Soil mechanics.. pdf

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Assala mu alykum My Name is saqib imran and I am the
student of b.tech (civil) in sarhad univeristy of
science and technology peshawer.
I have written this notes by different websites and
some by self and prepare it for the student and also
for engineer who work on field to get some knowledge
from it.
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Saqib imran.

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Soil Mechanics
Soil Mechanics Introduction and Definition
Soil mechanics is defined as the application of the laws and principles of mechanics and
hydraulics to engineering problems dealing with soil as an engineering material. Soil has
many different meanings, depending on the field of study. To a geotechnical engineer, soil
has a much broader meaning and can include not only agronomic material, but also broken-
up fragments of rock, volcanic ash, alluvium, Aeolian sand, glacial material, and any other
residual or transported product of rock weathering.
As the name Soil Mechanics implies the subject is concerned with the deformation and
strength of bodies of soil. It deals with the mechanical properties of the soil materials and
with the application of the knowledge of these properties to engineering problems. In
particular it is concerned with the interaction of structures with their foundation
material. This includes both conventional structures and also structures such as earth dams,
embankments and roads which are their-selves made of soil.
Unconfined Compression (UC) Test

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Purpose:
The primary purpose of this test is to determine the unconfined compressive strength,
which is then used to calculate the unconsolidated undrained shear strength of the clay
under unconfined conditions. According to the ASTM standard, the unconfined
compressive strength (qu) is defined as the compressive stress at which an unconfined
cylindrical specimen of soil will fail in a simple compression test. In addition, in this test
method, the unconfined compressive strength is taken as the maximum load attained per
unit area, or the load per unit area at 15% axial strain, whichever occurs first during the
performance of a test.
Standard Reference:
ASTM D 2166 - Standard Test Method for Unconfined Compressive Strength of Cohesive
Soil
Significance:
For soils, the undrained shear strength (su) is necessary for the determination of the bearing
capacity of foundations, dams, etc. The undrained shear strength (su) of clays is commonly
determined from an unconfined compression test. The undrained shear strength (su) of a
cohesive soil is equal to one half the unconfined compressive strength (qu) when the soil
is under the f = 0 condition (f = the angle of internal friction). The most critical condition
for the soil usually occurs immediately after construction, which represents undrained
conditions, when the undrained shear strength is basically equal to the cohesion (c). This
is expressed as:
Then, as time passes, the pore water in the soil slowly dissipates, and the intergranular
stress increases, so that the drained shear strength (s), given by s = c + s‘tan f , must be
used. Where s‘ = intergranular pressure acting perpendicular to the shear plane; and s‘ = (s
- u), s = total pressure, and u = pore water pressure; c’ and φ’ are drained shear strength
parameters. The determination of drained shear strength parameters is given in Experiment
14
Equipment:
Compression device, Load and deformation dial gauges, Sample trimming equipment,
Balance, Moisture can.
Test Procedure:

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1. Extrude the soil sample from Shelby tube sampler. Cut a soil specimen so that the ratio
(L/d) is approximately between 2 and 2.5.
Where L and d are the length and diameter of soil specimen, respectively.
2. Measure the exact diameter of the top of the specimen at three locations 120° apart, and
then make the same measurements on the bottom of the specimen. Average the
measurements and record the average as the diameter on the data sheet.
3. Measure the exact length of the specimen at three locations 120° apart, and then average
the measurements and record the average as the length on the data sheet.
4. Weigh the sample and record the mass on the data sheet.
5. Calculate the deformation (∆L) corresponding to 15% strain (ε).
Where L0 = Original specimen length (as measured in step 3).
6. Carefully place the specimen in the compression device and center it on the bottom plate.
Adjust the device so that the upper plate just makes contact with the specimen and set the
load and deformation dials to zero.
7. Apply the load so that the device produces an axial strain at a rate of 0.5% to 2.0% per
minute, and then record the load and deformation dial readings on the data sheet at every
20 to 50 divisions on deformation the dial.
8. Keep applying the load until (1) the load (load dial) decreases on the specimen
significantly, (2) the load holds constant for at least four deformation dial readings, or (3)
the deformation is significantly past the 15% strain that was determined in step 5.
9. Draw a sketch to depict the sample failure.
10. Remove the sample from the compression device and obtain a sample for water content
determination. Determine the water content as in Experiment 1.
Analysis:
1. Convert the dial readings to the appropriate load and length units, and enter these values
on the data sheet in the deformation and total load columns. (Confirm that the conversion
is done correctly, particularly proving dial gauge readings conversion into load)
2. Compute the sample cross-sectional area
3. Compute the strain
4. Computed the corrected area,
5. Using A’, compute the specimen stress,
6. Compute the water content, w%.
7. Plot the stress versus strain. Show qu as the peak stress (or at 15% strain) of the test. Be
sure that the strain is plotted on the abscissa. See example data.
8. Draw Mohr’s circle using qu from the last step and show the undrained shear strength, su
= c (or cohesion) = qu/2. See the example data.
General Considerations and Assumptions in the
Soil Stability Analysis

