Soil Mechanics (CoTM 3171)
3rd YEAR cOTm
Lecture 1
CHAPTER-I
Origin and formation of Soils and Soil
Categories
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1. INTRODUCTION
1.1 Definition:
• The definition given to the word soil differs from one
discipline to another.
• In Engineering Geology, Soil is unconsolidated material,
composed of solid particles, produced by the disintegration
of rocks.
• For the agriculturalist (agronomist), on the other
hand, soil is the top thin layer of the earth where organic
forces are prevalent and which supports plant life.
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• The engineering definitions of soil is however quite
different from those given in geology and soil science. What do
you think?
• Soil is the un-cemented aggregate of mineral grains and
decayed organic matter (solid particles) with liquid and gas
in the empty spaces between the solid particles and which
can be separated by gentle mechanical means and
excavated without blasting unlike rock.
• ’Rock’’ is A natural aggregate of mineral particles bonded by
strong and permanent cohesive forces.
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Rock
Soil
Nomenclature of soil in Geology & in Soil Engineering
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1.2 The Discipline “Soil Mechanics”
• According to Dr. karl Terzaghi, Soil Mechanics is a branch of
Civil Engineering that deals with the ‘’application of laws
of mechanics and hydraulics to engineering problems dealing
with sediments and other unconsolidated accumulations of
solid particles produced by the mechanical and chemical
disintegration of rocks.
• Therefore; soil mechanics is branch of mechanics which deal
with the action of forces on soil and with flow of water in soil.
• Soil Mechanics is one of the youngest and fast developing
fields of engineering that deals with the application of soil
science, the laws of statics and dynamics and the principles of
mechanics and hydraulics to understand the behavior and use
of soil as an engineering material.
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Why Soil Mechanics became distinctive and separate
branch of engineering?
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1.3. Applications of Soil Mechanics in construction
Industry
Some of the important applications are discussed below:
1.Foundations : Every civil engineering structure is built on or
below the surface of the earth. The loads from any structure
have to be ultimately transmitted to a soil through the
foundation for the structure.
They are required to transmit the load from the super structure
safely and efficiently to the sub surface.
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2. Retaining Structures : are required
✓ To retain soil from spreading when there is no sufficient space.
✓ They are also required to maintain different elevation at either
side of the structure.
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3. Stability of Slope:
• Excavations require the knowledge of slope stability analysis;
deep excavations may need temporary supports—‘timbering’
or ‘bracing’, the design of which requires knowledge of soil
mechanics.
• Used to check the stability of slopes by determining the factor
of safety
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4. Underground Structures : Used in the design of
conduits and tunnels
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5. Highway Pavement : a hard layer placed over soil
subgrade to provide a smooth and strong surface on
which vehicles can move on.
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6. Earth Dams:
✓ Soil is used as a construction material in order to create a
barrier to a flowing water
✓ Knowledge of slope stability, effects of seepage, consolidation
and consequent settlement as well as compaction
characteristics for achieving maximum unit weight of the
soil in-situ, is absolutely essential for efficient design and
construction of embankments and earth dams
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1.4. Soil Formation process
Soil Mechanics
Conditions of formation
Causes (weathering)
Transportation agency
Deposition (Residual)
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Formation of soils
Weathering
▪ All soils originate, directly or indirectly, from solid rocks
by the process of weathering.
▪ Weathering is a process of physical breakdown or
disintegration and/or chemical decomposition or
alteration of a rock by the interaction of the
intrinsic/state variables
▪ The process of weathering of the rock decreases the
cohesive forces binding the mineral grains and leads to
the disintegration of bigger masses to smaller and smaller
particles.
▪ What types of weathering do you know?
1. Physical (Mechanical Weathering)
2. Chemical Weathering
3. Biological Weathering
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The weathering of the rocks might be
a. Physical (mechanical) – operates through physical
disintegration and fragmentation of rocks (breakdown) and
involves only physical change.
▪ No change in the chemical composition. The soil formed has
the properties of the parent rock.
• Among the many physical forces responsible for the
disintegration of rock
o Frost Wedging (Ice Pressure)
o Wetting and Drying (Slaking)
o Thermal Changes/ Temperature fluctuations
o Mechanical Exfoliation (Unloading)
o Impact and abrasive action of flowing water (Wearing)
o Freezing action of water
o Spreading of roots of plants
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b. Chemical (decomposition) - Operates through chemical
decomposition or alteration of a rock by the interaction of
the intrinsic/state variables and involves both physical and
chemical changes.
▪ original rock minerals are transformed into new minerals
by chemical reaction.
▪ The soils formed do not have the properties of the parent
rock.
Following chemical processes generally occur in nature.
