Geotechnical properties of soils

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GEOTECHNICAL PROPERTIES OF SOILS
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
Mysore-6

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Introduction: Soils and rocks have unique and
distinct engineering properties.
Engineering properties of soils and rocks are
very essential parameters to be analysed for
several technical reasons.
Properties of these materials may not only pose
problems but also give solutions to solve the
problems.

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Foundation and construction engineering,
structural engineering and excavation works for
various projects need a thorough investigations
of soils and their properties.
Engineering properties of soils:
Soils are loose fragments of organic and
inorganic constituents.
They possess typical physical, chemical,
engineering and biological properties.

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Soils are also characterised by their
mechanical properties,
index properties,
strength parameters (shear strength
parameters),
permeability characteristics,
consolidation properties,
modulus parameters, dynamic behavior, etc.
Sometimes these are called as geotechnical
properties.

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1. Soil texture and soil structure:
Both are unique properties of the soil that will
have a profound effect on the behavior of soils.
2. Specific Gravity :
The specific gravity of soil, Gs, is defined as the
ratio of the unit weight of a given material to the
unit weight of water.

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3. Bulk density:
Bulk density is the weight of soil in a given
volume.
Bulk density is dependent on soil texture and the
densities of soil mineral (sand, silt, and clay) and
organic matter particles, as well as their packing
arrangement.
Bulk density increases with compaction and
tends to increase with depth.

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Sandy soils are more prone to have high bulk
density.
Bulk density can be used to calculate soil
properties per unit area (e.g. kg/ha).
Bulk density is not an intrinsic property of a
material; it can change depending on how the
material is handled.

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The bulk density of soil depends greatly on the
mineral make up of soil and the degree
of compaction.
Bulk density typically increases with soil depth.
High bulk density is an indicator of low soil
porosity.
When eroded soil particles fill pore space,
porosity is reduced and bulk density increases.
Cultivation can result in compacted soil layers
with increased bulk density.

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a) Calculate the bulk density of a 400 cm3 soil
sample that weighs 575 g (oven dry weight).
r b = Ms/Vs = 575g/400cm3
= 1.44g/ cm3.
.

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b) Calculate the bulk density of a rectangular soil
sample with dimensions 12 cm by 6 cm by 4 cm,
that is 15% moisture content and weighs 320 g.
Vol. of soil = 12cm x 6cm x 4cm = 288cm3
Oven dry wt. = 320/1.15 = 272g.
r b = 272/288 = 0.97g/cm3
Bulk density influence water infiltration and
plant root health.

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4. Cohesion:
Cohesive soil means clay (fine grained soil), or
soil with a high clay content, which has cohesive
strength.
Cohesive soil does not crumble, can be
excavated with vertical sideslopes, and is plastic
when moist.
Cohesive soil is hard to break up when dry, and
exhibits significant cohesion when submerged.

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Cohesive soils include clayey silt, sandy clay, silty
clay, clay and organic clay.
It reflects the internal molecular
attraction(binding capacity) which resists the
rupture or shear(looseness) of a material.
Cohesion is derived in the fine grained soils from
the water films which bind together the
individual particles in the soil mass.

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The soil cohesion depends strongly on the
consistence, packing, and saturation condition.
Cohesion is the property of the fine grained soil
with particle size below 0.002 mm.
Cohesion of a soil decreases when the moisture
content increases.

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Cohesionless soil is any free-running type of soil,
such as sand or gravel, whose strength depends
on friction between particles.
5. Moisture Content:
The moisture content (w) is defined as the ratio
of the weight of water in a sample to the weight
of solids. The sample is oven-dried and is
considered as weight of dry soil.

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6. Soil plasticity& Elasticity :
Soil plasticity is a property that enables the
moist soil to change shape when some force is
applied over it and to retain this shape even
after the removal of the force from it.
The plasticity of soil depends on the cohesion
and adhesion of soil materials.

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Cohesion refers to the attraction of substances
of like characteristics, such as, that of one water
molecule for another.
Adhesion refers to the attraction of substances
of unlike characteristics.
Soil consistency depends on the texture and
amount of inorganic and organic colloids,
structure and moisture contents of soil.

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Elasticity :
This elastic behavior is a characteristic of peat.
A soil is said to be elastic when it suffers a
reduction in volume (or is changed shape & bulk)
while the load is applied, but recovers its initial
volume immediately when the load is removed.

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The most important characteristic of the elastic
behavior of soil is that the soil does not become
permanently deformed.
A. Plastic Limit(PL): The plastic limit (PL) is the
moisture content at which a soil transitions from
being in a semisolid state to a plastic state.

