Soil Mechanics - I
Lecture # 3,4
Chapter # 1. Introduction to Soil Mechanics
(Part 2)
Prepared by:
Engr Mamoon Kareem
Department of Civil Engineering
Swedish College
Of Engg & Tech Wah Cantt.

Introduction to Soil Mechanics

Weathering of Rocks

Soil and its Types

Physical Properties of Soil
•
•
•
•
•
•
•
Color
Soil Structure
Particle Shape and Size
Specific Gravity
Soil Phases
Porosity
Void Ratio
•
•
•
•
•
•
Moisture Content
Degree of Saturation
Air Content
Consistency Limit
Particle Size Distribution
Relative Density


Significance: Identification Purposes
Colour depends upon:
 Type of soil mineral
 Organic content
 Amount of coloring oxides
 Degree of oxidation

Examples:
 Black color
 Green or Blue
 Red, Brown or Yellow
 Grey
Manganese Compound
Ferrous Compounds
Iron
Organic matter

Soil Structure is defined as the grouping or
arrangement of soil particles with respect to one
another.

Factors that affect the structure are:
 Shape and Size
 Mineralogical Composition
 Nature and Composition of Water

Structures in Cohesionless Soil
 Single Grained
 Soil particles are in stable position
 The shape and size distribution of soil particles and their relative
positions influence the denseness of packing.
 Irregularity in the particle shapes generally yields an increase in the
void ratio
 Honeycombed
 Relatively small sand and silt form small arches with chains of
particles.
 They can carry an ordinary static load because of large inter-particle
spaces.

Structures in Cohesive Soil
 Flocculent Structure:
 The clay minerals are extremely flaky in shape and have a large surface
area-to-mass ratio.
 Flocculated structure is developed when the edge of one clay particle is
attracted to the flat face of another
 Dispersed Structure:
 Develops when the edges and faces of the clay particles have similar
electrical charge
 Also develops as a result of remolding by the transportation process (manmade earth fills )

Different shapes:

Nomenclature of material (soil type) and range of sizes


The ratio of the unit weight of a substance, to the unit
weight of water at 4oC
How many times a substance (or material) is heavier than
water


Significance:
Used for determination and calculation of many
other soil properties ,as
 Particle size analysis by hydrometer test
 Porosity and void ratio
 Unit weight
 Critical hydraulic gradient
 Degree of saturation or zero air void

Specific Gravity of some Minerals and Soil types

Any homogeneous part of a soil mass different from
other parts in the mass and clearly separated from
them is called a phase.

Fundamental phases:
1. Solid phase,
2. Liquid phase
3. Gaseous or vapour phase.
4. Ice phase (in cold regions)

The ratio of volume of all the voids “Vv” to the total
volume of the soil mass “V” is known as the
porosity.
Porosity falls in the range of
0 n  100
Where
V = Vs + Vv
V = Total volume of soil mass
Vs = Volume of solid particles of soil
Vv = Volume of voids, which may be filled with air
or water or both
The ratio of volume of voids present in a soil mass to
the volume of solid particles.
 It is denoted by “e”.

Vv
volume of voids in soil
e

volume of slids in soil
Vs

The void ratio is expressed as a number and the
limiting values can be within the range.

The ratio of the volume of air present in the voids to
the total volume of a soil mass.
Va Vv  Vw
Av or A 

V
Vv  Vs
Since; Vv = Va + Vw

Air content or Air Void Ratio fall within the range of
0  A  100  percent.

The condition when voids are partially filled with
water is expressed by the degree of saturation or
relative moisture content. It is the ratio of actual
volume of water in voids “Vw” to the total volume of
voids “Vv”.
Vw
Ww
m
S 


Vv
Wv
msat
Ww – is the weight of water actually present in the voids.
Wv – is wt of water that can fill all the voids.
m – actual moisture content.
msat – moisture content when all voids are totally filled with water.
The range of “S” 0  S  100.