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There are three distinct parts to an analysis of the stability of a slope. They are:
1. Testing of samples to determine the cohesion and angle of internal
friction
If the analysis is for a natural slope, it is essential that the sample be undisturbed. In such
important respects as rate of shear application and state of initial consolidation, the
condition of testing must represent as closely as possible the most unfavorable conditions
ever likely to occur in the actual slope.
2. The study of items which are known to enter but which cannot be
accounted for in the computations
The most important of such items is progressive cracking which will start at the top of the
slope where the soil is in tension, and aided by water pressure, may progress to considerable
depth. In addition, there are the effects of the non‐homogeneous nature of the typical soil
and other variations from the ideal conditions which must be assumed.
3. Computation
If a slope is to fail along a surface, all the shearing strength must be overcome along that
surface which then becomes a surface of rupture. Any one such as ABC in Fig. 2 (b)
represents one of an infinite number of possible traces on which failure might occur.

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It is assumed that the problem is two dimensional, which theoretically requires a long
length of slope normal to the section.
The shear strength of soil is assumed to follow Mohr Coulomb's law Τ or s = c̕+ σ' tan φ̕
Where, c' ‐ effective unit cohesion
σ'= effective normal stress on the surface of rupture = (σ ‐ u)
σ ‐ total normal stress on the surface of rupture
u ‐ pore water pressure on the surface of rupture
φ̕= effective angle of internal friction.
The item of great importance is the loss of shearing strength which many clays show when
subjected to a large shearing strain. The stress‐strain curves for such clay show the stress
rising with increasing strain to a maximum value, after which it decreases and approaches
an ultimate value which may be much less than the maximum. Since a rupture surface
tends to develop progressively rather than with all the points at the same state of strain, it
is generally the ultimate value that should be used for the shearing strength rather than the
maximum value.
Active Earth Pressure on Retaining Wall
Active earth pressure on retaining wall:
The following cases of active earth pressure on cohesionless backfill will now be
considered:
1. Dry or moist backfill
2. Submerged backfill
3. Partly submerged backfill
4. Backfill with uniform surcharge
5. Backfill with sloping surcharge
Dry or moist backfill:
Consider an element at a depth z below the ground surface, When the wall is at the point
of moving away (Outwards) from the backfill, there are two kinds of pressure acting on
it.
1. The lateral (horizontal) pressure, it is the minimum principal stress.
2. The vertical pressure, it is the maximum principal stress.

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Submerged back fill:
In this case the sand back-fill behind the retaining wall is saturated with water. The lateral
pressure is made up of two components:
1. Lateral pressure due to submerged weight (gamma) of the soil
2. Lateral pressure due to water
Partly submerged backfill
If the back-fill is partly submerged i.e. the backfill is moist up to depth H1 below the
ground level and the it is submerged.
Back-fill with uniform surcharge
If the backfill is horizontal and carries a surcharge load of uniform intensity q per unit
area.
The vertical pressure increment at any depth will increase by q. The increase in the lateral
pressure due to this will be kaq.
Coefficients of earth pressure
On a small unit at depth Z in the back there are two kinds of pressure.
1. Vertical Earth pressure:
The pressure applied in the vertical direction due to the backfill lying above it.
2. Horizontal Earth pressure:
The pressure applied in the horizontal direction due to backfill is called the
horizontal pressure or lateral earth pressure
Coefficient of active earth pressure at rest:
When the retaining wall is at rest then the ratio between the lateral earth pressure and the
vertical pressure is called the co-efficient of the earth pressure at rest,
Ko =lateral pressure / vertical pressure
Coefficient of active earth pressure
When the retaining wall is moving away from the backfill the the ratio between lateral
earth pressure and vertical earth pressure is called coefficient of active earth pressure.

