SOIL MECHANICS
By
Eng. Albert Niyonzima (MSc)
A.Lecturer at UR, College of Science and Technology.
Indicative content
Chapter I: Introduction
Chap II: Soil Phase Relationship, Index
properties & soil classification
Chap III: Soil Compaction
Chap IV: Flow of water through the soil
Chap V: Effective stress and pore water
pressure
Indicative content
• Chap VI: Stress distribution in soils due to
surface loads
• Chap VII: Compressibility and consolidation
• Chap VIII: Shear strength of soils
• Chap IX: Lateral earth pressure
Core Text
1.
“PRINCIPLES
AND
PRACTICES
OF
SOIL
MECHANICS
AND
FOUNDATION ENGINEERING BY V.N.S MURTHY”.
2.
AN
INTRODUCTION
TO
THE
MECHANICS
OF
SOILS
AND
FOUNDATIONS BY JOHN ATKINSON
3.
PRINCIPLES OF GEOTECHNICAL ENGINNERING 5TH EDITION (BY
BRAJA M.DAS)
4.
PRINCIPLES OF FOUNDATION ENGINEERING 7TH EDITION (BY
BRAJA DAS)
ETC.
Assessment Pattern
1. Continuous Assessment: 50pts
– Quizzes/10
– Cats/20
– Assignments & Lab reports/20
2. End of Trimester Examination: 50pts
Chap I Introduction to soil
mechanics
• Soil Mechanics is defined as the branch of engineering science
which enables an engineer to know theoretically or
experimentally the behavior of soil under the action of ;
1. Loads (static or dynamic),
2. Gravitational forces,
3. Water and,
4. Temperature.
• According to Karl Terzaghi, Soil Mechanics is the applications
of Laws of Hydraulics and Mechanics to engineering problem
dealing with sediments and other unconsolidated accumulations
of solid particles produced by Mechanical and Chemical
Disintegration of rocks.
Introduction
 Soil Mechanics is the branch of science that deals with study of
physical properties of soil and behavior of soil masses subjected to
various types of forces.
 Civil Engineer must study the properties of Soil, such as its origin,
grain size distribution, ability to drain water, compressibility, shear
strength, and load bearing capacity.
Soil
Mechanics
GeoTech.
Engg.
Civil Engg.
 Geotechnical Engineering is the sub discipline of Civil Engineering
that involves applications of the principles of Soil Mechanics and
Rock Mechanics to design foundations, retaining structures and earth
structures.
Introduction
Why we study Soil Mechanics?
 Virtually every structure is supported by soil or rock.
=>Those that aren’t either fly, float or fall over.
 Various reasons to study the properties of Soil:
1. Foundation
to
support
Structures
and
Embankments
2. Construction Material
3. Slopes and Landslides
4. Earth Retaining Structures
5. Special Problems
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
1. Foundation to support Structures and Embankments
• Effects of static loading on soil mass
 Shear failure of the foundation soil
 Settlement of structures
•




Stability criteria (Solution)
There should be no shear failure of the foundation soil.
The settlement should remain within permissible limits.
Firm Soil -> Spread Footing (Spread Foundation)
Soft Soil -> Pile Foundation (Vertical members
transferring load of structure to ground i.e. rock)
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
1. Foundation to support Structures and Embankments
• Effects of dynamic loading on soil mass
• For Design and construction of roads following
must be considered:
 Compaction Characteristics
 Moisture Variation
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
2. Construction Material
• Subgrade of highway pavement
• Land reclamation
• Earthen dam
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
3. Slopes and Landslides
 Major cause is the moisture variation resulting in;
• Reduction of shear strength
• Increase of moisture
• Increase in unit weight
• Excavation of trenches for buildings require braced
excavation.
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
4. Earth Retaining Structures
• Earth retaining structure (e.g., Retaining walls)are
constructed to retains (holds back) any material
(usually earth) and prevents it from sliding or eroding
away.
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
5. Special Problems
i. Effects of river water on soil mass
a) Scouring
Causes:
• Increased flow velocity due to obstruction
• Fineness of riverbed material
Stability criteria:
• The foundation of pier must be below the scour depth
ii. Land Erosion
Introduction
Why we study Soil Mechanics?
