1. Explain the soil characteristics related to
2. Explain basic soil test
Soils are formed by the disintegration (or more
precisely, evolution) of rock material of the earth’s
relatively deeper crust, which itself is formed by the
cooling of volcanic magma.
The stability of crystalline structure governs the rock
As the temperature falls, new and often more stable
minerals are formed. For instance, one of the most
abundant minerals in soils known as quartz acquires
a stable crystalline structure when the temperature
drops below 573°C.
The intermediate and less stable minerals (from
which quartz has evolved) lend themselves to easy
disintegration during the formation of soils.
The disintegration process of rocks leading to the formation of
soils is called weathering.
It is caused by natural agents; primarily wind and water (note
that these are the same agents that aid the evolution and life in
The specific processes responsible for weathering of rocks are:
i. Erosion by the forces of wind, water, or glaciers, and
alternate freezing and thawing of the rock material.
ii. Chemical processes, often triggered by the presence of water.
Hydrolysis (reaction between H-
ions of water and
the ions of the rock minerals),
Chelation (complexation and removal of metal ions),
Cation exchange between the rock mineral surface and
the surrounding medium
Oxidation and reduction reactions,
Carbonation of the mineral surface because of the
presence of atmospheric CO2
iii. Biological processes which, through the presence of organic
compounds, affect the weathering process either directly or
Once the rock material is weathered, the
resultant soil may either remain in place or
may be transported by the natural agencies
of water, air, and glaciers.
In the former case, the soils are called
Depending on the natural agent involved, the
transported soils are called alluvial or fluvial
(water-laid), aeolian (wind-laid), or glacial
Several subdivisions are often made based
on the transportation and deposition
Five independent variables may be viewed as governing soil
Climate - Amount of moisture available,
temperature, chemical reaction speed and
rate of plant growth
- Organisms influence the soil's physical
and chemical properties and furnish
organic matter to soil
Topography - Angle: like Steep is poorly developed
soils but flat to undulating surface is the
best. Orientation (direction the slope is
facing) - soil temperature and Moisture
The nature of
- Original mineral makeup and important in
young soils. Residual soil–from bedrock.
Transported soil–carried from elsewhere
Five mains groups of mineral composition in soil
(regular structure elements and atomic elements) are :
i. CARBONATES - calcite and dolomite usually use
– gibbsite and brucite minus OH’s
sheet in clay minerals
PHOSPHATE – using for fertilizer
v. SILICATE – 90% of all soil
Silicates constitute well over 90% of
the earth's crust.
The fundamental unit of all silicate
structures is the SiO4 tetrahedron.
It consists of four O2-
ions at the apices
of a regular tetrahedron coordinated to
at the center.
The individual tetrahedral are linked
together by sharing O2-
ions to form
more complex structures.
Silica tetrahedron: The silica tetrahedron consists of four
oxygen ions and one silica ion.
The molecular arrangement is such that the four oxygen
ions are spaced at what would be the corners and tip of a
three-dimensional, three-sided pyramid, with the silicon
located within the pyramid.
Oxygen ions at the base are shared by adjacent
tetrahedrons, thus combining and forming a sheet.
Commonly found in soil and the
mineral composition SiO2.
The Quartz shape are in three
dimensions and each of quartz cannot
absorb in acid and cannot break
There is no isomorphous substitution
in quartz, and each silica
tetrahedronis firmly and equally
braced in all directions.
As a result, quartz has no planes of
weakness and is very hard and highly
resistant to mechanical and chemical
Quartz is not only the most common
mineral in sand and silt-sized particles
of soils, but quartz or amorphous silica
some of the silicon atoms are replaced by
aluminum. This results in a negative charge and
in distortion of the crystal structure, because Al
atoms are larger than Si atoms.
The negative charge is balanced by taking in
cations such as K+
, and Ca+
albite, and anorthite feldspars, respectively. The
distortion of the lattice and the inclusion of the
cations cause cleavage planes that reduce the
resistance of feldspars to mechanical and
For these reasons, feldspars are not as common
as quartz in the sand-, silt-, and claysized
fractions of soils, even though feldspars are the
most common constituent of the earth's crust.
Common micas such as muscovite and
biotite are often present in the silt- and
sand-sized fractions of soils.
In a unit sheet of mica, which is 1 nm
thick, two tetrahedral layers are linked
together with one octahedral layer.
In muscovite, only two of every three
octahedral sites are occupied by
aluminum cations, whereas in biotite
all sites are occupied by magnesium.
In well-crystallized micas one fourth of
the tetrahedral Si+4
are replaced by
The resulting negative charge in
common micas is balanced by
intersheet potassiums. In a face-to-face
stacking of sheets to form mica plates,
the hexagonal holes on opposing
tetrahedral surfaces are matched to
The alumina octahedron consists of six-oxygen and one-
aluminum.3 oxygen is in the top place of the octahedrons,
and three are in the bottom plane. The aluminum is within
the oxygen grouping. It is possible that the aluminum ion
may be replaced with magnesium, iron, or other neutral
ions. The aluminum sheet is 5 x 10-7
mm thick. Oxygen from
the tip of a silica tetrahedron can share an alumina sheet,
thus layering sheets. Different sheet arrangements are then
combined to form the different clay minerals. The
composition and typical properties of the more commonly
occurring clay minerals are Kaolinite, Illite and
is a common mineral in soils and is the most common member
of this subgroup. A Kaolinite is the most prevalent clay mineral
and is very stable, with little tendencies for volume change
when exposed to water. Kaolinite layers are stack together to
form relatively thick particle. Particles are plate shaped. The
composition is one-silica, one alumina sheet that is very
strongly bonded together. Kaolinites have very little
isomorphous substitution in either the tetrahedral or
octahedral sheets and most kaolinites are close to the ideal
formula Al2Si2O5 (OH) 4.
