of the soil in a terminology for classification that is understood by all concerned parties. To that end, it is apparent that soil classification schemes
should be universal: that is, a large majority of stakeholders would be able


Environmental Soil Properties and Behaviour

to utilize the schemes in toto or as a starting point. With experience gained

from associated laboratory tests, one could associate the identified soil with
certain characteristic properties or behavioural patterns. Particle-Size Differentiation and Textural Classification
The two most common types of soil classification for engineering purposes
are particle-size differentiation and textural. Particle-size or grain-size differentiation of soils classifies or identifies soils on the basis of ranges of limiting
sizes. In the left-hand bar chart shown in Figure1.3, the distinction between
gravel, sand, silt, and clay (called separates) is made on the basis of a limiting
maximum or minimum size. Thus, for example, the limiting maximum size
for clay, first proposed by Atterberg (1908, 1911), is 2 m; or one could say that
the limiting minimum size for silts is 2 m. Sands and silts are also differentiated in terms of fine, medium, and coarse sizes. Fine sands are greater than
0.06 mm and less than 0.2 mm in particle size. Sand grains with sizes greater
than 0.2 mm and less than 0.6 mm are classed as medium sands, and coarse
sands are particles with sizes ranging from 0.6 to 2 mm. Particles greater
than 2 mm are called gravels. We should point out that whilst the limiting


>> 2 mm


0.002 mm, (2m)








Sandy clay



Clay loam


Silt loam
Sandy loam
Percent sand


















2 mm

Increasing particle size, mm





<< 0.002 mm, (2m)

The left-hand bar chart shows the grain-size (particle-size) differentiation for classification of
soils. The right-hand chart shows the textural classification of soils based on proportions of
sand, silt, and clay in a soil.


Origin and Function of Soils

size boundaries for these separates are considered by some as arbitrary, they
have nevertheless been accepted in common practice.
The triangular chart shown in the right-hand side of Figure1.3 is a textural
chart that identifies soil type according to the proportions of sand, silt, and
clay in the soil. As with the particle-size classification scheme shown on the
left-hand side, textural charts classify soils based on particle sizes, meaning
that these kinds of charts and classification schemes are more applicable for
granular soils (gravels, sands, and silts). It is important to understand that
the clay classification used in both the particle-size classification scheme and
the textural chart refers to clay-sized particles. Some confusion may arise if
one does not distinguish this from clay minerals as a particular type of soil.
We will be discussing this in greater detail in the next chapter (Chapter 2). Particle-Size Distribution Curves
A particle-size distribution curve is useful as an indicator of the kind of soil
under consideration. This does not require an arbitrary division of particles
into separate sizes (separates). Figure1.4 shows some typical particle-size distribution curves in a semilog plot. The ordinate shows the weight, in terms of
percentages, of particles finer than a particular size, and the abscissa shows
the effective particle diameter.






Percent Finer, by Weight







Eective Particle Diameter, mm



Particle-size distribution curves. Curve A represents a clay. Curve B is a clayey silt, and Curve
C is a silty sand. Note that identification of soil types is based solely on proportions of separates
in the soil sample and not on any particular behaviour characteristic.


Environmental Soil Properties and Behaviour

To obtain a particle-size distribution curve, it is important to obtain a representative soil sample that is completely dispersed; that is, all the particles
must be able to act individually and separately. Dispersion of a soil sample
requires removal of cementing or bonding materials responsible for aggregating the individual particles, that is, deflocculating the soil sample. Typical
cementing materials in surficial soil samples include carbonates, iron and
aluminium oxides, and organic matter. Note that after removal of cementing
material, the soil sample must be deflocculated or peptized. This is especially
critical for clays since the forces of attraction that are always present between
particles (see Chapter 2) will cause flocculation of the clay particles. Since
the forces of attraction cannot be easily decreased, they need to be overcome
by increasing the forces of repulsion between the clay particles (Chapter 2),
generally accomplished by having a monovalent exchangeable ion on the
clay and a low salt concentration in the solution. A common procedure is to
add sodium ions (sodium metaphosphate) and washing the soil suspension
free of soluble salt. The advantage of using sodium metaphosphate as the
source of sodium ions is that the metaphosphate will form complexes with
any remaining calcium or magnesium ions. Atterberg Limits Classification
The initial work in agricultural science undertaken by Atterberg (1911) on
classification of soils, designed to provide an assessment of the state of a soil
in relation to its water content, serves as the basis for classification of soils in
respect to its state of consistency. The consistency limits devised by Atterberg,
shown in Figure1.5, provide an indication of the plasticity or plastic behaviour of soils (mainly clays) in relation to this water content. These limits are
most often referred to as the Atterberg limits, and the tests are part of the
group of index tests that include determination of particle-size distribution.
The shrinkage limit (SL) shown in Figure 1.5 represents the water content
boundary between a solid soil and a semisolid soil. Using the measured volume of the test sample as a guide, the shrinkage limit is the water content
at which a further reduction in water content will not result in any observable decrease in volume of the test soil sample. In terms of a water contentvolume change definition, the shrinkage limit is the only limit that can be
rigorously defined. Using the relationship shown in Figure1.5, the SL can be
defined as the point where volume change is no longer linearly proportional
to the change in water content. The SL is sometimes referred to as the lower
plastic limit. The total shrinkage limit shown in Figure1.5 is the point where
the volume of the test soil sample is defined by its oven-dry volume.
The plastic limit (PL) is the water content at which any increase in water
content beyond this limit would result in a plastic state for the test soil. A
semisolid state of the test soil exists at water contents between the SL and PL.
Increasing the water content of the test soil above the PL would produce a more
plastic soil, to the point where the soil would exhibit liquid-like behaviour;