 Various reasons to study the properties of Soil:
5. Special Problems
iii. Effects of frost action on soil mass
• Reduction Of Shear Strength
• Settlement Of Structure In Summer
• Lifting Up Of Structure In Winter
Causes:
• Heaving (due to formation of ice lenses)
• Increase of moisture due to thawing (MELTING)
Introduction
• Soil is understood to be the weathered material in the
upper layers of the earth’s crust.
• The non-weathered material in this crust is denoted as
rock.
How is soil formed?
 Soils are formed by the process of weathering of the
parent rock.
 The process of weathering of the rock decreases the
cohesive forces binding the mineral grains and leads to
the disintegration of bigger masses to smaller and
smaller particles.
Introduction
 Weathering of rocks might be as a result of mechanical
disintegration or/and chemical decomposition.
 Mechanical weathering also known as physical weathering is
the disintegration or breakdown of rocks by mechanical agents
such as water, wind, and ice or glaciers.
 Chemical weathering (decomposition) transform hard rock
minerals into soft and easily erodible matter. The principal
types of decomposition are hydration, oxidation and
carbonation.
Introduction
General Soil Types
• The properties of the soil materials depend upon the
properties of the rocks from which they are derived.
• A brief discussion of the parent rocks is, therefore,
quite essential in order to understand the properties
of soil materials.
• A rock can be defined as a compact, semi-hard to
hard mass of natural material composed of one or
more minerals.
Introduction
Types of Soils (cont’)
 Soil types, based on geological and engineering view
points, are separately discussed below:
1. Geological consideration:
Geologist classify soil into two major categories:
residual soils and transported soil
i. Residual Soils:
When the rock weathering is faster than the
transport process
induced by water, wind and
gravity, much of the soil remains in
place. It is
known as residual soil.
Introduction
Types of Soils (cont’)
ii. Transported Soil:
a. Glacial Soil: This type of soil is developed,
transported and deposited by the actions of
glaciers. These deposits consists of rocks
fragments, boulders, gravels, sand, silt and clay in
various proportions (i.e., a heterogeneous mixture
of all sizes of particles).
b. Alluvial Soil: This type of soil (also known as
fluvial soil or alluvium) is transported and
deposited to their present position by streams and
rivers.
Introduction
Types of Soils (cont’)
c. Aeolian Soil: The soil transported by geological agent
‘wind’ and subsequently deposited is known as wind
blown soil or Aeolian Soil.
d. Colluvial Soil: A colluvial soil is one transported
downslope by gravity. There are two types of downslope
movement – slow (creep – mm/yr) and rapid (e.g.,
landslide)
e. Lacustrine and Marine Soil:
i. Lacustrine Soil is deposited beneath the lakes.
ii. Marine Soil is also deposited underwater i.e., in the
Ocean.
Introduction
Types of Soil (cont’)
2. Engineering consideration:
Introduction
Types of Soils (cont’)
2. Engineering consideration (MIT):
i. Clay: ( < .002mm)
– In moist condition, clay becomes sticky and can be
rolled into threads.
– High dry strength, low erosion, low permeability, good
workability and compaction under moist condition. Also
susceptible to shrinkage and swelling.
ii. Silt: (.002mm < Size < .06mm)
– High capillarity, no plasticity and very low dry strength
– It possesses properties of both clay and sand.
Introduction
Types of Soils (cont’)
iii. Sand: (.06mm < Size < 2mm)
– Particle shape varies from rounded to angular
– No plasticity, considerable frictional resistance, high
permeability and low capillarity
– Abundant quantities of sand are available in deserts
and riverbeds
Introduction
Types of Soils (cont’)
iv. Gravels: (2mm < Size < 60mm)
– They form a good foundation material.
– The gravels produced by crushing of rocks are
angular in shape while those taken from riverbeds
are sub-rounded to rounded.
v. Cobbles and Boulder:
– Particles larger than gravels are commonly known
as cobbles and boulders.
– Cobbles generally range in size 60mm t0 200mm.