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Ka =lateral pressure / vertical pressure
Coefficient of passive earth pressure:
When the retaining wall is moving towards the back-fill, then the ratio between the lateral
earth pressure and the vertical earth pressure is called the Coefficient of passive earth
pressure.
Ka =lateral pressure / vertical pressure
Effects of Density and Moisture on Soil
Behaviour
Effect of Moisture on Performance of Soil
Particles of fine grained soil are more influenced by variation in moisture content then by
any other course. Soils that have ample supporting power under one set of moisture
condition may be entirely unsatisfactory if the percentage of moisture change.
From study of soil it in observed that between the dry state and where moisture demand is
satisfied the volume increases as:
Sand 2%
Silt 16%
Clay 160%
Colloids950%
(i.e. more effect on fine grained soil)
The general properties of soil composed largely of course material are primarily controlled
by the characteristics of the particles. But soil composed of clay & colloids the properties
are primarily controlled
By moisture film (i.e. chemistry and surface charges of soil)
Atterberg's limits can be used to describe the different states of soil with varying degree
of moisture
Effect of Density on the Behavior of soil

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The density of a soil is its weight per unit volume.
It is sometime expressed as “Wet weight” or total weight including water. It is more
commonly expressed as “dry weight” which in the weight of soil alone excluding the
weight of the contained water.
As a particle of soil become denser, it will contain a greater number of particles and the
(pore) volume remaining for air and water will be decreased. In the increased density and
decreased moisture content physical of soil improve.
Strength in increased, consolidation under load and the rate of water movement through
the soil and the changes under variation of moisture content all are decreased. Composed
of sub grades and bases for high density to obtain above advantages is an accepted practice.
Characteristics of Soil Particles
PURPOSE
The characteristics of soil particles are important in predicting the sub grade performance
of the given soil. As the purpose of the sub grade is to given adequate support to the

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pavement thus the sub grade should posses’ sufficient stability under the adverse climate
and loading conditions.
CHARACTERISTICS OF SOIL PARTICLES
What is Soil
Consists largely of mineral matter formed by the disintegration or decomposition of rocks,
caused by the action of water, ice, frost or temperature changes or by plant and animal life.
Some soils near surface may contain humus organic acid resulting from the decay of
vegetation.
In a road there is wide variation in soil type along the road (compared to buildings).
Broad classification rather then bearing at a specific point is taken for finding soil
properties.
GRAIN SIZE
The grains which soil is composed have been classified by AASHTO as follows.
S. No. Classes
Particle size
(Diameter) mm
Sieve Size
Passing
Sieve Size
Retained
1 Gravel 80mm to 2mm 3 No 10
2 Coarse sand 2mm to 0.4 No 10 No 40
3 Fine sand 0.4 to 0.08 No 40 No. 200
4 Silt 0.08 to 0.002 No. 200 -
5 Clay 0.002 to 0.001 - -
6 Colloidal Clay smaller than 0.001 - -
Grain shape
Shape has importance in terms of strength & toughness for large soil particles.
1. Rounded: particles found in stream bed have undergone considerable weal and are
probably quite strong.
2. Flat & Flaky: particles probably have not been subjected to such treatment and may be
weak and friable and not suitable for highway use.
3. are useful in granular soil mixtures. Which contain little clay and large soil particles
having on important influence on their behavior (used in base course) have, an angular
particle shape produced by crushing strong, tough rock increase the resistance of soil
mass to formation under load since the individual grains tend to lock together.