Soil Volume in Proportion to

Oven-dry Volume

Origin and Function of Soils



limit (SL)
Total shrinkage



limit (PL)
Water Content, Percent

limit (LL)

Pictorial representation of the Atterberg consistency limits. The ordinate represents the soil
sample volume at any particular water content as a proportion of the oven-dry volume of the
test sample.

that is, a liquid-like state of the soil would be obtained. At this point, the
water content defines the liquid limit (LL) of the soil. The range of water contents between the plastic limit (PL) and the liquid limit (LL) is defined as the
plasticity index (PI). Laboratory test procedures have been devised and standardized for determination of the various consistency limits. Unified Soil Classification System
The Unified Soil Classification System (Waterways Experiment Station, 1953)
is a soil classification scheme that uses field identification procedures and
laboratory classification criteria to organize the soils under consideration
into specific groups. Particle-size distribution information allows for initial
grouping of soils into two major groups: coarse-grained and fine-grained
soilsand a third group (highly organic soils) that is not differentiated by
particle-size distribution. These highly organic soils are readily identified by
colour, odour, spongy feel, and their fibrous nature.
For the two major groups, coarse-grained soils contain more than 50% of
soil particles greater than the number 200 sieve size, and fine-grained soils
contain more than 50% of particles passing the number 200 sieve size. The
following notation is used for the soils in the various groups: gravels (G),
sands (S), fine sands and inorganic silts (M), inorganic clays (C), organic silts,


Environmental Soil Properties and Behaviour

and organic clays (O). Since soils, as a rule, contain mixtures of differentsized particles, further refinement of the two broad groups into subgroups
can be made with (a) information on proportions of fines in the coarsegrained group, using secondary notations of W for well graded and P for
poorly graded and (b) the addition of Atterberg limits information for the
fine-grained soils.
Laboratory classification criteria in the classification system use the coefficient of uniformity Cu (also called the Hazen coefficient) as an aid in grouping
the coarse-grained soils into the W and P subgroups, where Cu = D60/D10, and
D60 and D10 refer to grain sizes for which 60% is finer than, and for which 10%
of the soil is finer than, respectively. For the other coarse-grained soils subgroups, the Atterberg limits are used in conjunction with a plasticity chart
with the Casagrande A line (Figure1.6). For fine-grained soils, the identification procedures focus on the soil particle sizes less than the number 40 sieve
size using three particular indicators: (a) dry strength, indicative of crushing
characteristics; (b) dilatancy, indicative of reaction to shaking; and (c) toughness, indicative of the consistency of the sample near the plastic limit. The
types of soils classified under the Unified Soil Classification System are shown
in the figure in relation to their plastic and liquid limits. Detailed descriptions of the various types of soils and the Casagrande A line can be found
in Waterways Experiment Station (1953) and Casagrande (1937), respectively.






MH and OH





Plastic Limit, Percent




Liquid Limit, Percent



Plasticity chart used in conjunction with laboratory classification criteria in the Unified Soil
Classification System. Detailed descriptions of the System and the Casagrande A line can be
found in Waterways Experiment Station (1953) and Casagrande (1937), respectively.