– The materials larger than 200mm is designated as
boulders.
Chapter II.
Chap II Soil phase relations, index
properties and classification
• Soil mass is generally a three phase system. It consists of
solid particles, liquid and gas. For all practical purposes, the
liquid may be considered to be water (although in some
cases, the water may contain some dissolved salts) and the
gas as air.
• The phase system may be expressed in SI units either in
terms of mass-volume or weight-volume relationships.
• The inter relationships of the different phases are important
since they help to define the condition or the physical makeup of the soil.
Soil Phase Relations
Mass-Volume Relationship
• In SI units, the mass M, is normally expressed in kg and the
density ρ in kg/m3.
• Sometimes, the mass and densities are also expressed in g
and g/cm3 or Mg and Mg/m3 respectively. The density of
water at 4 °C is exactly 1.00 g/cm3 (= 1000 kg/m3 = 1
Mg/m3).
• Since the variation in density is relatively small over the
range of temperatures encountered in ordinary engineering
practice, the density of water ρ w at other temperatures may
be taken the same as that at 4 °C. The volume is expressed
either in cm3 or m3.
Soil Phase Relations
Weight-Volume Relationship
• Unit weight or weight per unit volume
𝜸𝜸 = 𝝆𝝆 ∗ 𝒈𝒈
• The 'standard' value of g is 9.807 m/s2 (= 9.81 m/s2
for all practical purposes)
Volumetric
ratios (Soil
Phase Relations)
Volumetric
ratio
• There are three volumetric ratios that are very useful in
geotechnical engineering and these can be determined
directly from the phase diagram below.
Fig.2.1. Block diagram—three phases of a soil element
Soil Phase Relations
Volumetric ratios
1. The void ratio, e is defined as :
Vv
where Vv is the volume of voids
e=
Vs
e is always expressed in decimal
2. The porosity n is defined as:
Vv
where V is total volume of the
*100%
n=
V
sample
3. The degree of saturation S is defined as:
Vw
*100%
S =
Vv
Where Vw volume of water. When S=0%=> soil is dry
and when S=100%=> is completely saturated
Soil Phase Relations
Soil Physical Parameters
1. Water content
• The water content, w, of a soil mass is defined as the
ratio of the mass of water, Mw, in the voids to the mass
of solids, Ms, as
• The water content, which is usually expressed as a
percentage, can range from zero (dry soil) to several hundred
percent.
Soil Phase Relations
2. Density
The density (or, unit weight) is expressed as mass per unit
volume,
 The total (or bulk)
ρt , or moist density,
 The dry density ρ d
 The saturated density, ρ sat
 The density of the particles, solid density, ρ s
 Density of water ρ w
Soil Phase Relations
2. Density (cont’)
• Dry density:
• Saturated density:
• Density of solids:
• Density of Water :
𝑀𝑀𝑆𝑆
𝜌𝜌𝑑𝑑 =
𝑉𝑉
ρ sat
M
=
V
ρS
MS
=
VS
ρW
MW
=
VW
for S = 100%
Soil Phase Relations
3. Specific Gravity
• The specific gravity of a substance is defined
as the ratio of its mass in air to the mass of an
equal volume of water at reference temperature
.
• The specific gravity of a mass of soil
(including air, water and solids) is termed as
bulk specific gravity Gm.
ρt
M
=
Gm =
ρ w Vρ w
Soil Phase Relations
3. Specific Gravity (Cont’)
• The specific gravity of solids, Gs, (excluding air
and water) is expressed by:
ρs M s
=
GS =
ρ w Vs ρ w
• Gs of solid particles is approximately equal to 2.65
• Gs of soil forming minerals ranges between 2.5 and 2.8
• Gs can be used to calculate the density or unit weight
of solid particles:
𝜌𝜌𝑠𝑠 = 𝐺𝐺𝑠𝑠 . 𝜌𝜌𝑤𝑤
𝛾𝛾𝑠𝑠 = 𝐺𝐺𝑠𝑠 . 𝛾𝛾𝑤𝑤
Soil Phase Relations
4. Specific Volume
• The total volume of the soil must consist of the
sum of the solid volume plus the void volume.
