Lecture 1 introduction & properties of soil

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303322: Soil Mechanics
Introduction &Properties of Soil
Dr. Abdulmannan Orabi
Lecture
1

Das, B., M. (2014), “ Principles of geotechnical
Engineering ” Eighth Edition, CENGAGE
Learning, ISBN-13: 978-1-133-10867-2.
Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil
Mechanics” Eighth Edition, Spon Press, ISBN: 978-
0-415-56125-9.
References
2Dr. Abdulmannan Orabi IUST

Dr. Abdulmannan Orabi IUST 3
Introduction
Definition of Soil
The term soil according to engineering point of view
is defined as the material, by means of which and upon
which engineers build their structures. The term soil
includes entire thickness of the earth’s crust (from
ground surface to bed rock), which is accessible and
feasible for practical utilization as foundation support
or construction material. It is composed of loosely
bound mineral particles of various sizes and shapes
formed due to weathering of rocks.

Soil Mechanics is a discipline of Civil Engineering
involving the study properties of soil, behavior of soil
masses subjected to various types of forces, and its
application as an engineering material.
Introduction
Definition of Soil Mechanics

Soil Mechanics is the application of laws of
mechanics and hydraulics to engineering problems
dealing with sediments and other unconsolidated
accumulations of solid particles, which are produced
by the mechanical and chemical disintegration of
rocks, regardless of whether or not they contain an
admixture of organic constituents.
According to Terzaghi (1948):
Introduction
Definition of Soil Mechanics

Why do you need to learn about soils?
Almost all structures are either constructed of
soil, supported on soil, or both.
Introduction

1. Foundation to support Structures and
Embankments
2. Construction Material
3. Slopes and Landslides
4. Earth Retaining Structures
5. Special Problems
Various reasons to study the properties of Soil:
Introduction
Why do you need to learn about soils

Who must be concerned with soils?
Civil engineers (structural, environmental
and geotechnical) must have basic
understanding of the soil properties in order
to use them effectively in construction.
Introduction

Problems in Geotechnical Engineering
‫ي‬ ‫ا‬ ‫ا‬
‫ا‬ ( Al-naser Dome)

Shear Failure-Loads have exceeded shear strength capacity
of soil!
Transcosna Grain Elevator, Canada Oct. 18, 1913

Shear Failure-Loads have exceeded shear strength capacity
of soil!

Shear Failure-Loads have exceeded shear strength
capacity of soil!

Settlement
Leaning Tower, Pisa

Seepage Problems

Teton Dam Failure
Dam Failure - Seepage

Soil subjected to dynamic load

All soils originate, directly or indirectly, from
different rock types.
Soil Formation
Soils are formed from the physical and
chemical weathering of rocks.
Soil is generally formed by disintegration and
decomposition (weathering) of rocks through
the action of physical (or mechanical) and
chemical agents which break them into smaller
and smaller particles.

Soil Formation
Physical weathering
Involves reduction of size without any change in the
original composition of the parent rock. The main
agents responsible for this process are exfoliation,
erosion, freezing, and thawing.
Physical or mechanical processes taking place on the
earth's surface include the actions of water, frost,
temperature changes, wind and ice. They cause
disintegration and the products are mainly coarse soils.

Soil Formation
Physical weathering

Soil Formation
Chemical weathering causes both reduction in
size and chemical alteration of the original
parent rock. The main agents responsible for
chemical weathering are hydration, carbonation,
and oxidation.
Rain water that comes in contact with the rock
surface reacts to form hydrated oxides, carbonates
and sulphates.
The results of chemical weathering are generally fine
soils with altered mineral grains.

Soil Formation
Chemical weathering

Soils as they are found in different regions can be
classified into two broad categories:
(1) Residual soils
(2) Transported soils
Soil Types

Residual Soils
Residual soils are found at the same location where they have
been formed. Generally, the depth of residual soils varies
from 5 to 20 m.
Chemical weathering rate is greater in warm, humid regions
than in cold, dry regions causing a faster breakdown of
rocks. Accumulation of residual soils takes place as the rate
of rock decomposition exceeds the rate of erosion or
transportation of the weathered material. In humid regions,
the presence of surface vegetation reduces the possibility of
soil transportation.
Residual Soil