Origin and Function of Soils


For coarse-grained soils, the following subgroups are obtained:


GW for well-graded gravels, gravelsand mixtures, little or no fines

GP for poorly graded gravels, gravelsand mixtures, little or no fines
GM for silty gravels, gravelsandsilt mixtures
GC for clayey gravels, gravelsandclay mixtures

The sands in the coarse-grained soils group are classified in the same manner:

SW for well-graded sands, gravelly sands, little or no fines

SP for poorly graded sands, gravelly sands, little or no fines
SM for silty sands or mixtures of sands and silts
SC for clayey sands or mixtures of sand and clay

The fine-grained soils (M and C) in this classification scheme are soils where
more than 50% of the soil particles pass through the number 200 sieve size.
For these soils, subgrouping is made on the basis of the water content at the
liquid limit. For fine-grained soils with liquid limits less than 50%, the following subgroups are obtained:
r ML for inorganic silts and very fine sands, rock flour, silty or clayey
fine sands, or clayey silts with slight plasticity
r CL for inorganic clays of low to medium plasticity, gravelly clays,
sandy clays, silty clays, lean clays
r OL for organic silts and organic silty clays of low plasticity
For fine-grained soils with liquid limits greater than 50%, the following subgroups are obtained:
r MH for inorganic silts, micaceous or diatomaceous fine sandy or
silty soils, elastic silts
r CH for inorganic clays of high plasticity, fat clays
r OH for organic clays of medium to high plasticity, organic silts

1.4 Basic Soil Functions

We define basic soil functions to mean the basic purposes or roles of soil in
the ecosphere. In essence, a functioning soil is a soil that fulfils its natural
or planned purpose, and soil functionality defines the capability of a soil
to function in a role according to its nature, circumstance, and ecosystem


Environmental Soil Properties and Behaviour

constraints. For the purposes of the discussion on basic soil functions, we

will consider soil in the context of geoenvironment as defined by the schematic
shown in Figure1.7. The geoenvironment, which is a specific compartment
of the environment, includes a significant portion of the geosphere and portions of both the hydrosphere and biosphere. The earth surface material component from the geosphere shown in the figure is defined as soil and other natural
land surface materials (organics, debris, etc.) from the geosphere: that is, soils
in the A, B, and C horizons and the overlying surface material. From the
hydrosphere and the biosphere, we include the receiving waters in the land
environment and the living or life zone in the earth. Considering only the
soil aspects, all of these contribute to the soil ecosystem.
There are a variety of roles for soil in the geoenvironment. For convenience,
these can be grouped into three main categories:
r Natural in situ role of soilThe role of soil is in its natural state in all
the settings (situations) found in the geoenvironment. This includes
residual and naturally transported soils. Soils transported in support of anthropogenic activities are not included in this group.
r In situ role of soil with external interventionThe function of soil as
a result of human intervention.
Includes solid
continental mantle and
continental and oceanic
crust and extending

Earth surface material

Soil and organic surface
material overlying source
rocks (D horizon);
Soils in A, B, and C



Includes all the water

on the earths surface

Life zone of the earth;

includes all living

All receiving waters on land;

includes lakes, rivers, ponds,
wetlands, estuaries,
groundwater, aquifers

Habitats and
other life zones
in the earth
surface material

The various constituents of the ecosphere and their relationship to the geoenvironment. Note
that the definition of earth surface material in the left-hand column of the figure applies to all
references to the term earth surface material.


Origin and Function of Soils

r Soil as a resource materialThis includes soil used by itself as a

resource material, for metal recovery, and for minerals extracted
from the soil for use as ingredients for various uses.
In turn, these main groups will spawn numerous subgroups and sub-subgroups. One should expect that there will be overlap between main and subgroups because of the variety of situations and requirements for soil functions.
A number of these will be described in the following summary discussion.
1.4.1 Natural in Situ Role
As we have seen in Figure1.7, the geoenvironment is a natural resource base
that features soil as its primary component, and the natural resources in the
geoenvironment can be considered as the natural capital of the geoenvironment. Figure1.8 shows some of the principal ecosystems within the geoenvironment together with some of their major derivatives. The term ecosystems
refers to a system where the various individual elements and organisms
interact singly or collectively to the advantage or disadvantage of the whole.
We consider the natural in situ role of soil to be one where the soil, which is
in its natural state in the geoenvironment, functions without human or any

Geoenvironment Ecosystems

Agro ecosystem

Forest ecosystem

Habitat for fauna, biota, and

other microbial species

Soil ecosystem

Groundwater and aquifers

Soil as a resource material

Fuel and energy
Urban ecosystem

Building materials
Metals, minerals, etc.