Thus the total volume of the soil, termed the
specific volume is given by,
V = Vs(1 + e) = 1+e
Inter-relationships between soil
parameters
•
Since the sectional area perpendicular to the plane of the paper
is assumed as unity on the Fig 2.2, the heights of the blocks will
represent the volumes
Fig. 2.2
Soil Phase Relations
• The volume of solids may be represented as
Vs = 1 . When the soil is fully saturated, the
voids are completely filled with water.
1. Relationship between e and n
Soil Phase Relations
2. Relationship Between e, Gs and S
Case 1: When partially saturated (S<100%)
Vw Vw
=
S=
Vv
e
M w wM s wG sVs ρ w
Vw =
=
=
= wG s
ρw
ρw
ρw
Soil Phase Relations
2. Relationship Between e, Gs and S (Cont’)
Case 2: When saturated (S=100%)
From
We have (for S=1),
Soil Phase Relations
3. Relationships between Density ρ and Other
Parameters
Case 1: For S < 100%:
(General Equation)
Case 2: For S= 100%:
From the general Equation,
3. Relationships between Density ρ and Other
Parameters (Cont’)
Case 3: For S = 0%:
From the general Equation,
Case 4: When the soil is submerged:
• If the soil is submerged, the density of the submerged soil
ρ’or ρb, is equal to the density of the saturated soil (𝜌𝜌𝑠𝑠𝑠𝑠𝑠𝑠 )
reduced by the density of water (𝜌𝜌𝑤𝑤 ) , that is
Soil Phase Relations
4. Relative Density
• The looseness or denseness of sandy soils can be
expressed numerically by relative density Dr, defined by
the equation
Soil Phase Relations
4. Relative Density (Cont’)
• A general equation for void ratio (e) may be written as,
• Now substituting the corresponding dry densities for
emax, emin and e in Eq. (2.20) and simplifying, we have
Soil Index Properties
• The index properties of soils can be studied in a general
way under two classes. They are:
1. Soil grain properties.
2. Soil aggregate properties
 The principal soil grain properties are the size and shape
of grains and the mineralogical character of the finer
fractions (applied to clay soils).
 The most significant aggregate property of cohesionless
soils is the relative density, whereas that of cohesive soils
is the consistency.
Index Properties
The Size and Shape of Particles
• The shapes of particles as conceived by visual inspection
give only a qualitative idea of the behavior of a soil mass
composed of such particles. Since particles finer than 0.075
mm diameter cannot be seen by the naked eye, one can
visualize the nature of the coarse grained particles only.
• Coarser fractions composed of angular grains are capable of
supporting heavier static loads and can be compacted to a
dense mass by vibration.
• The classification according to size divides the soils
broadly into two distinctive groups, namely, coarse grained
and fine grained.
Index Properties
The Size and Shape of Particles (Cont’)
• Soil particles which are coarser than 0.075 mm are
generally termed as coarse grained and the finer ones as
silt, clay and peat (organic soil) are considered fine
grained.
 The physical separation of a sample of soil by any method
into two or more fractions, each containing only particles of
certain sizes, is termed fractionation.
 The determination of the mass of material in fractions
containing only particles of certain sizes is termed
Mechanical Analysis.
Index Properties
The Size and Shape of Particles (Cont’)
• 2 methods used for particle sizes distribution analysis:
 Sieve Analysis: method adopted for separation of particles
in coarse-grained soils and,
 Hydrometer Analysis: adopted for fine-grained soils
• The particle size distribution analysis provides the basic
information for revealing the uniformity or gradation of the
materials within established size ranges and for textural
classifications.
Index Properties
Sieve Analysis
• Sieve analysis is carried out by using a set of standard
sieves.
• Sieves are made by weaving two sets of wires at right
angles to one another.
• The square holes thus formed between the wires
provide the limit which determines the size of the
particles retained on a particular sieve.
• The sieve sizes are given in terms of the number of
openings per inch (Eg. An ASTM 60 sieve has 60
openings per inch width with each opening of 0.250
mm. Table 3.2 gives a set of ASTM Standard Sieves
(same as US standard sieves).