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B. Liquid Limit (LL) :
The liquid limit (LL) is defined as the moisture
content at which a soil transitions from a plastic
state to a liquid state.
Plasticity Index :
The plasticity index (PI) is defined as the
difference between the liquid limit and the
plastic limit of a soil , PI = LL − PL.

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The liquid limit, plastic limit, and shrinkage limit
are extremely useful in correlating anticipated
soil behavior with previous experience on soils in
similar consistency states.
Each limit represents a water content at which
the soil changes from one state to another.

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7. Atterberg Limits :
When a clayey soil is mixed with an excessive
amount of water, it may flow like a semiliquid.
If the soil is gradually dried, it will behave like a
plastic, semisolid, or solid material depending on
its moisture content.

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The moisture content, in percent, at which the
soil changes from a liquid to a plastic state, is
defined as the liquid limit (LL).
Similarly, the moisture contents, in percent, at
which the soil change from a plastic to a
semisolid state and from a semisolid to a solid
state are define as the plastic limit (PL) and the
shrinkage limit (SL), respectively. These limits are
named as a factor. It is called as Atterberg
limits.

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The behavior of the soil is therefore related
directly to the amount of water which is present.
In 1911, A. Atterberg defined the boundaries of
four states of consistency in terms of limits.
8. Consistency:
A fine-grained soil can exist in any of several
states of consistency.

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The particular state of consistency of any
particular soil depends primarily upon the
amount of water present in the soil-water
system.
The behavior of the soil is therefore related
directly to the amount of water which is present.

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9 . Soil Strength :
The shear strength is the internal resistance per
unit area that the soil can handle before failure
and is expressed as a stress.
The shear strength is the internal resistance per
unit area that the soil can handle before failure
and is expressed as a stress.

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Strengths are of various kinds.
Uniaxial Compressive Strength (UCS) :
Compressive strength is the capacity of a
material to withstand axially directed
compressive forces.
The most common measure of compressive
strength is the uniaxial compressive strength
(a.k.a. unconfined compressive strength).- apply
Force from top till it creates cracks.

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Triaxial Compression :
The triaxial compression test involves a soil
specimen subjected to an axial load from all
sides until it attains the state of failure.
10. Compressibility:
When a soil mass is subjected to a compressive
force, its volume decreases.

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The property of the soil due to which a decrease
in volume occurs under compressive force is
known as the compressibility of soil.
Gravels, sands & silts are incompressible, i.e. if a
moist mass of those materials is subjected to
compression; they suffer no significant volume
change.

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Clays are compressible, i.e. if a moist mass of
clay is subjected to compression, moisture & air
may be expelled, resulting in a reduction in
volume which is not immediately recovered
when the compression load is withdrawn.
The decrease in volume per unit increase of
pressure is defined as the compressibility of soil,
and a measure of the rate at which consolidation
proceeds is given by the ‘co-efficient of
consolidation’ of the soil.

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Compressibility of sand & silt varies with density
& compressibility of clay varies directly with
water content & inversely with cohesive
strength.
11. Compaction and Consolidation:
Compaction is a process in which with the help
of mechanical pressing air present in void of soil
is expelled resulting closely packed soil
structure.

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Compacted soil has better shear strength and
hence better bearing capacity and stability of
soil.
Both terms are very important in
foundation engineering.
Consolidation is any process which involves a
decrease in water content of
saturated soil without replacement of water by
air.

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All soils undergo elastic compression and
primary and secondary consolidation.
Consolidation and compaction are totally
different process.
Though both process results a reduction in
volume.

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12. Direct Shear:
The direct shear test is the oldest and simplest
form of shear test.
A soil sample is placed in a metal shear box and
undergoes a horizontal force.
The soil fails by shearing along a plane when the
force is applied.

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12. Shrinkage and Swell :
Certain soil types (highly plastic) have a large
potential for volumetric change depending on
the moisture content of the soil.
These soils can shrink with decreasing moisture
or swell with increasing moisture.

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Shrinkage can cause soil to pull away from
structure thus reducing the bearing area or
causing settlement of the structure beyond that
predicted by settlement analysis.
Swelling of the soil can cause an extra load to be
applied to the structure that was not accounted
for in design.

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Therefore, the potential for shrinkage and
swelling should be determined for soils that
have high plasticity.
Shrinkage and Swell :
Certain soil types (highly plastic) have a large
potential for volumetric change depending on
the moisture content of the soil.
These soils can shrink with decreasing moisture
or swell with increasing moisture.

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Shrinkage can cause soil to pull away from
structure thus reducing the bearing area or
causing settlement of the structure beyond that
predicted by settlement analysis.
Swell : Swelling of the soil can cause an extra
load to be applied to the structure that was not
accounted for in design.