Physiographic ecosystems

Aquatic ecosystems
The soil ecosystem as part of some of the major ecosystems, resources, and features of the
geoenvironment. Note that the physiographic ecosystems include, for example, coastal, alpine,
desert, and arctic ecosystems.


Environmental Soil Properties and Behaviour

other external intervention. In this main group, two main subgroups and
perhaps some other lesser subgroups stand out prominently:
r Habitat for fauna, other biota, and microbial species. Loss of biodiversity is perhaps one of the best-known concerns in seeking active
means for habitat preservation.
r Growing medium for plants, trees, and flora. Agricultural activities
that involve the use of soil amendments or control of insecticides
and pesticides are not directly included in this main group. These
activities are grouped under the next major group: in situ role of soil
with external intervention.
r Physical and chemical buffering system. Particular examples of these
are the filtration capability, especially in transmission of groundwater to receiving waters, and contaminant partitioning or accumulative capacity of soils in respect to transport of contaminants.
r Groundwater storage and quality determination. Surface hydrology,
hydrogeology, and the roles of aquifers and aquitards are particularly significant.
r CO2 sink. Soil, organic matter, plants, trees, and others are important
players in the carbon cycle.
r Heat sink and source for climate and air temperature control.
These groups and subgroups feature prominently in establishing ecosystem functioning. By this, we mean the activities of flora, fauna, plants, biota,
microorganisms, animals, and so forth within the community defined by
the ecosystem. In the broader sense, we can define a functioning ecosystem to
include biological, chemical, and physicochemical activities that are characteristic of the type of ecosystem under consideration. The living organisms
in a specific ecosystem represent the biodiversity in the ecosystem and are
important in maintaining atmospheric composition and production of oxygen by photosynthesis and fixation of CO2 and recycling of carbon, nitrogen,
phosphorous, and other elements in the soil.
1.4.2 In Situ Role with Human Intervention
The subgroups describing the basic soil functions within this main group
(and the next main group in Section 1.4.3) are by and large associated with
the major activities and industries involved in the production of food, shelter,
and clothing. These include
r Growing mediumThe major activities and industries involved
are agroindustries and other activities associated with production
of raw material such as cotton and timber. What distinguishes
this from the previous natural in situ role is the use of soil

Origin and Function of Soils






amendments and other technological aids designed to increase

food and material production. In situ soilwater relationships,
physical and mechanical properties, and fluid transmissivity, heat
conductance, and transmission are some of the more important
Foundation support and other ground engineering-associated systemsMajor activities and products include foundation support for
structures and infrastructure, tunnels and buried structures, buried
pipelines, and transmission lines. The major sets of concerns revolve
around the physical, fluid transmissivity, and physicochemical and
mechanical properties of the in-place soil.
Containment systemsWe include waste landfills, mine tailings ponds, and repositories in the B, C, or D horizons in this subgroup. The major functional requirements revolve around natural
soil buffering potential capacity, contaminant accumulation, and
hydraulic permeability. The processes involved in situ are similar to
those described in wastewater natural treatment systems (the next
Wastewater natural treatment systemsSlow-rate treatment, rapid
infiltration, and natural wetlands are some of the natural treatment
techniques used. The in situ processes involved include physical
actions such as sedimentation; chemical reactions such as oxidationreduction, ion exchange, and precipitation; and biological activities
such as biological conversion and degradation.
In situ groundwater harvesting, hydrocarbon resource extractionexploitation, and related activitiesThese include the various in
situ means for extraction of heavy oil, storage systems, and carbon

1.4.3 Soil as a Resource Material

The subgroups in this main group are perhaps the most varied of the three
main groups. As in the previous main group, these subgroups are best
associated with activities and industries involved in the production of food,
shelter, and clothing. Some examples of these include
r Engineering constructionThis subgroup includes the use of soil
as a construction material in varied forms, for example, sand and
gravel for Portland cement concrete and bituminous concrete,
embankments, sub-bases, and buffer-barrier systems.
r Mining industry (metal, mineral, aggregate, hydrocarbon resources,
etc.)Examples include extraction of oil such as tar sands, metals
(bauxite industry), minerals such as quartz, calcite, and clay minerals such as kaolinite, montmorillonite, and illite.


Environmental Soil Properties and Behaviour

r Paper, packaging, and paint industryPaper coating, packaging,

and paint pigment.
r Pharmaceutical-cosmetic industryLaxatives and creams.
r MiscellaneousPottery, ceramics, oxidizing agents, catalysts, filter
for UV emissions, thickeners, chalk, and so forth.
There are many more uses for soils other than the examples shown in this
section. Suffice to say that the importance of soil, either in place of or as a
resource material, cannot be underestimated. It is a living dynamic system
and its properties and behaviour will inevitably change with time.