Index Properties
Sieve Analysis (Cont’)
Index Properties
Sieve Analysis (Cont’)
• The usual procedure is to use a set of sieves which will
yield equal grain size intervals on a logarithmic scale.
• A good spacing of soil particle diameters on the grain size
distribution curve will be obtained if a nest of sieves is used
in which each sieve has an opening approximately one-half
of the coarser sieve above it in the nest.
• the coarsest sieve that can be used to separate out gravel
from sand is the No. 4 Sieve (4.75 mm opening).
• To separate out the silt-clay fractions from the sand
fractions, No. 200 sieve may be used.
Index Properties
Sieve Analysis (Cont’)
• The nest of sieves consists of Nos 4 (4.75 mm), 8 (2.36
mm), 16 (1.18 mm), 30 (600 μm), 50 (300 μm), 100 (150
μm), and 200 (75 μm).
• The sieve analysis is carried out by sieving a known dry
mass of sample through the nest of sieves placed one below
the other so that the openings decrease in size from the top
sieve downwards, with a pan at the bottom of the stack as
shown in Fig. 3.3.
Index Properties
Sieve Analysis (Cont’)
• By determining the mass of soil sample left on each sieve,
the following calculations can be made
.
• The results may be plotted in the form of a graph on semilog paper with the percentage finer on the arithmetic scale
and the particle diameter on the log scale as shown in Fig.
3.4.
Grain Size Distribution Curve
Index Properties
Grain Size Distribution Curve (Cont’)
• The shape of the curve indicates the nature of the soil tested.
On the basis of the shapes we can classify soils as:
1. Uniformly graded or poorly graded.
2. Well graded.
3. Gap graded.
• Uniformly graded soils are represented by nearly vertical
lines. Such soils possess particles of almost the same
diameter.
• A well graded soil possesses a wide range of particle sizes
ranging from gravel to clay size particles.
• A gap graded soil, has some of the sizes of particles
missing
Index Properties
Grain Size Distribution Curve (Cont’)
 To determine whether a material is uniformly graded or well
graded, Hazen proposed the following equation:
Where;
• D60 is the diameter of the particle at 60 per cent finer on
the grain size distribution curve.
• D10 is the effective grain size corresponds to 10 per cent
finer particles.
• Cu is the uniformity coefficient,
For all practical purposes we can consider the following values for
granular soils.
Cu > 4 for well graded gravel
Cu > 6 for well graded sand
C < 4 for uniformly graded soil containing particles of the same size
Index Properties
Grain Size Distribution Curve (Cont’)
 There is another step in the procedure to determine the
gradation of particles. This is based on the term called the
coefficient of curvature which is expressed as.
where D30 is the size of particle at 30% finer on the
gradation curve. The soil is said to be well graded
if Cc lies between 1 and 3 for gravels and sands.
 Two samples of soils are said to be similarly graded if their
grain size distribution curves are almost parallel to each other
on a semi logarithmic plot. When the curves are almost
parallel to each other the ratios of their diameters at any
percentage finer approximately remain constant. Such curves
are useful in the design of filter materials around drainage
pipes.
Index Properties
Grain Size Distribution Curve (Cont’)
Index Properties
Hydrometer Analysis
• This method depends upon variations in the density
of a soil suspension contained in a 1000 mL
graduated cylinder.
• The density of the suspension is measured with a
hydrometer at determined time intervals;
• Then the coarsest diameter of particles in suspension
at a given time and the percentage of particles finer
than that coarsest (suspended) diameter are
computed.
