ME3OW: SOIL AND WATER
ENGINEERING
COURSE
INTRODUCTION
Details of Lecturer
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Course Lecturer: Dr. E.I. Ekwue
Room Number: 216 Main Block,
Faculty of Engineering
Email: ekwue@eng.uwi.tt ,
Tel. No. : 662 2002 Extension 3171
Office Hours: 9 a.m. to 12 Noon. (Tue,
Wed and Friday)
Course Outline

Soil constituents, texture, structure and
plasticity.
Phase relations.
Soil water
content and potential. Soil compression,
strength
and
stress-strain
relations.
Prediction of forces on soil engaging tools.
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Hydrologic cycle. Rainfall measurement and
analysis. Stream flow measurement. Runoff
analysis. Open channel flow and channel
design for steady uniform flow. Introductory
ground water hydrology.
Computer
applications.
Course Objectives
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On Completion of this course, students
should be able to:
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(i) Understand the basic engineering
properties of soils and how these affect the
trafficability of soils.
(ii) Understand the principles related to the
occurrence of water on the earth's surface
and how this affects the flow of water in soils
thereby
affecting
the
strength
and
deformation behaviour of soils.
(iii) Predict forces required to pull
mechanical implements through the soil.
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Course Objectives Contd.
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(iv) Measure hydrologic processes and be
able to analyze problems relating to these
processes like rainfall and water runoff.
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(v) Design channels required for irrigation,
drainage, soil conservation, flood control and
other water-related projects.
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(vi) Have a basic understanding of the
occurrence, flow and yield of groundwater
Teaching Strategies
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The course will be taught via
Lectures.
Lectures will also
involve the solution of tutorial
questions. Tutorial questions are
designed to complement and
enhance both the lectures and the
students appreciation of the
subject.
Course work assignments will be
reviewed with the students.
Lecture Times
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Monday: 11.10 a.m. to 12.00 noon
Tuesday: 1.00 to 1.50 p.m.
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Attendance at the Lectures is Compulsory.
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Special Lab Sessions and Tutorials may be
arranged.
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Reading Material
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(i) Smith, G.N. (1990). Elements of
Soil Mechanics, Blackwell Science,
6th Edition or latest Edition.
(ii) Wanielista, M., Kersten, R., and
Eaglin, R. (1997). Hydrology, Water
Quantity and Quality Control, John
Wiley, 1st Edition.
(iii) Course comprehensive note
book and other handouts and
tutorial sheets.
COURSE WORK
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1. One Mid-Semester Test (20%);
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2. End of Semester 1 Examination
(80%).
CHAPTER ONE
INTRODUCTION TO SOIL
PHYSICS
1.1 RELEVANCE OF SOIL
PHYSICS IN ENGINEERING
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The objective of studying Soil Physics
in Engineering is to present the basic
scientific principles influencing the
nature and behaviour of soils as well as
to discuss those management aspects
which are particularly relevant to the
soil design Engineer.
Food Production Chain
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Looking at the food production chain, two food types
exist:
(a) From plants – carbohydrates, proteins etc;
(b) Animal Food
Plants have roots and the roots are in the soil.
Development of plants depends on three soil factors:
(a) Soil Type: texture (the relative sizes of soil
mineral components)
(b) Soil Structure: how soils are bonded together
(c) Soil Fertility: Largely depends on soil organic
matter
Soil Texture and Structure
Introduced
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Soil texture and soil structure are very important
because they determine the kind of crops to be
grown, whether there will be erosion, whether to
apply water to crops, the kind of machinery to use
and so on.
Organic matter from plants and animals helps to
build up or improve soil structure.
Organic matter also helps to improve the water
holding capacity of the soil.
The soil type can be controlled using farm
machinery.
Soil structure can be changed with soil machinery
. Soil structure studies is important in order to determine the
1.1 SOIL CONSTITUENTS
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There are Four basic Soil Constituents:
(a) Mineral Material: Sand, clay and silt
(b) Organic matter - very valuable
(c) Water
(d) Air
SOIL CONSTITUENTS
Soil Constituents
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The mineral matter and organic matter make up the
solid contents of the soil while the water and the air
represent the soil pores.
From a structural point of view, air is irrelevant but it
helps in aeration for plant growth.
In an average agricultural soil, half will be solid
solids, 45% minerals and about 5% organic matter.