25
Residual Soils
As leaching action due to percolating surface water
decreases with depth, there is a corresponding decrease
in the degree of chemical weathering from the ground
surface downwards. This results in a gradual reduction
of residual soil formation with depth, until unaltered
rock is found.
Residual soils comprise of a wide range of particle
sizes, shapes and composition.
Residual Soil
Dr. Abdulmannan Orabi IUST

Transported Soils
Weathered rock materials can be moved from
their original site to new locations by one or
more of the transportation agencies to form
transported soils. Transported soils are classified
based on the mode of transportation and
the final deposition environment.
Transported Soil

Transported Soil
DESERT SOIL Contains soluble salts.
Originated by Mechanical disintegration & wind
deposit. Porous and coarse. 90% sand & 5% clay..

Transported Soil
DESERT SOIL Rich in Nitrates &
Phosphates. Poor in Nitrogen.

Transported Soils
Transported soils are classified based on the mode of
transportation and the final deposition environment.
(a) Soils that are carried and deposited by rivers are
called alluvial deposits.
(b) Soils that are deposited by flowing water or surface
runoff while entering a lake are called lacustrine deposits.
Alternate layers are formed in different seasons depending
on flow rate.
Transported Soil

Transported Soils
(c) If the deposits are made by rivers in sea water, they are
called marine deposits. Marine deposits contain both
particulate material brought from the shore as well as
organic remnants of marine life forms.
(d) Melting of a glacier causes the deposition of all the
materials scoured by it leading to formation of glacial
deposits.
(e) Soil particles carried by wind and subsequently deposited
are known as Aeolian deposits.
Transported Soil

Gravity Soils
Gravity can transport materials only for a
short distance.
Gravity soils are termed as talus these soils
are generally loose and porous.
Transported Soil

Soil is not a coherent solid material like steel and
concrete, but is a particulate material. Soils, as they
exist in nature, consist of solid particles (mineral
grains, rock fragments) with water and air in the
voids between the particles.
The water and air contents are readily changed by
changes in ambient conditions and location.
Phases System of Soils

As the relative proportions of the three phases vary
in any soil deposit, it is useful to consider a soil
model which will represent these phases distinctly
and properly quantify the amount of each phase. A
schematic diagram of the three-phase system is
shown in terms of weight and volume symbols
respectively for soil solids, water, and air.
The weight of air can be neglected.

Ground surface
Voids
Air
Water
Solids
The compositions of natural soils may include diverse components
which may be classified into three large groups:
1. Solid phase ( minerals,
cementations and organic
materials)
2. Liquid phase (water
with dissolved salts)
3. Gaseous phase (air
or other some gas)

Ground surface
Voids
Air
Water
Solids
The spaces between the solids ( solid particles) are called voids.
Water is often the predominant liquid and air is the predominant gas.
We will use the terms water and air instead of liquid and gases.

Soils can be partially saturated (with both air and
water present), or be fully saturated (no air
content) or be perfectly dry (no water content).
In a saturated soil or a dry soil, the three-phase
system thus reduces to two phases only, as shown.
Three Phases System

Three Phases System
Partially saturated soil
Solid Particles
Voids (air
or water)
Idealization:
Three Phases Diagram
Water
Air
Solid Particles

Two - Phases System
Fully saturated soil
Solid Particles
Idealization:
Two Phases Diagram
Water

Two - Phases System
Dry soil
Idealization:
Two Phases Diagram
Air
Solid Particles

The soil model is given dimensional values for the solid, water
and air components.
Weight SymbolsVolume Symbols
Va
VS
VT
VW
VV
WT
WSWWWa≈0
Phase Relations of Soils
Water
Air
Solid Particles

For the purpose of engineering analysis and
design, it is necessary to express relations between
the weights and the volumes of the three phases.
The various relations can be grouped into:
Weight relations
Volume relations
Inter-relations
Three - Phases System

WT
WSWWWa≈0
Water
Air
Solid Particles
Weight Relations
= +
where,
(1-1)
= ℎ
= ℎ
= ℎ
= ℎ ≈ 0
The following are the basic weight relations:
water content or moisture content
specific gravity (Gs)

Weight Relations
Water content
The ratio of the mass of water present to the mass
of solid particles is called the water content ( ), or
sometimes the moisture content.
% =

100% (1-2)
The water content of a soil is found by weighing a sample
of the soil and then placing it in an oven at
until the weight of the sample remains constant , that is, all
the absorbed water is driven out.
110 ∓ 5 !