Index Properties
Hydrometer Analysis (Cont’)
• These computations are based on Stokes‘ formula
which is described below. Stokes (1856), an English
physicist, proposed an equation for determining the
terminal velocity of a falling sphere in a liquid. If a
single sphere is allowed to fall through a liquid of
indefinite extent, the terminal velocity, v can be
expressed as, ν = γ s − γ w D 2
18µ
D=
18µ
(Gs − 1)γ w
L
t
Index Properties
Hydrometer Analysis (Cont’)
• If L is in cm, t in min, Yw in g/cm2, μ in (gsec)/cm2 and D in mm, the Eq (3.22) may be
written as
• Or
18µ
D(mm)
=
10
(Gs − 1)γ w
30 µ
D=
(Gs − 1)γ w
L
t * 60
L
L
=K
t
t
(3.24)
Index Properties
Hydrometer Analysis (Cont’)
• Where,
30 µ
K=
(Gs − 1)
𝜌𝜌𝑤𝑤 = 1𝑔𝑔/𝑐𝑐𝑚𝑚3
(3.25)
By assuming
• It may be noted here that the factor K is a
function of temperature T, specific gravity Gs
of particles and viscosity of water. Table 3.4a
gives the values of K for the various values of
Gs at different temperatures T
Index Properties
Hydrometer Analysis (Cont’)
Index Properties
Hydrometer Analysis (Cont’)
Index Properties
Hydrometer Analysis (Cont’)
Index Properties
Relative Density of Cohesionless Soils
• The density of granular soils varies with the
shape and size of grains, the gradation and the
manner in which the mass is compacted.
• If all the grains are assumed to be spheres of
uniform size and packed as shown in Fig.
3.8(a), the void ratio of such a mass amounts
to about 0.90. However, if the grains are
packed as shown in Fig. 3.8(b), the void ratio
of the mass is about 0.35.
Index Properties
Relative Density of Cohesionless Soils (Cont’)
• The soil corresponding to the higher void ratio is called
loose and that corresponding to the lower void ratio is called
dense.
• If the soil grains are not uniform, then smaller grains fill in
the space between the bigger ones and the void ratios of
such soils are reduced to as low as 0.25 in the densest state.
• If the grains are angular, they tend to form looser structures
than rounded grains because their sharp edges and points
hold the grains further apart.
Index Properties
Index Properties
Consistency of clay soil
• Consistency is a term used to indicate the degree of
firmness of cohesive soils,
• The consistency of natural cohesive soil deposits is
expressed qualitatively by such terms as very soft,
soft, stiff, very stiff and hard.
• The physical properties of clays greatly differ at
different water contents. A soil which is very soft at
a higher percentage of water content becomes very
hard with a decrease in water content
Index Properties
Consistency of clay soil (Cont’)
• However, it has been found that at the same water
content, two samples of clay of different origins may
possess different consistency.
• Water content alone, therefore, is not an adequate
index of consistency for engineering and many other
purposes.
• Consistency of a soil can be expressed in terms of:
1. Atterberg limits of soils
2. Unconfined compressive strengths of soils
Index Properties
Atterberg Limits
• Albert Atterberg, a Swedish scientist, considered the
consistency of soils in 1911, and proposed a series of tests
for defining the properties of cohesive soils.
• He showed that if the water content of a thick suspension of
clay is gradually reduced, the clay water mixture undergoes
changes from a liquid state through a plastic state and
finally into a solid state.
• The water contents corresponding to the transition from one
state to another are termed as Atterberg Limits and the tests
required to determine the limits are the Atterberg Limit
Tests.
Index Properties
Atterberg Limits (Cont’)
• He showed that if the water content of a thick
suspension of clay is gradually reduced, the clay
water mixture undergoes changes from a liquid state
through a plastic state and finally into a solid state.
• The water contents corresponding to the transition
from one state to another are termed as Atterberg
Limits and the tests required to determine the limits
are the Atterberg Limit Tests.
Index Properties
Atterberg Limits (Cont’)
Index Properties
Atterberg Limits (Cont’)
• Liquid Limit: The transition state from the liquid
state to a plastic state is called the liquid limit, Wl.
=>At this stage all soils possess a certain small
shear strength.
• Plastic Limit: The transition from the plastic state
to the semi-solid state is termed the plastic limit, Wp
=> At this state the soil rolled into threads of about 3
mm diameter just crumbles
=> Plastic state (plasticity is defined as the property of
cohesive soils which possess the ability to undergo
changes of shape without rupture) and other states.
Index Properties
Atterberg Limits (Cont’)
• Shrinkage Limit: Further decrease of the water contents of
the same will lead finally to the point where the sample can
decrease in volume no further.