The other 50% which is called soil pores or voids
will occupy 50%.
Voids are dynamic and we can get 20 – 30% air and
30 – 20% water by volume.
Excess water will lead to soil saturation and will
cause death of plants as poor aeration is obtained.
PROPORTIONS OF SOIL
CONSTITUENTS
20%
45%
30%
5%
MINERALS
OM
Water
Air
Mineral (Inorganic) Component of the
Soil
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Except in the case of organic soils, most of a
soil’s solid framework consists of mineral
particles.
(a) They are variable in size and
composition. They can vary from small rock
particles to colloids.
(b) The mineral can be raw (quartz and other
primary materials – coarse fractions which
have not changed from parent material)
(c)They can also be silicate clays and iron oxides
formed by the breakdown and weathering of less
resistant minerals as soil formation progressed.
These are called secondary minerals.
Mineral Component Contd.
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The mineral particles present in soils are
extremely variable in size.
The larger soil particles, which include
stones, gravel and coarse sands, are
generally rock fragments of various kinds.
Excluding the larger rock fragments such as
stones and gravel, soil particles range in size
over four orders of magnitude: from 2 mm to
less than 0.0002 mm in diameter.
Sand Particles
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(a) Sand particles are visible to the
naked eye and vary in size from (2.0 to
0.05 mm).
They are gritty when rubbed between
fingers. Sand particles do not adhere
to one another and are therefore not
sticky.
Silt and Clay Particles
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(b) Silt particles are smaller than sand and
range from (0.05 to 0.002 mm).
The silt particles are too small to be seen
without a microscope. It feels smooth but
not sticky, even when wet.
(c) Clays are the smallest class of mineral
particles (< 0.002 mm).
They adhere together to form a sticky mass
when wet and form hard clods when dry.
Classification of Soil Constituents
USDA
ISSS
ROCKS
> 2 mm
> 2 mm
SAND
0.05 to 2 mm
0.02 to 2 mm
SILT
0.002 to 0.05
mm
< 0.002 mm
0.002 to 0.02
mm
< 0.002 mm
CLAY
Soil Texture
This refers to the relative proportions of the
various soil separates (sand, silt and clay) in
a soil.
Terms such as sandy loam, silty clay, and
clay loam are used to identify soil texture.
The various soil components can be
separated using Mechanical Analysis.
Sand can be separated by sieving.
For silt and clay, separation is by the rate
of settling which can be obtained in a pipette
or hydrometer analysis.
Soil Texture Contd.
They are put in a solution and the time of
settling is noted.
From
the
Mechanical
analysis,
the
proportions of sand, silt and clay are obtained.
The actual soil texture is determined using
the Soil Textural Triangle ( Fig. 1.1.)
e.g. for a soil with 50% sand, 20% silt and
30% clay, the texture is Sandy Clay Loam.
1.1.1 COLLOIDAL
MATERIAL
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The smaller particles (< 0.001 mm) of clay (
and similar sized organic particles) have
colloidal properties and can be seen with an
electronic microscope.
The colloidal particles have a very large area
per unit weight so there are enough surface
charges to which water and ions can be
attracted.
These charges make them adhere together.
Humus improves the water holding capacity
of the soil.
WATER
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Water is held in the soil at various degrees of
tenacity which varies with the amount of water
present.
The tension force of water in unsaturated soils has
been described by several expressions such as soilpull, the force of suction and capillary tension.
Suction can be defined as the force per unit area
that must be exerted to remove water from the soil.
Suction or tension is measured in bars.
When the soil is saturated or nearly so, the amount
of suction is almost zero, but as the soil water
depletes, greater amount of energy must be applied
to extract water.
The soil can be initially saturated and if it is drained,
field capacity can be reached. Some water is still
held by surface tension.
Field Capacity
Field Capacity is greatest amount of
water the soil can hold under drainage.
For most soils, it is obtained after two
days of drainage after the soil was
saturated by heavy rain or irrigation.
It is the optimum amount of water
needed for agriculture.
Permanent Wilting Point and Available
Water
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Below Field capacity, the plant finds it more
and more difficult to extract water until the
suction or tension reaches 15 atmospheres
permanent wilting point is obtained, which is
the maximum tension the plant can exert on the
soil to extract water.
Available water is the difference between the
moisture contents at field capacity and
permanent wilting point.