1. Oxidation
2. Carbonation
3. Hydration/Hydrolysis
4. Leaching
c. Biological Weathering: Operates through the action of
biological organisms and involves physical or chemical changes
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1.5. Soil categories
• Categories of soils based on
o Formation
o Chemical composition
o Common soil types based on grain size (physical
composition)
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A. Soil categories based on Formation
Residual and Transported Soils
▪ Soils, which are formed by mechanical or chemical
weathering, may be classified as residual or
transported soils.
▪ Residual soils:- still located in the place of their origin
(remain at the site of weathering) (e.g Saprolitic,
Allophanic, Halloysitic and lateritic soils)
▪ Transported soils:- are formed by sorting processes of
erosion, transportation, deposition and upheaval by water,
wind, ice and deposited in an area different from their
place of origin.
▪ Transported soils are loose, soft and non-homogeneous.
▪ While Residual soils are usually homogeneous and stiff
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The products of weathering are picked up by agencies of
transportation, such as water and wind, and are carried to new
locations where they are deposited.
Exercise # 6 : At what regions of the globe do you think we
can find residual soils?
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✓ Transported soils may be classified according to the mode
of their transportation and deposition such as
a. Alluvial soils( fluvial soils) - transported by running
water (rivers and streams) and deposited along streams
b. Aeolian soils - transported and deposited by wind
c. Lacustrine soils - deposited from suspension in quite
fresh water lakes.
d. Colluvial soils - deposited by movement of soil by
gravity , such as during landslides
e. Marine soils - deposited from suspension in seawater.
f. Glacial soils - deposited as a result of glacial activities
(large body of ice moving slowly down a slope).
g. Loess is a wind blown, uniform, fine-grained soil.
❖Texture refers to the appearance or feel of a soil.
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B. Soil categories based on Chemical Composition of
soils
Based on their chemical composition, soil can be
broadly classified as Inorganic and Organic soils
• Inorganic soils are formed by the disintegration
(weathering) of rocks.
• Organic soils are mixtures in which a significant part
is derived from the growth and decay of plant life.
• Since most soil types fall under the inorganic
category we shall focus our discussion towards this
group.
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C. Soil categories based on physical properties (Common soil
types)
• Sand, gravel, cobbles and boulders – are coarsegrained cohesion less soils. Grain-size ranges are used to
distinguish between them.
• Particle size 0.06 to 2mm are sand, 2 to 60mm are gravel,
Fragments with diameters 60mm to 200mm are cobbles and
size >200mm are boulders.
• Silts - microscopic soil fractions that consist of very fine
quartz grains and some particles that are fragments of
micaceous minerals fine grained soil with little or no
plasticity. Grain sizes are in the range of 0.06 to 0.002mm
• Clays – are microscopic particles of weathered rock. They
have been defined as those particles which develop plasticity
when mixed with a limited amount of water. They also are
generally defined as particles smaller than 0.002mm
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1.6 Clay Mineralogy
• Minerals are naturally formed crystalline materials composed
of one or more chemical elements
• Most minerals of interest to geotechnical engineering are
composed of oxygen, silicon and aluminum, three of the most
abundant elements on earth.
• Silicates are a group of minerals with a structural unit called the
silica tetrahedron.
• A central silica cation (positively charged ions) is surrounded by
four oxygen anion (negatively charged ions) one at each corner
of the tetrahedron
• Silica tetrahedrons combine to form sheets, called silica sheets,
• Alumina sheets are formed by combination of alumina
minerals, which consists of aluminum ion(Al+) surrounded by
six oxygen hydroxyl atoms in an octahedron.
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• The main groups of crystalline materials that make up clays
are the minerals: kaolinite, illite, and montmorillonite
• Kaolinite is formed by bondage of one silica sheet and one
alumina sheet that are stacked repeatedly. The layers are
held together by hydrogen bonds.
• Illite consists of repeated layers of one alumina sheet
sandwiched by two silicate sheets. The layers are held
together by potassium ions.
▪
Montmorillonite has a structure similar to illite, but the
layers are held together by weak van der Walls forces(water
molecule) and exchangeable ions.
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1.7. Comparison of coarse and fine grained soils
▪ Coarse-grained soils
o have good load bearing capacities and good drainage
qualities, and their strength and volume change
characteristics are not significantly affected by change in
moisture conditions.
o They are practically incompressible when dense, but
significant volume change can occur when they are loose.
▪ Fine-grained soils
o have poor load bearing capacities compared with coarsegrained soils, practically impermeable, and change in
strength and volume with variations in moisture
conditions and they are in general compressible
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Comparison
Parameter
Coarse grained Soils
Fine grained Soils
Bearing Capacity
and Drainage
Good
Poor load bearing
capacities
Strength and
volume change
Not significantly affected
by change in moisture
conditions
Variations in
moisture conditions
Practically
incompressibility
When dense, but
significant volume change
can occur when they are
loose
Impermeable
Engineering
properties
Controlled mainly by the
grain size of the particles
and their structural
arrangement
Controlled by
mineralogical factors
rather than grain
size.
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Thank you!!
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