The amount of water present in the voids of a soil in
its natural state.
weight of water
m
 100
weight of dry soil
The common range of moisture content for most
soil is 20-40 percent.
 Oven dried soil has zero percent moisture and the
soils which appear dry (i.e., air dried soil) often have
2 to 4 percent moisture content.
 The range of water content is:


The moisture/water in the voids of a soil mass can
occur in a variety of forms. Depending upon the
form of occurrence they are given different names
e.g.,
 Hygroscopic Moisture
 Film Moisture
 Capillary Moisture
 Chemically Bound Moisture
Hygroscopic Moisture:
1.
 Also known as adsorbed moisture, contact moisture or surface




bound moisture.
This form of soil moisture exists as a very thin film of moisture
surrounding the surfaces of individual soil particles and is held
by the forces of adhesion.
It depends upon temperature and humidity.
It is not affected by gravitational forces, capillary forces and air
drying at ordinary ordinary temperature.
The approximate values of hygroscopic moisture for various
soils are as under:
123-
Sand
Silt
Clay
1-2 %
7-9 %
17-20 %
2.
Film Moisture:
 The moisture film attached to the soil particles, above the
layer of hygroscopic moisture film, is known is film
moisture.
 It is held by the molecular forces and is not affected by
gravity.
 The amount of film moisture depends on the specific
surface i.e., higher the specific surface higher will be the
film moisture and vice versa.
3.
Capillary Moisture:
 The moisture which in held within the voids of capillary
size. The capillary moisture is continuously connected to
the groundwater table.
 Capillary water can be removed from the soil by drainage
4.
Chemically Bound Moisture:
 Moisture contained chemically within the mineral particles
and can be removed only by chemical processes of the
substance when the crystalline structure of the mineral
breaks.
 Chemically bound moisture is not important for common
soil engineering problems and therefore is not
determined.

The percentage of various particle sizes present in a
soil is known as particle size distribution or
gradation.

Particle size analysis is made by sieving or by
sedimentation.
 Sieving method – when particle size > .074 mm
 Sedimentation method – when particle size < .074mm

The sieves normally required are as follows:

Significance:
 Engineering classification of soils.
 Selection of the most suitable soil for construction of
roads, airfields, levees, dams and other embankments.
 To predict the seepage through soil (although
permeability tests are more generally used)
 To predict the susceptibility to frost action.
 Selection of most suitable filter material.

The gradation curve:

A gradation curve is drawn by plotting the percentage finer
(%age passing) on ordinate against the particle sizes on
abscissa.

The gradation curves indicate the type of soil, and provide
very important information related to the properties and
behavior of soil

The gradation curves have great importance in civil
engineering and are extensively used for the following
purposes.
 Determination of Effective Grain (Particle) Size.
 Determination of Uniformity co-efficient.
 Determination of co-efficient of Curvature.
 Determination of percentage of different soil types in a soil




sample e.g., sand, silt, clay.
Determination of percentage larger or finer than a given size.
Classification of soil.
Design of filters.
Concrete mix design.

Well-Graded Soil:

A soil containing an assortment of particles with a wide
range of sizes.
A well-graded soil has following merits:

1. Higher shear strength 2. Higher density 3. Reduced Compressibility 4.
Higher stability 5. Higher Bearing Capacity 6. Low permeability
well graded
uniformly graded
Ideal packing, due to particles
Loose packing, as smaller
ranging from large to small
particles to fill voids are
sizes
missing

Uniformly-Graded Soil:

A uniformly graded soil is defined as a soil containing
particles having a limited range of sizes (Almost the same
sizes)

Poorly-Graded Soil:
A poorly graded soil is defined as a soil containing particles
of varying sizes with intermediate particle sizes missing.
 Such soils give lower density and lower strength.
 The gradation curve of a poorly graded soil show steps
indicating an excess of certain particle sizes, and a deficiency
of others

 The gradation curves:
a) well graded soil
b)
b) poorly graded soil.
uniformly graded soil

Co-efficient of uniformity:

When the value of Cu is less than 4, the soil is generally
considered as uniformly graded.

A higher value of Cu represents a wide range of particle sizes
and the soil is termed as well graded.
Cu
D60

D10

Co-efficient of curvature:

It is also known as coefficient of gradation (Cg) or
Co-efficient of Concavity.
( D30 ) 2
Cc 
( D60 )( D10 )
 Cc = 1, represents that all the soil particles have the same size, and the
soil is uniformly graded.
 Cc between 0.2 and 2.0 indicate well graded or poorly graded soil.
The term relative density (also called density index,
ID) is used to express the state of compactness of a
granular soil.
 The following relationship between the void ratio
values is termed as the relative density.


The range of values for relative densities (Dr) and
the commonly referred state of compaction for
granular soil.

The consistency of a soil means its physical state
with respect to the moisture content present that
time.

Consistency states are:
1. Solid state
2. Semi solid state
3. Plastic state
4. Liquid state.