=> At this point the sample begins to dry at the surface,
saturation is no longer complete, and further decrease in water
in the voids occurs without change in the void volume. The
color of the soil begins to change from dark to light.
=>This water content is called the shrinkage limit, Ws.
 The limits expressed above are all expressed by their
percentages of water contents.
Index Properties
Atterberg Limits (Cont’)
• Plasticity Index: The range of water content
between the liquid and plastic limits, which is an
important measure of plastic behavior, is called the
plasticity index, Ip.
=> The difference between the liquid limit and the
plastic limit of a soil is defined as the plasticity index :
I p = wt − w p
Index Properties
Atterberg Limits (Cont’)
• Liquidity Index: The relative consistency of a cohesive soil in the
natural state can be defined by a ratio called the liquidity index, which is
given by
Where w = in situ moisture content of soil.
 The in situ moisture content for a sensitive clay may be greater than
the liquid limit. In this case, LI >1
 These soils, when remolded, can be transformed into a viscous form
to flow like a liquid.
 Soil deposits that are heavily over consolidated may have a natural
moisture content less than the plastic limit. In this case, LI < 0
Index Properties
Atterberg Limits (Cont’)
• Activity: Activity is used as an index for identifying
the swelling potential of clay soils.
Because the plasticity of soil is caused by the
adsorbed water that surrounds the clay particles, we
can expect that the type of clay minerals and their
proportional amounts in a soil will affect the liquid
and plastic limits.
Skempton (1953) observed that the plasticity index
of a soil increases linearly with the percentage of
clay-size fraction (% finer than 2 μm by weight)
present.
Index Properties
Atterberg Limits (Cont’)
• The correlations of PI with the clay-size fractions for
different clays plot separate lines. This difference is due to the
diverse plasticity characteristics of the various types of clay
minerals.
• On the basis of these results, Skempton defined a quantity
called activity, which is the slope of the line correlating PI and
% finer than 2 μm. This activity may be expressed as
Index Properties
Atterberg Limits (Cont’)
Soil Classification
Broad Classification
• A classification scheme provides a method of
identifying soils in a particular group that would likely
exhibit similar characteristics.
• Soil classification is used to specify a certain soil
type that is best suitable for a given application.
• Soil is broadly classified according to grain size
under:
1. Coarse grained
2. Fine grained
Soil Classification
Broad Classification (Cont’)
1. Coarse grained soils: These include sands, gravels and
larger particles. For these soils the grains are well defined
and may be seen by the naked eye. The individual particles
may vary from perfectly round to highly angular reflecting
their geological origins.
2. Fine grained soils: These include the silts and clays and
have particles smaller than 60 μm.
• Silts: these can be visually differentiated from clays because
they exhibit the property of dilatancy. If a moist sample is
shaken in the hand water will appear on the surface. If the
sample is then squeezed in the fingers the water will
disappear. Their gritty feel can also identify silts.
Soil Classification
Broad Classification (Cont’)
• Clays: exhibit plasticity, they may be readily remolded
when moist, and if left to dry can attain high strengths
=> The precise boundaries between different soil types are
somewhat arbitrary, but the following scale is now in use
worldwide.
Note : most soils contain mixtures of sand, silt and clay
particles, so the range of particle sizes can be very large so
the use of logarithmic scale.
Soil Classification
Soil Classification Systems
• There several classification schemes available. Each was devised
for a specific use.
• These systems have two main purposes:
1. To determine the suitability of different soils for various
purposes
2. To develop correlations with useful soil properties, for
example, compressibility and strength
• For example ,The American Association of State Highway and
Transportation Officials (AASHTO) developed one scheme that
classify soils according to their usefulness in roads and highways
while the Unified Soil Classification System (USCS) was
originally developed for use in airfield construction but was later
modified for general use.
Soil Classification
Soil Classification Systems (Cont’)
• Soil classification systems divide soils into groups and
subgroups based on common engineering properties such as
the grain-size distribution, liquid limit, and plastic limit.
• The two major classification systems presently in use are
(1) the American Association of State Highway
Transportation Officials (AASHTO) System and
and
(2) the Unified Soil Classification System (also ASTM).