Clay holds more water but the plants exert
more tension to extract water more than sand.
SOIL AIR
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(a)From a mechanical point of view, air is
irrelevant as it cannot support load. Soil air
is different from atmospheric air. It is found
in voids around the soil solids.
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(b)Soil air has a very high relative humidity
when compared with atmospheric air
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c) It has high carbon dioxide levels and low
oxygen levels in comparison with
atmospheric air
Aeration Effects
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(a) Aeration affects root development by three main
factors
(i) By oxygen content: There is restricted root
development if oxygen falls below 9%. If it drops to
less than 5%, root development ceases.
(ii) Carbon dioxide content: the roots will survive up
to 9 to 10%. The optimum is < 1%.
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By-Products of anaerobic decomposition: Hydrogen
sulphide is very toxic at low concentrations and kills
roots. Methane and hydrogen are all right at
reasonable concentration
SOIL STRUCTURE
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This is a field term that describes the
aggregation of the soil primary particles.
Managing soil is managing soil structure.
The soil structural types include:
(a)Massive Structure: No aggregation e.g.
big block of clay or mass of sand grains. In
clay, the more the dryness, the deeper the
cracks from the surface and the following
structure is obtained:
Soil Structural Types Contd.
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(b) Prismatic or Columnar Structure :
Caused by vertical cracking
If there is secondary horizontal cracks, the
following structure is obtained:
(C) Blocky Structure:
(b) If the blocky structure is worked upon by
man, the spheriodal or granular or crumb
structure is obtained.
(c) Platelike Structure
Methods of Describing Soil
Structure:
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1.3.2
Describing structure is difficult but the
following methods can be used:
(i) Virtual Account using the above terminologies
and stating size ranges can be adopted. Absolute
quantification is not possible.
(ii) Clod Size Distribution:
Especially using
photographs. The sizes can be read using a scale.
Aerial, digital or stereo photographs can also be
used.
(iii) Dry Sieving: This is aggregate grading and can
only be adopted for dry soils as wet soils cannot be
dry-sieved. Obtain a grading curve.
Stack of Sieves For Dry Sieving
Aggregate Stability:
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This is practically very important due
to the following effects:
(i) Rainfall
(iii) Tillage tools and wheels
(iv) Water tables
There is a threshold value above which
it is impossible to have aggregate
stability. Crops need small aggregates
so that they can be stable.
Tests For Aggregate Stability
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(i) Wet Sieving:
Preparing soil
aggregates of a given size e.g. 4 – 5
mm, putting them on a set of sieves
and agitating in water. The percentage
of soil aggregates that are retained on
each sieve are obtained so that water
stable aggregates > a particular sieve
size e.g. 5 mm can be computed.
Tests For Aggregate Stability Contd.
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(i) Drop Tests: Water drops, say 3 mm
diameter are dropped on a soil aggregate of
a given size say 4 to 5 mm placed on a
sieve of about 3 mm size.
The number of water drops needed to break
down each aggregate so that it can be
washed through the sieve size is used as an
index of water aggregate stability. Many
replications are needed for each soil.
(ii) Childes/Haines Method: Flood with
water from below and compare for various
soils. This is not a popular method.
SOIL PHASE RELATIONS
Air
Va
Vp
Water
Solids
Vw
V
Vs
Va is the Volume of air, Vw is the Volume of water
Vs is the Volume of solids, Vp is the Volume of pores (water & air)
V is the total volume (air, water and solids)
Definitions:
Density of Solid
Particles, Dp (Particle density):
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(a) It is defined as the mass per unit volume of soil
solids usually expressed as g/cm3 or Mg/m3 .
i.e. Particle density = Ms/Vs where Ms is the mass
of dry soil
Particle density is essentially the same as the
specific gravity of a soil substance and varies within
a narrow range of 2.60 to 2.75 g/cm3 with the
average being assumed as 2.65 g/cm3 for a typical
soil with 1 to 5% organic matter content.
Montmorillonite can have a particle density of 2.74
g/cm3, quartz , 2.65 and kaolinite, 2.61 g/cm3.
(b) Bulk Density, Db:
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It is defined as the mass of a unit volume of dry soil.
This includes both solids and pores. i.e. bulk
density = Ms/V ; Ms is the mass of dry soil and V is
the total volume of undisturbed soil.