Weight Relations
Specific Gravity,
The mass of solid particles is usually expressed in
terms of their particle unit weight or specific
gravity (Gs) of the soil grain solids
The specific gravity of a solid substance is the ratio
of the weight of a given volume of material to the
weight of an equal volume of water (at 20°C).
" =

=
# $
# $
=
#
#
(1-3)
# = %& ℎ = 9.81
*+
,

For most inorganic soils, the value of Gs lies
between 2.60 and 2.80.
The presence of organic material reduces the
value of Gs.
Weight Relations
Specific Gravity,
The specific gravity of soil solids is often needed
for various calculations in soil mechanics.

The following are the basic volume relations:
Volume Relations
1. Void ratio (e)
2. Porosity (n)
3. Degree of saturation (S)
4. Air content (a)
Volume Symbols
Va
VS
VT
VW
VV
Water
Air
Solid Particles$ = $ + $ + $ (1-4)
$- = $ + $

Void ratio (e) is the ratio of the volume of voids
(Vv) to the volume of soil solids (Vs), and is
expressed as a decimal.
Volume Relations
Void ratio (e)
The void ratio of real coarse grained soils vary
between 0.3 and 1. Clay soils can have void ratio
greater than one.
=
$.
$
(1-5)

Porosity (n) is the ratio of the volume of voids to the
total volume of soil (Vt ), and is expressed as a
percentage.
Volume Relations
Porosity (n)
The range of porosity is 0 %< n < 100%
& 100% =
$.
$
100% (1-6)

Void ratio and porosity are inter-related to each
other as follows:
Volume Relations
Void ratio (e) & Porosity (n)
& =
$.
$/ + $.
=
$.
$/ 1 +
$.
$/
=
1 +
=
$.
$
=
$.
$ − $.
=
$.
$ 1 −
$.
$
=
&
1 − &
(1-7)
(1-8)

The volume of water (Vw) in a soil can vary
between zero (i.e. a dry soil) and the volume of
voids. This can be expressed as the degree of
saturation (S) in percentage.
Volume Relations
Degree of saturation (S)
Degree of saturation is the ratio of the volume
of water to the volume of voids.
1 100% =
$
$.
100% (1-9)

Volume Relations
Degree of saturation (S)
The degree of saturation tell us what percentage
of the volume of voids contains water .
For fully saturated soil, VV = VW, S =1 or 100%
For a dry soil, S = 0 and
For partially saturated soil 1<S<0
1 =
$
$.
$/
$/
=
1
#
#
=
"
(1-10)

Volume Relations
Air content (a)
The air content, a, is the ratio of air volume to
total volume .
The air- voids, Va , is that part of the voids
space not occupied by water
For a perfectly dry soil : a = n
For a saturated soil : a = 0
100% =
$
$
100% (1-11)
100% = & 1 − 1 (1-12)

Weight –volume relationship
Density is a measure of the quantity of mass in a
unit volume of material. Unit weight is a measure
of the weight of a unit volume of material.
Both can be used interchangeably. The units of
density are ton/m³, kg/m³ or g/cm³.
The unit of unit weight is kN/m³.
Unit weight ( )#

Unit weight ( )#
The unit weight of a soil is the ratio of the weight
of soil to the total volume.
# =

$
(1-13)
In natural soils the magnitude of the total unit
weight will depend on how much water happens to
be in the voids as will as the unit weight of the
mineral grains themselves.

Dry unit weight ( ) #2
The dry unit weight of a soil is the ratio of the
weight of solids to the total volume.
(1-14)#2 =

$
#2 =

$ 1 +
=
#
1 +
=
# "
1 +
(1-15)
# =

$
=
1 +
$
= #2 1 + (1-16)
The dry unit weight can also be determined as

Saturated unit weight ( ) #
For a saturated soil, the unit weight becomes
(1-17)
(1-18)
# =

$
# =
1 +
$ 1 +
=
# 1 + "
1 +
=
# " 1 + "
1 +
# =
# " +
1 +

Submerged unit weight ( ) # 34
The submerged unit weight of the soil is
given as
(1-19)# = # 34 + # # 34 = #5
= # − #
G.W.T
Ground SurfaceS = 0
S =( 0 to 1)
S = 1
#2
#
#
# 34

Use
Summary
In summary, for the easy solution of phase
problem, you don’t have to memorize lots of
complicated formulas. Most of them can easily be
derived from the phase diagram. Just remember
the following simple rules:
1. Remember the basic definitions of properties
2. Draw a phase diagram
3. Assume either VS = 1 or VT = 1.