Illite - has irregular plate shape, more plastic than
Its does not expand when exposed to water unless
potassium deficiency exists. This clay is most prevalent in
The composition is an alumina sheet sandwiched between
two silica sheets to form a layer. Potassium provides the
bonds between the layers.
has irregular plate shapes or is fibrous because of the
weak bond between layers this clay readily absorbs
water between layers.
This mineral has a great tendency for large volume
change. The composition is an alumina sheet
sandwiched between two silica sheets to form a layer.
Iron or magnesium may replace the alumina in the
The soil type or category is based on particle size,
however, where the soil particle size is too small to be
observed, an additional physical property, known as
plasticity is utilized as a criterion for evaluation
Soil is all the material located above bedrock and can be
grouped into four major categories or types including
gravel, sand, clay and silt.
These four categories can be reduced to two groups
termed coarse-grained soil and fine-grained soil.
Particle size and shape affects the mechanical behavior of
soils, however, the effect of varies for coarse-grained and
The size and shape of the granular soil particles can
increase or decrease the tendency of particles to fracture,
crush and degrade.
The grading of gravels and sands may be qualified in the
field as well graded (good representation of all particle
sizes from largest to smallest).
Poorly graded materials may be further divided into
uniformly graded (most particles about the same size) and
gap graded (absence of one or more intermediate sizes).
Soil structure is the shape that the soil takes based
on its physical and chemical properties; it is the
geometric arrangement of soil particles with respect
to one another.
The process of sedimentation or rock weathering
creates the initial soil structure.
Among the many factors that effect soil structure is
the shape, size, and mineral composition of the soil
particles, and the nature and composition of soil
The basic terminology used to define the soil
structure are single-grained, honeycombed,
flocculated and dispersed with variations dependent
upon the composition of the soil.
The particle arrangement of cohesionless soils
(gravel, sand and silt) has been likened to
arrangements attained by stacking marbles, or
In single grained structures soil particles are in a
stable position, with each particle in contact with
the surrounding ones.
For similar sized particles large variations in the
void ratio are related to the relative position of the
The term cohesive is used for clay soils, which have
an inherent strength, based on their particle
structure which provides considerable strength in an
The cohesiveness of a clay is due to its’ mineralogy
and is a controlling factor determining the shapes,
sizes, and surface characteristics of a particle in a
It determines interaction with fluids.
Together, these factors determine plasticity,
swelling, compression, strength, and fluid
Fine grained soils are identified on the basis of some simple tests for :
Dry strength is a qualitative measure of how hard it is to
crush a dry mass of fine grained soil between the fingers.
Clays have very high dry strength and silts have very low
ii. Dilatancy Dilatancy is an indication of how quickly the moisture
from a wet soil can be brought to the surface by vibration.
In silty soils, within a few strikes water rises to the
surface making it shine. In clays, it may require
considerable effort to make the surface shiny. In other
words, dilatancy is quick in silts and slow in clays.
Toughness is a qualitative measure of how tough the soil
is near its plastic limit (where the soil crumbles when
rolled to a 3 mm diameter thread). Toughness increases
with plasticity. Silty soils are soft and friable (crumble
easily) at Plastic Limits (PL), and clays are hard at PL. The
fines can also be identified by feeling a moist pat; clays
feel sticky and silts feel gritty. The stickiness is due to
the cohesive properties of the fines, which is also
The grain size distribution of a coarse grained soil is generally
determined through sieve analysis, where the soil sample is
passed through a stack of sieves and the percentages passing
different sizes of sieves are noted.
The grain size distribution of the fines are determined through
hydrometer analysis, where the fines are mixed with distilled
water to make 1000 ml of suspension and a hydrometer is used
to measure the density of the soil-water suspension at different
which is the water content at which
the soil passes from solid to
ii. Plastic limit which is the water content at which
transition from semisolid to
plastic state takes place
iii. Liquid limit which indicates the water content
required in order for the clayey soil
to begin exhibiting flow
characteristics like liquids
The BS 5930:1999 (Code of Practice for Site
Investigations) summarizes the purposes of laboratory
testing to be to describe and classify the samples, to
investigate the fundamental behavior of the soils in
order to determine the most appropriate method to be
used in the analysis, and to obtain soil parameters
relevant to the technical objectives of the
The laboratory tests for soils commonly carried out
• Moisture content, which read in conjunction with liquid
and plastic limits gives an
• indication of undrained strength;
• Liquid and plastic limits to classify fine grained soil
and the fine fraction of mixed
Particle size distribution to give the relative proportions of
gravel, sand, silt and clay;
Organic matter which may interfere with the hydration of
Mass loss of ignition which measures the organic content in
soil, particularly peat;
Sulfate content which assesses the aggressiveness of the soil
or groundwater to buried concrete;
pH value which is usually carried out in conjunction with sulfate
California bearing ration (CBR) used for the design of flexible
Soil strength tests such as Triaxial compression, unconfined
compression and vane shear;
Soil deformation tests;
Soil permeability tests.