1.5 Concluding Remarks

We consider soil to be an important resource material that possesses a large
variety of functions. Soil functionality is dependent on several factors and
circumstances, some of which are controlled by natural forces and others by
humans and their activities. In this chapter and in this book, we are concerned
with the engineering properties and behaviour of soils. The brief discussion on
soil formation (origin of soil) is designed to remind us about the importance of
understanding where soil comes from because we need to appreciate that the
properties and behaviour of a soil are intimately tied into its origin, and ultimately, its composition. Readers interested in learning more about soil genesis
and formation should consult specialized textbooks dealing with these subjects.
We classify soils because we want to have a quick appreciation (idea) of
a soil under consideration. The purpose or role of the soil will tell us what
kinds of soil property and soil behaviour information we require. We recognize that many of the pieces of information require detailed and laborious
laboratory tests. However, to sort out the various kinds of soils as candidates
for the detailed tests, it is first necessary to choose appropriate candidates,
and therein lies the dilemma: What kinds of simple tests and what kinds
of information are needed? The discussion on soil classification has chosen to consider the role of soil in the context of soil engineering as the basis
for classification. It is understood that detailed laboratory tests as befits the
requirements of specific projects/plans must be undertaken to provide for a
complete classification scheme.
1.5.1 Soil Functionality
The concept of soil functionality proposed herein for use in soil engineering
is in essence similar in spirit to the concept of soil quality used in agriculture.
Soil quality has been defined by the Soil Science Society of America (SSSA) as


Origin and Function of Soils

ecosystem boundaries, to sustain plant and animal productivity, maintain

(Karlen et al., 1997). To ensure that the definition of soil functionality covers
the various soil functions, we define soil functionality (SF) as the capacity of
a specific soil to function under designed circumstances to meet its planned
intentions or requirement without any loss of its original functional capability. The use of the concept of soil functionality as a measure of the soil
and its functional capability essentially establishes a base that allows one to
measure changes in specific soil function capability, that is, specific soil functionality indices (SFI), to ensure that design or intended goals are met. This
subject will be discussed in detail in Chapter 5.
To illustrate how one might use soil functionality and soil functionality
indices in a soil engineering project, we use the example of the performance
or role of a bottom engineered clay barrier (of a multibarrier system) of a
nonhazardous waste landfill, as shown in Figure1.9. The required properties
of the clay barrier as placed in a multibarrier system is shown in the grey box
located at the right of the figure. These are the properties required for the


te Landfill



Functional capability of
engineered clay barrier
defined by
Specified values for required design
performance of engineered clay barrier

Filter membrane
Leachate collection &
leak detection system

Waste pile

Maximum allowable permeability

coecient k
Maximum compressibility Cc

Minimum shear strength and values of

cohesion c and internal friction
Maximum swelling pressures
 Minimum density of compacted clay

Engineered clay

Natural base soil

Partition coecients koc and kd

Diusion coecients for target


Soil functionality (SF) is defined by

how well functional capability is met

Typical landfill waste bottom engineered-barrier system for nonhazardous solid wastes. The
functional capability of the clay used for the barrier is defined by the specified values of the
various engineering properties required to fulfill design performance of the clay barrier. Soil
functionality at any time after onset of service life is defined by how well functional capability
of the clay barrier is met.


Environmental Soil Properties and Behaviour

clay barrier to fulfil its design function. Any degradation of any of the properties will detract from the functionality of the clay. In essence, for this particular example, one uses these specified values as indicators to determine soil
functionality indices, which in this case will be clay functionality indices.

Atterberg, A.M., 1908, Studien auf dem Gebiet der Bodenkunde, Landw, Versuchsanstalt, 69.
Atterberg, A.M., 1911, ber die physikalishe Bodenuntersuchung und ber die
Plastiszitt der Tone, Int. Mitt. fr Bodenkunde, 1:1043.
Casagrande, A., 1937, Classification and identification of soils, Proc. Amer. Soc. Civil
Engrs., 73:783810.
G.E., 1997, Soil quality: A concept, definition, and framework for evaluation,
Soil Sci. Soc. Am. J., 61:410.
Keller, W.D., 1957, The Principles of Chemical Weathering, Lucas Brothers Publishers,
Columbia, MO, 111 pp.
Waterways Experiment Station, 1953, Unified soil classification system, U.S. Corps of
Engineers, Tech. Memo, 3-337, Vicksburg, MS.

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