Engineering properties of soil

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ENGINEERING PROPERTIES OF SOIL
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
India

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1. Introduction:
Engineering properties of soil comprises of
physical properties, index properties,
strength parameters (shear
strength parameters), permeability
characteristics, consolidation properties,
modulus parameters, dynamic behavior etc.

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2. Methods of Determining Soil Properties
Geotechnical soil and rock properties of
geologic strata are typically determined using
one or more of the following methods:
• In-situ testing data from the field exploration
program;
• Laboratory testing; and
• Back analysis based on site performance data.

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Laboratory Testing :
Laboratory soil testing is used to estimate
strength,
stressstrain,
compressibility, and
permeability characteristics.

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3. Cohesion :
It is the internal molecular attraction which
resists the rupture or shear 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.
Cohesion is the property of the fine grained soil
with particle size below 0.002 mm.

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Cohesion of a soil decreases as the moisture
content increases.
Cohesion is greater in well compacted clays and
it is independent of the external load applied.
4. 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
.
5. 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.
6. 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’.

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The rate of flow is directly proportional to the
head of water.
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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7. Soil plasticity :
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.
Cohesion refers to the attraction of substances
of like characteristics, such as, that of one water
molecule for another.

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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.
8 . Elasticity :
This elastic behavior is characteristic of peat.

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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. The most important characteristic of
the elastic behavior of soil is that no matter how
many repetitions of load are applied to it,
provided that the stress set up in the soil do not
exceed the yield stress, the soil does not become
permanently deformed.

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9. Compressibility:
Gravels, sands & silts are incompressible, i.e. if
a moist mass of those materials is subjected to
compression; they suffer no significant volume
change.
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.

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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.
Compressibility of sand & silt varies with
density & compressibility of clay varies directly
with water content & inversely with cohesive
strength.

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10. Uniaxial Compressive Strength (UCS)
Test
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).

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Triaxial Compression :
The triaxial compression test is a more
sophisticated testing procedure for determining
the shear strength of a soil.
The test involves a soil specimen subjected to
an axial load until failure while also being
subjected to confining pressure that
approximates the in-situ stress conditions.

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11. 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.
12. Atterberg Limits : When a clayey soil is
mixed with an excessive amount of water, it
may flow like a semi-liquid. 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.

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These limit a referred to as Atterberg limits.
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.
13. The consistency limits of the soil are
controlled by the pore fluid pressure.

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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.
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.
C. 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.. The PI
represents the range of moisture contents within
which the soil behaves as a plastic solid.

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PI Range Description
0 Nonplastic
1 – 5 Slightly Plastic
5 – 10 Low Plasticity
10 – 20 Medium Plasticity
20 – 40 High Plasticity
> 40 Very High Plasticity
The liquid limit, plastic limit, and shrinkage
limit are extremely useful in correlating
anticipated soil behavior with previous

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experience on soils in similar consistency states.
Each limit represents a water content at which
the soil changes from one state to another.
14. Activity :
The degree of plasticity related to the clay
content is called the Activity of the soil. In most
natural clays, the actual clay-sized material
(smaller than 2 μm) comprise only a fraction of
the total.

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15. Specific Gravity of Soils
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.
Weight-Volume Relationships: In nature,
soils are three-phase systems consisting of solid
soil particles, water, an air (or gas). To develop
the weight-volume relationships for a soil, the
three phases can be separated.

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16. 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. There are two components of shear
strength; the cohesive element (expressed as the
cohesion, c, in units of force/unit area) and the
frictional element (expressed as the angle of
internal friction, φ).

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Soil strength tests are performed on high
quality, relatively undisturbed in-situ
specimens.
However, it is difficult and frequently
impossible to sample, transport, extrude and set-
up testing for granular, cohesionless soils (Sand
or Gravel) without excessively disturbing or
completely obliterating the soil specimen.

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It is difficult to obtain good strength values
through lab testing of disturbed (remolded)
specimens since the soil matrix (i.e., cohesion/
bonding of soil particles) is destroyed and the
in-situ density and moisture content are very
difficult to recreate.

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17. Consolidation Test :
The amount of settlement induced by the
placement of load bearing elements on the
ground surface or the construction of earthen
embankments will affect the performance of the
structure. The amount of settlement is a
function of the increase in pore water pressure
caused by the loading and the reduction of this
pressure over time.

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The reduction in pore pressure and the rate of
the reduction are a function of the permeability
of the in-situ soil. All soils undergo elastic
compression and primary and secondary
consolidation.
18. Shrinkage and Swell :
Certain soil types (highly plastic) have a large
potential for volumetric change depending on
the moisture content of the soil.

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These soils can shrink with decreasing moisture
or swell with increasing moisture.
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.
19. Bulk density :
Bulk density data are used to compute shrink-
swell potential, available water capacity, total
pore space, and other soil properties. The moist
bulk density of a soil indicates the pore space
available for water and roots.

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A bulk density of more than 1.6 can restrict
water storage and root penetration. Moist bulk
density is influenced by texture, kind of clay,
content of organic matter, and soil structure.
20. Erodibility:
It refers to the ease with which soil materials
can be removed by wind or water. Easily eroded
materials include unprotected silt, sand and
other loosely consolidated materials,

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Cohesive soils (with more than 20% clay) and
naturally cemented soils are not easily removed
from its place by wind or water and, therefore,
have a low erosion factor.

Engineering properties of soil

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