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Surface Texture
The surface texture of large soil particles greatly influences their performance in granular
soil mixtures and Rough surface results in high coefficient of friction among the particles.
As a result, the particles cannot slide over each other and provide resistance to deformation
under load. (Freshly crushed rock)
Rough surface of freshly crushed rock is more important than the angular shape in
developing strength.
Desirable properties of Soil
i. Stability (Resistance to permanent deformation)
ii. Incompressibility
iii. Permanency of strength
iv. Minimum change in volume & stability under adverse condition of weather & ground
water.
v. Good drainage
vi. Ease of compaction (denser)
To Determine Liquid Limit of Soil & Plastic Limit
of Soil

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Apparatus
1. Liquid Limit Device - a mechanical device consisting of a brass cup suspended
from a carriage designed to control its drop onto a hard rubber base. The device
may be operated by either a hand crank or electric motor.
2. Cup - brass with mass (including cup hanger) of 185 to 215 g.
3. Cam - designed to raise the cup smoothly and continuously to its maximum
height, over a distance of at least 180o of cam rotation, without developing an
upward or downward velocity of the cup when the cam follower leaves the cam.
4. Flat Grooving Tool - a tool made of plastic or non-corroding metal having
specified dimensions.
5. Gage - A metal gage block for adjusting the height of the drop of the cup to 10
mm.
6. Ground Glass Plate - used for rolling plastic limit threads.
Significance and Use
This testing method is used as an integral part of several engineering classifications
systems to characterize the fine-grained fractions of soils and to specify the fine-grained
fraction of construction materials. The liquid limit, plastic limit and plasticity index of
soils are also used extensively, either individually or together, with other soil properties

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to correlate with engineering behavior such as compressibility, permeability,
compactibility, shrink-swell and shear strength.
Scope
This test method covers the determination of the liquid limit, plastic limit and plasticity
index of soils. The liquid and plastic limits of soils are often referred the as the Atterberg
limits.
 Workability of Concrete
 Atterbergs Limits
 Plastic limit of soil
 Sieve Analysis
 Civil Lab Tests
 Liquid Limit
 Liquid Limit in Sub grade
 Plastic Limit Test
Procedure for Liquid Limit Test
1. Place a portion of the prepared sample in the cup of the liquid limit device at the
point where the cup rests on the base and spread it so that it is 10mm deep at its
deepest point. Form a horizontal surface over the soil. Take care to eliminate air
bubbles from the soil specimen. Keep the unused portion of the specimen in the
storage container.
2. Form a groove in the soil by drawing the grooving tool, beveled edge forward,
through the soil from the top of the cup to the bottom of the cup. When forming the

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groove, hold the tip of the grooving tool against the surface of the cup and keep the
tool perpendicular to the surface of the cup.
3. Lift and drop the cup at a rate of 2 drops per second. Continue cranking until the
two halves of the soil specimen meet each other at the bottom of the groove. The
two halves must meet along a distance of 13mm (1/2 in).
4. Record the number of drops required to close the groove.
5. Remove a slice of soil and determine its water content, w.
6. Repeat steps 1 through 5 with a sample of soil at a slightly higher or lower water
content. Whether water should be added or removed depends on the number of
blows required to close the grove in the previous sample.
Note: The liquid limit is the water content at which it will takes 25 blows to close the
groove over a distance of 13 mm. Run at least five tests increasing the water content each
time. As the water content increases it will take less blows to close the groove.
Observations & Calculations:
Sample number 01 02 03
Container number 24 21 25
Number of Blows 17 25 34
Mass of empty container (M1), gm 44.9 46 44.6
Mass of container + wet soil (M2), gm 78.3 81.3 76.8
Mass of container + dry soil ( m3
), gm 70 75.30 74.10
Water content= w = (M2 - m3
/ m3
- M1) x 100,
%
33.07 20.30 10.00
Standard Values for Liquid Limit Test
Liquid Limit, LL
Plot the relationship between the water content, w, and the corresponding number of
drops, N, of the cup on a semi-logarithmic graph with water content as the ordinates and
arithmetical scale, and the number of drops on the abscissas on a logarithmic scale. Draw
the best fit straight line through the five or more plotted points. Take the water content
corresponding to the intersection of the line with the 25 drop abscissa as the liquid limit,
LL, of the soil.