The major method of measuring bulk density in the
field is to collect a known volume of undisturbed soil
(V) in a soil core, and drying it in the oven to
remove all the water to obtain Ms. The values of
bulk density range from 1.0 for loose open soil to 1.7
g/cm3 for compacted soil.
Values of bulk density are mainly affected by soil
texture (sandy soils have more density than silty and
clay soils), degree of soil aggregation and is reduced
by soil organic matter content.
c) Porosity
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It is defined as the proportion of the volume
of soil pores (air and water) in comparison
with the total volume of soil.
i.e. Porosity, P = Volume of Pores, Vp / V
It normally ranges from 0.2 (20%) to 0.6
(60%).
One of the main reasons for measuring soil
bulk density is that this value can be used to
calculate soil porosity.
For the same particle density, the lower the
bulk density, the higher the porosity.
Derivation of Formula Used to
Calculate Porosity of Soil
By definition: Particle density, Dp 
Ms
Vs
and Bulk density , Db 
Solving for Ms gives: Ms = Dp x Vs
Therefore: Dp x Vs
`Since;
Ms
Vs  Vp
and Ms = Db (Vs + Vp)
= Db (Vs + Vp) and
Vs
D
 solid space then solid space  b
Vs  Vp
Dp
Since: Pore space + Solid space = 1
(Porosity) = 1 – Solid space , then:
Db
Porosity, P  1 
Dp
Vs
D
 b
Vs  Vp Dp
and
Pore space
More Definitions of Soil Phase
Relations
(d) Void Ratio (e): It is defined as the volume of
Pores (Vp) divided by the volume of solids, Vs. It
can be shown that:
e
P
1 P
and
P
e
1 e
(e)
Degree of Saturation, S: This is the volume of
water divided by the volume of voids. A soils is said to
be saturated when all pores are filled with water i.e.
when:
Vw = Vp
Soil Wetness
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This can be expressed by mass or by
volume.
(i) By Mass (Pm) – gravimetric system: This
is equal to:
Weigh soil, put in oven at 105 oC for about
48 hours; weigh again and obtain the weight
of water by subtraction.
A good soil should have moisture contents
between 5 and 60% and for peat or organic
soils, it can be greater than 100%.
Soil Wetness by Volume
(ii) Volumetric water content, Pv. This is equal to:
Pv 
Vw
Vw


Vs  Va  Vw
V
Volume of water
Total volume of undisturbed soil sample
Recall that volume = mass/density i.e.
Pv 
M w / Dw
M s / Db
and
Pv 
Mw
x
Ms
Db
Since Dw = 1
i.e.
Pv = Pm x Db
where Db is the bulk density of the soil.
Example
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A moist sand sample has a volume of
450 cm3 and a wet mass of 786 gm.
The particle density is 2.65 g/cm3 and
the dry mass is 731 gm. Determine the
void ratio, porosity, percentage water
content and the degree of saturation.
Solution:
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Mass of water = 786 – 731 = 55 gm
Volume of water = Mw/Dw = 55 cm3,
assuming Dw = 1gm/cm3
Mass of solids = 731 gm and Volume of
solids = Ms/Ds = 731/2.65 = 275.85 cm3
Volume of air, Va = V – Vs – Vw = 450 –
275.85 – 55 = 119.15 cm3.
Volume of pores, Vp = Vw + Va = 55 +
119.15 = 174.15 cm3
Solution Concluded
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(a) Void ratio, e = Vp/Vs = 174.15/275.85
= 0.63
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(b) Porosity, P = Vp/V = 174.15/450 =
0.30
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c) Percentage water content = Mw/Ms =
(55/731) x 100 = 7.52%
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(d) Degree of saturation = Vw/Vp = 55 / 175.15
= 31.4%
SOIL PROPERTIES AND
MOISTURE CONTENT
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Moisture content is a critical factor for soil
behaviour.
In a soil tillage context, it is not a good
measure e.g. 40% moisture content does not
say anything about the degree of wetness.
For clay, the soil may be moist at 40%
moisture content, but for sandy loam soil, it
can be flowing as a liquid.
Soil tension is also not useful.
Soil Consistency
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Soil consistency is the best parameter for
predicting soil behaviour during tillage
operations.
For instance, all soils behave in the same
way at the same consistency limit.
If the plastic limit of the soil is 35% and it
is cultivated at 30%, then the condition is
friable.
