lecture 1 introduction & properties of soil

INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY

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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 Material3. Slopes and Landslides4. Earth Retaining Structures5. 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

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�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, Pisa14Dr. Abdulmannan Orabi IUST

� 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 SoilsResidual 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 SoilsAs 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 SoilsWeathered 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 SoilsTransported 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 soilsare 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 Symbols Volume Symbols

Va

VS

VT

VW

VV

WT

WS

WW

Wa ≈

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

WS

WW

Wa ≈

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

Watere

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 15.6Find 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/m³Determine the voids ratio e and the specific gravity Gs of this soil.

Worked Examples

Example 12