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Graph
Following is the
graph of relation
between water
content and
number of blows
Precautions
 After
performing
each test the
cup and
grooving
tool must be
cleaned.
 The number of blows should be just enough to close the groove.
 The number of blows should be between 10 and 40.
Applications
 The value of liquid limit helps in classification of fine grain soil.
 The values of liquid limit are required to calculate flow index, toughness index etc.
Limitations of Liquid Limit Test
Procedure for Determination of the Plastic Limit
1. From the 20g sample select a 1.5 to 2 g specimen for testing.
2. Roll the test specimen between the palm or fingers on the ground glass plate to
from a thread of uniform diameter.
3. Continue rolling the thread until it reaches a uniform diameter of 3.2mm or 1/8 in.
4. When the thread becomes a diameter of 1/8 in. reform it into a ball.
5. Knead the soil for a few minutes to reduce its water content slightly.
6. Repeat steps 2 to 5 until the thread crumbles when it reaches a uniform diameter
of 1/8 in.
7. When the soil reaches the point where it will crumble, and when the thread is a
uniform diameter of 1/8", it is at its plastic limit. Determine the water content of
the soil.
Note: Repeat this procedure three times to compute an average plastic limit for the
sample.

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Calculations
Plastic Limit, PL
Compute the average of the water contents obtained from the three plastic limit tests. The
plastic limit, PL, is the average of the three water contents.
Plasticity Index
Calculate the plasticity index as follows: PI = LL - PL where:
LL = liquid limit, and PL = plastic limit.
Precautions
 The apparatus required for the experiment should be clean.
 All the readings should be noted carefully.
 Practical applications
 The value of liquid limit and plastic limit are used to classify fine grained soil.
 The values of liquid limit and plastic limits are used to calculate flow index, toughness
index and plasticity index of the soil.
Observations and calculations:
Number of container 24 25
Weight of empty container(M1) gm 45.7 44.5
Weight of container + wet soil (M2) gm 50.6 49.5
Weight of container + dry soil ( m3
) gm 48 46
Water content = (M2- m3
/ m3
-M1)100 % 1.13 1.17
Average plastic limit = (1.13 + 1.7)/2 =1.15%
To Determine The Shrinkage Limit of Soil

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Apparatus
Shrinkage dish, electric oven, mercury, electric balance, sieve#40, spatula and containers.
Procedure
 Take a soil sample passing through sieve#40 and add some amount of water in it
to form a thick uniform paste.
 Take the shrinkage dish, weigh it, and put some of the soil mixture in it by spatula,
fill it and again weigh it.
 Place the shrinkage dish in the oven for 24hours at 110-115C.
 Find the volume of the shrinkage dish using mercury this will be equal to the
volume of the saturated soil sample.
 Take mercury in container and weigh it, put dry soil in it the mercury is displaced.
 Collect carefully the displace mercury and weigh it with great accuracy.
 The volume of dry soil is then determined by dividing the weight by the unit
weight of mercury.
 The shrinkage limit is then calculated using the formula.
S.L = {{(w1-wd)-(v1-vd) γw}/ wd] x 100
Where,
W1 = M2-M1

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WD = m3
-M1
Precautions
The displaced mercury should be carefully collected in order to get exact weight of
mercury displaced.
Rankine's Assumptions for Earth Pressure
Theory for Active/Passive Pressure
Rankine's Theory for Earth pressure
As originally proposed, Rankine's theory is applied to uniform cohesion-less soil only.
Later it was extended to include cohesive soil by Bell in 1915.
Assumptions of Rankine's theory:
Rankine approached the lateral earth pressure problem with the following assumptions:
1. The soil is homogeneous and isotropic, which means c, φ and γ have the same values
everywhere, and they have the same values in all directions at every point (i.e., the