Soil Consistency Contd.
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Soil Consistency describes how a soil
behaves in contact with water.
It can be referred to as rheology.
Consistency is the nature and
behaviour of a soil.
It states whether the soil is hard,
friable, firm, plastic or liquid when it
comes in contact with water.
Diagram Depicting Soil Consistency
Very
Hard
Increasing Moisture Content
Cemented
Harsh
Hard
Shrinkage
limit
Friable
Very Wet
Sticky
Plastic Liquid
Mud
Upper Plastic
Limit (Liquid Limit)
Lower Plastic
Limit (Plastic Limit)
Soil Consistency
Soil Consistency Contd.
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The soil can behave as a solid or fluid
depending on moisture content.
When dry, the soil has a cemented, harsh or
hard consistency.
When very wet, it is sticky, liquid and muddy.
The two extremes are not good for soil
cultivation, one too wet and the other too dry.
In-between is better.
First, there is the friable zone in which the
soil can be manipulated easily. It is the ideal
soil working zone.
Soil Consistency Concluded
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Most soils with clay have plastic limits
between the friable and sticky or liquid
zones.
Sand has not got any plastic limit because
sand particles are inert and have no links
with water.
Same goes for silt.
For a soil with up to 10% clay content, clay
begins to dominate soil behaviour and
plasticity sets in.
Atterberg Limits
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Atterberg defined limits of the plastic limits as
follows:
(a) Lower Plastic Limit: (LPL): Also called Plastic
Limit. Significant water has been added to provide a
film around each particle. Film cohesion is at a
maximum just above the lower plastic limit. It is the
minimum moisture content at which the soil may be
puddled.
In the laboratory, the LPL or plastic limit is
determined by adding soil moisture and at each
point, rolling the soil out and remoulding up until it
breaks out at 10 mm length and 3 mm diameter. For
higher moisture content, it can be longer.
Diagrams For Lower Plastic Limit
Cohesion
LPL
UPL
Moisture Content
3 mm diameter
10 mm
Upper Plastic Limit (UPL)
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This is also called liquid limit. This
signifies the moisture content at which
the water films are so thick that the film
cohesion is decreased and the soil
mass can flow under an applied load.
The point of maximum adhesion
called the sticky point is almost the
same as the UPL.
Diagram of Upper Plastic Limit
Force
Sticky Point
Adhesion Lubrication
Friction
Phase
Moisture Content
Determination of Upper Plastic
Limit
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(i) Drop Cone Penetrometer: The soil is prepared
in a core. A cone penetrometer is dropped and
allowed to penetrate the soil. The UPL is the soil
state or condition where the depth of penetration is
20 mm measured with standard methods.
(ii) Cassangrande Apparatus: The soil is put
into a cup.
A V-shaped
groove of given
dimensions is made on the soil and the cup is
cranked so that it falls through 1 cm onto a rubber
base, twice per second. The impact blows causes
the soil to flow and close the groove. The UPL is
reached when the soil is closed a distance of 12.5
mm by 25 blows.
Liquid Limit Apparatus
Plasticity Number or Index
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It is the difference between the upper and
lower plastic limits.
It gives the range of soil moisture where the
soil will behave plastically.
The larger the plasticity index, the smaller
the apparent friable zone.
If it is ten percent for instance, the soil is
good for cultivation.
If it is 100 to 150, then the soil is bad for
working.
Factors that Affect the Atterberg
Limits
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(a) Clay Content: The higher the clay
content, the higher the LPL, UPL and the
plasticity number.
Increase in plasticity
number is due to the greater rate of increase
of UPL more than LPL.
(b) Nature of Exchangeable Cations: The
ions linked to clay affect the orientation of
water molecules.
Some like phosphate lowers LPL as it
discourages thick oriented water layers while
others like sodium encourages thick water
layers and therefore increase the three
Atterberg limits.
Factors that Affect the Atterberg
Limits Concluded
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(c) Nature of the Clay Mineral: The nature
of the clay mineral affects the swelling and
shrinking nature of the clay and the Atterberg
limits. For montmorillonite, there is a lot of
swelling and shrinkling and the plastic limits
are very high in comparison with the nonswelling illite or kaoline clays.
(d) Organic Matter: For higher Organic
matter content of the soil, higher lower
plastic limits are obtained. The soil becomes
difficult to work as organic matter level falls.