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Therefore, the potential for shrinkage and
swelling should be determined for soils that
have high plasticity.
There are certain types of soils that can swell,
particularly clay in the montmorillonite (a very
soft phyllosilicate group of minerals that
typically form in microscopic crystals, forming a
clay) family.

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Swelling occurs when the moisture is allowed to
increase causing the clay soil to increase in
volume
13. Permeability :
Permeability of a soil is the rate at which water
flows through it under action of hydraulic
gradient.

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The passage of moisture through the inter-
spaces or pores of the soil is called ‘percolation’.
Soils having porous enough for percolation to
occur are termed ‘pervious’ or ‘permeable’,
while those which do not permit the passage of
water are termed ‘impervious’ or ‘impermeable’.
The rate of flow is directly proportional to the
head of water.

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Permeability is a property of soil mass and not of
individual particles.
The permeability of cohesive soil is, in general,
very small.
Knowledge of permeability is required not only
for seepage, drainage and ground water
problems but also for the rate of settlement of
structures on saturated soils.

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Permeability, also known as hydraulic
conductivity, has the same units as velocity and
is generally expressed in ft/min or m/sec.
Coefficient of permeability is dependent on void
ratio, grain-size distribution, pore-size
distribution, roughness of mineral particles, fluid
viscosity, and degree of saturation.

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14. Electro-Chemical Test :
Electro-chemical tests provide quantitative
information related to the aggressiveness of the
subsurface environment, the surface water
environment, and the potential for deterioration
of foundation materials.
Electro-chemical testing includes pH, resistivity,
sulfate, and chloride contents.

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15. pH Testing :
pH testing is used to determine the acidity or
alkalinity of the subsurface or surface water
environments.
Acidic or alkaline environments have the
potential for aggressively corroding structures
placed within these environments.

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16. Resistivity Testing :
Resistivity testing is used to determine the
electric conduction potential of the subsurface
environment.
The ability of soil to conduct electricity can have
a significant impact on the corrosion of steel
piling.

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If a soil has a high potential for conducting
electricity, then sacrificial anodes may be
required on the structure.
17. Chloride Testing :
Most salts are active participants in the
corrosion process.
Chlorides, sulphates and sulfides have been
identified as being chief agents in promoting
corrosion.

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Subsurface soils and surface water should be
tested for chloride if the presence of sea or
brackish water is suspected.
18. Durability :
A soundness test determines a granular
materials resistance to disintegration by
weathering and, in particular, freeze-thaw
cycles.

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Aggregates that are durable (resistant to
weathering) are less likely to degrade in the field
and cause premature failure.
A soundness test involves an initial sieve analysis
to determine the distribution of particle sizes.
19. Angle of Internal Friction: The resistance in
sliding of grain particles of a soil mass depends
upon the angle of internal friction.

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It is usually considered that the value of the
angle of internal friction is almost independent
of the normal pressure but varies with the
degree of packing of the particles, i.e. with the
density.
The soils subjected to the higher normal stresses
will have lower moisture contents and higher
bulk densities at failure than those subjected to
lower normal stresses and the angle of internal
friction may thus change.

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The true angle of internal friction of clay is
seldom zero and may be as much as 260
. The
angle of internal friction fro granular soils may
vary in between 280
to 500
.
20. Capillarity :
It is the ability of soil to transmit moisture in all
directions regardless of any gravitational force.
Water rises up through soil pores due to
capillary attraction.

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The maximum theoretical height of capillary rise
depends upon the pressure which tends to force
the water into the soil, and this force increases
as the size of the soil particles decreases.
The capillary rise in a soil when wet may equal as
much as 4 to 5 times the height of capillary rise
in the same soil when dry.

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Coarse gravel has no capillary rise;
coarse sand has up to 30 cm;
fine sand and soils have capillary rise up to 1.2 m
but dry sand have very little capillarity.
Clays may have capillary rise up to 0.9 to 1.2 m
but pure clays have very low value.

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21. Moisture Content & Available water
capacity :
The moisture content (w) is defined as the ratio
of the weight of water in a sample to the weight
of solids. Available water capacity refers to the
quantity of water that the soil is capable of
storing for use by plants.

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The capacity varies, depending on soil properties
that affect the retention of water and the depth
of the root zone.
The most important properties are the content
of organic matter, soil texture, bulk density, and
soil structure.
Available water capacity is an important factor in
the choice of plants or crops to be grown and in
the design and management of irrigation
systems.

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Available water capacity is not an estimate of
the quantity of water actually available to plants
at any given time.
22. The consistency limits of the soil are
controlled by the pore fluid pressure.
The fluid in a unconsolidated material promotes
inter-granular cohesion.
Fluid in a soil will promote excess pressure to
cause fluid like behavior of the soil.

Geotechnical properties of soils

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