Note: The AASHTO system is used mainly for the
classification of highway subgrades. It is not used in
foundation construction.
Soil Classification
Soil Classification Systems (Cont’)
1. AASHTO SYSTEM
• The AASHTO Soil Classification System was originally
proposed by the Highway Research Board’s Committee on
Classification of Materials for Subgrades and Granular Type
Roads (1945).
• According to the present form of this system, soils can be
classified according to eight major groups, A-1 through A-8,
based on their grain-size distribution, liquid limit, and
plasticity indices. Soils listed in groups A-1, A-2, and A-3
are coarse-grained materials, and those in groups A-4, A5, A-6, and A-7 are fine-grained materials. Peat, muck,
and other highly organic soils are classified under A-8.
Soil Classification
Soil Classification Systems (Cont’)
1. AASHTO SYSTEM (Cont’)
Soil Classification
Soil Classification Systems (Cont’)
1. AASHTO SYSTEM (Cont’)
Soil Classification
Soil Classification Systems (Cont’)
2. UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)
• The standard system used worldwide for most major
construction projects is known as the Unified Soil
Classification System (USCS). This is based on an original
system devised by Cassagrande. Soils are identified by
symbols determined from sieve analysis and Atterberg
Limit tests.
• In the Unified System, the following symbols are used for
identification:
Soil Classification
Soil Classification Systems (Cont’)
USCS for coarse-grained materials (Cont’)
• If more than half of the material is coarser than the 75 μm
sieve, the soil is classified as coarse. The following steps
are then followed to determine the appropriate 2 letter
symbol:
 Determine the prefix (1st letter of the symbol):
=> If more than half of the coarse fraction is sand then use
prefix S
=> If more than half of the coarse fraction is gravel then use
prefix G
Soil Classification
Soil Classification Systems (Cont’)
USCS for coarse-grained materials (Cont’)
 Determine the suffix (2nd letter of symbol):
This depends on the uniformity coefficient Cu and the coefficient of
curvature Cc obtained from the grading curve, on the percentage of fines,
and the type of fines.
First determine the percentage of fines, which is the % of material
passing the 75 μm sieve.
Then if % fines is
• < 5% use W or P as suffix
• >12% use M or C as suffix
• Between 5% and 12% use dual symbols. Use the prefix
from above with first one of W or P and then with one of M
or C.
− If W or P are required for the suffix then Cu and Cc must be evaluated
Soil Classification
Soil Classification Systems (Cont’)
USCS for coarse-grained materials (Cont’)
If prefix is G then suffix is W =>if Cu > 4 and Cc is between 1 and 3
Otherwise use P
If prefix is S then suffix is W => if Cu > 6 and Cc is between 1 and 3
Otherwise use P
− If M or C are required, they have to be determined from the procedure
used for fine grained materials discussed below. Note that M stands
for Silt and C for Clay. For a coarse grained soil which is
predominantly sand the following symbols are possible
SW, SP, SM, SC or SW-SM, SW-SC, SP-SM, SP-SC
The following symbols are used in case of soil dominated by gravel
GW, GP, GM, GC or GW-GM, GW-GC, GP-GM, GP-GC
Soil Classification
Soil Classification Systems (Cont’)
USCS for fine-grained materials
• These are classified solely according to the results from the
Atterberg Limit Tests. Values of the Plasticity Index and
Liquid Limit are used to determine a point in the plasticity
chart. The classification symbol is determined from the
region of the chart in which the point lies.
Examples
CH High plasticity clay
CL Low plasticity clay
MH High plasticity silt
ML Low plasticity silt
OH High plasticity organic soil (Rare)
Pt Peat
Soil Classification
Soil Classification Systems (Cont’)
USCS for fine-grained materials (Cont’)
A- Line is defined
by the equation:
Fig. 2.5 Plasticity chart for laboratory classification of fine grained soils
Chap III Soil Compaction
ASSIGNMENT No.1
Find the details of the assignment on a separate sheet
sent via class email.
TO BE SUBMITTED ON …../ …../…. at ……
• Hard copy
• Soft copy via: niyonzima.albert@yahoo.com
Chapter IV.