Worked Examples
Example 1
An undisturbed sample of saturated clay has been
found to have a moisture content of 24 %. The
specific gravity of the solid particles was
determined as 2.7. By deriving any
relationships needed using the basic definitions
and a phase diagram for this soil, determine the
void ratio and the bulk unit weight.

Worked Examples
Solution of example 1
Vt =1+e
Volume
Solid
Water e
Vs =1
Weight
GS γw
e γw
(GS +e) γw

Worked Examples
Solution of example 1
e = 0.24 * 2.7 = 0.648
γ = (2.7 + 0.648) 9.81/(1+0.648)
γ =19.93 kN/m3
1 = 1 =
"
# =
# " +
1 +

Use
Worked Examples
Example 2
Prove the following relationships:
#2 = 1 − & # "
# = " − & " − 1 #
( ) =
& #
# − & #
" =
#
# − # − #
a)
b)
c)
d)

A soil has void ratio = 0.72, moisture content = 12%
and Gs= 2.72. Determine its
(a) Dry unit weight
(b) Moist unit weight, and the
(c) Amount of water to be added per m3 to make the
soil saturated.
Use
Worked Examples
Example 3

The dry unit weight of a sand with porosity
of 0.387 is 15.6
Find the void ratio of the soil and the specific
gravity of the soil solids
Worked Examples
Example 4
*+/ ,

Worked Examples
A cubic meter of soil in its natural state weighs
17.75 kN; after being dried it weighs 15.08
kN. The specific gravity of the solids is 2.70.
(a) Determine the water content, void ratio,
porosity and degree of saturation for the soil as
it existed in its natural state.
(b) What would be the bulk unit weight and
water content if the soil were fully saturated at
the same void ratio as in its natural state ?
Example 5

Worked Examples
Example 6
For a given soil , the following are given : GS = 2.67;
wet unit weight ; γ = 16.8 kN/m³ moisture content
WC = 10.8 % . Determine :
1. Dry unit weight
2. Void ratio
3. Porosity
4. Degree of saturation

Worked Examples
Example 7
For a soil ; given γd = 16.8 kN/m3 ; e = 0.51,
determine:
1. Specific gravity
2. Saturated unit weight
3. Unit weight when the degree of saturation is 45%.
4. Saturated water content
5. Porosity.

Worked Examples
Example 8
Determine the weight of water (in kN) that must be
added to a cubic meter of soil to attain a 95 % degree
of saturation, if the dry unit weight is 17.5 kN/m³,
the moisture content is 4 % and the specific gravity is
2.65.

Worked Examples
A project engineer receives a laboratory report with
tests performed on marine marl calcareous silt). The
engineer suspects that one of the measurements is in
error. Are the engineer’s suspicions correct? If so,
which one of these values is wrong, and what should
be its correct value? ( Gs = 2.65 )
Given: γ = 18.6 kN/m^3 , wc = 40.08 %,
e = 1.18 , and S = 90 %
Example 9

Worked Examples
The bulk unit weight of the soil has been
measured as 19.17 kN/m³, the moisture content
as 25.3% and the Gs of the solid particles as
2.70. Calculate:
a) the degree of saturation, S.
b) the porosity, and
c) air content.
Example 10

Worked Examples
For a saturated soil; given
γd = 15.3 kN/m^3 ; and WC = 27 %; Determine:
1. Saturation unit weight
2. Void ratio
3. Specific gravity
4. Wet unit eight when the degree of saturation
is 50 %.
Example 11

A soil sample has a unit weight of 16.62 kN/m³
and a saturation of 50%. When its saturation
is increased to 75%, its unit weight raises to
17.72 kN/m³
Determine the voids ratio e and the specific
gravity Gs of this soil.
Worked Examples
Example 12

Lecture 1 introduction & properties of soil

Lecture 1 introduction & properties of soil

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