Boundaries of the above four states are:
 Shrinkage Limit: It is the moisture content at which a soil
changes from solid state to semi-solid state.
 Plastic Limit: It is the moisture content at which a soil
changes from semi-solid state to plastic state.
 Liquid Limit: It is the moisture content at which a soil
changes from plastic state to liquid state.
1.
Shrinkage Limit
 It is that moisture content at which a reduction in moisture will not
cause a decrease in the total volume of soil mass, but an increase in
moisture will result in an increase in volume of soil mass.
 At Shrinkage Limit The Degree Of Saturation is 100%.
 At certain point during drying process, air begins to enter the soil
mass and the volume decrease becomes appreciably less than the
volume of water lost.
 The shrinkage limit is not given much importance since it is not used
in soil classification.
1.
Shrinkage Limit
 Concept of surface tension forces and induced compressive stresses
(a) Particle separated due to thick moisture film
(b) Meniscus contracting due to drying process
(c) Meniscus tending to tear off
(d) Meniscus fully torn off allowing air entry

Relationship between volume and moisture content:
The soils which show higher shrinkage upon drying also swell more upon
wetting and are known as expansive soils. Expansive soils are very dense
and hard in dry state due to very high shrinkage stresses
Shrinkage cracks at Rawal lake which dried due to drought
Plastic Limit
2.

The moisture content at which a soil can be rolled into
threads of 1/8” (3.2mm) diameter without cracking and
crumbling.

Threads thinner than 1/8” (3.2 mm) diameter are
possible, if the moisture is higher than the plastic limit.

And if the moisture is less than plastic limit the thread
will crumble before reaching the required diameter of
1/8” (3.2 mm).
2.
Plastic Limit
Liquid Limit
3.

The moisture content at which 25 blows of Cassagrande
apparatus closes a standard groove cut in the soil paste
along a distance of 12.7 mm (0.5 in).

The moisture content which gives a penetration depth of
20mm of the standard cone (fall cone test) into the soil,
when the cone is released for 5 seconds.
3.
Liquid Limit

Plasticity Index
 Plasticity Index indicates the range of moisture through
which a cohesive soil behaves as a plastic material
 It is the numerical difference between liquid and plastic
limits. It is expressed as:

Range of Plasticity Index
 P.I. = 0
The soil is non-plastic and non-cohesive.
 P.I. < 7
The soil is low plastic and partly cohesive.
 P.I. 7 - 17
The soil is medium plastic and cohesive.
 P.I. > 17
The soil is highly plastic and very cohesive.

Change of liquid, plastic and shrinkage limits with plastic properties (not to
scale, just to show comparison).

Liquidity Index
 The ratio of difference between the moisture content and
plastic limit to the plasticity index.
m  P.L
m  P.L
L.I 

L.L  P.L
P.I
 L.I < 0, (i.e. negative value) the field moisture content is
less than the plastic limit, and hence the soil is in a semisolid state.
 Consistency of a soil at its natural moisture content:
▪ L.I < 0,
the soil is in a semi-solid or solid state (hard)
▪ 0.00 < L.I ≤ 0.25,
the consistency is stiff or hard
▪ 0.25 < L.I ≤ 0.50,
the consistency is medium
▪ 0.5 < L.I ≤ 0.75,
the consistency is soft
▪ 0.75 < L.I ≤ 1,
the consistency is very soft
▪ L.I > 1,
the soil is in a liquid state

Flow Index
 The slope of the flow curve (graph between log N and moisture
content drawn for the determination of liquid limit) is known as the
flow index and is equal to:
F.I =
F.I =
 Any two soils, although having the same plasticity indices and/or the
liquid limits may have different values of flow index, and hence may
possess varying degree of cohesiveness and shear strength.

Flow Index
 The slope of the flow curve (graph between log N and moisture
content drawn for the determination of liquid limit) is known as the
flow index and is equal to:
F.I =
F.I =
 Any two soils, although having the same plasticity indices and/or the
liquid limits may have different values of flow index, and hence may
possess varying degree of cohesiveness and shear strength.
Case-I: Two soils having the same values of plasticity index
No. of blows are indicative
of the resistance to
deformation or shear
strength. For the same
drop of moisture ∆m, the
No. of blows for flat curve
increase very much,
indicating higher shear
strength. Therefore, the
soils with same plasticity
index may posses
different shear strength.
Case-I: Two soils having the same values of plasticity index
No. of blows are indicative
of the resistance to
deformation or shear
strength. For the same
drop of moisture ∆m, the
No. of blows for flat curve
increase very much,
indicating higher shear
strength. Therefore, the
soils with same liquid limit
may posses different
shear strength.

Toughness Index
 Soils having same values of plasticity indices may vary in
toughness. This property of a soil is expressed by the
toughness index.
 Toughness and dry strength increases with increase in
toughness index.
T ..I . 
P.I .
F .I .
(1.31)
Thank You … for paying your attention