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strength on a vertical plane is the same as that on a horizontal plane). This discussion
will be expanded later to consider layered soils, where each layer has different
values of c, φ and γ∙
2. The most critical shear surface is a plane. In reality, it is slightly concave up, but
this is a reasonable assumption (especially for the active case) and it simplifies the
analysis.
3. The ground surface is a plane (although it does not necessarily need to be level).
4. The wall is infinitely long so that the problem may be analyzed in only two
dimensions. Geotechnical engineers refer to this as a plane strain condition.
5. The wall moves sufficiently to develop the active or passive condition.
6. The resultant of the normal and shear forces that act on the back of the wall is
inclined at an angle parallel to the ground surface (Coulomb's theory provides a
more accurate model of shear forces acting on the wall).
Active Condition
With these assumptions, we can treat the wedge of soil behind the wall as a free body and
evaluate the problem using the principles of statics, as shown in Figure 1a. This is similar
to methods used to analyze the stability of earth slopes, and is known as a limit equilibrium
analysis, which means that we consider the conditions that would exist if the soil along the
base of the failure wedge was about to fail in shear. Weak seams or other non-uniformities
in the soil may control the inclination of the critical shear surface. However, if the soil is
homogeneous, Pa/b is greatest when this surface is inclined at an angle of 45 + φ/2 degrees
from the horizontal, as shown in the Mohr's circle. Thus, this is the most critical angle.
Solving this free body diagram for Pa/b and Va/b gives:
The magnitude of Ka is usually between 0.2 and 0.9. Equation is valid only when β ≤ φ.
If β= 0, it reduces to:

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A solution of Pa/b as a function of H would show that the theoretical pressure distribution
is triangular. Therefore, the theoretical pressure and shear stress acting against the wall, α
and τ, respectively, are:
Where;
σ = soil pressure imparted on retaining wall from the soil
τ = shear stress imparted on retaining wall from the soil
Pa/b = normal force between soil and wall per unit length of wall
Va/b = shear force between soil and wall per unit length of wall
b = unit length of wall (usually 1 ft or 1 m)
Ka = active coefficient of lateral earth pressureσz' = vertical effective stress
β = inclination of ground surface above the wall
H = wall height
However, observations and measurements from real retaining structures indicate that the
true pressure distribution, as shown in Figure 2, is not triangular. This difference is because
of wall deflections, arching, and other factors. The magnitudes of Pa/b and Va/b are
approximately correct, but the resultant acts at about 0.40H from the bottom, not 0.33H as
predicted by theory (Duncan et al., 1990).

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Figure 1: Free body diagram of soil behind a retaining wall using Rankine’s
solution: (a) active case; and (b) passive case.
Figure 2: Comparison between (a) theoretical and (b) observed
distributions of earth pressures acting behind retaining structures
Passive Condition
Rankine analyzed the passive condition in a fashion similar to the active condition except
that the shear force acting along the base of the wedge now acts in the opposite direction
(it always opposes the movement of the wedge) and the free body diagram becomes as
shown in Figure 1b. Notice that the failure wedge is much flatter than it was in the active
case and the critical angle is now 45 – φ/2 degrees from the horizontal. The normal and
shear forces, Pp/b and Vp/b, respectively, acting on the wall in the passive case are:

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Equation is valid only when β≤ φ. If β = 0, it reduces to:
Methods to Determine Stress in Soil
Both immediate and consolidation settlement analysis requires estimate of increase in
pressure (ΔHσ) in the soil layers from the applied loads. Several methods are available to
estimate the increase in pressure at any depth z from the applied load. We will discuss:
1. 2:1 Slope method
2. Boussinesq Method.

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2h: 1v Slope Method
An early method is to use 2 (horizontal): 1 (vertical) slope as shown in figure. For
rectangular footing (B x L), the vertical pressure increase ΔHσ(z) at a depth z beneath the
loaded area due to base load P is:
For square footing the above equation simplifies to
The 2:1 method compares reasonably well with more theoretical methods from z1 = B to
about z2 = 4B but should not be used in the depth zone from z = 0 to B. The average stress
increase in a stratum from H = Z2 – Z1 is
Boussinesq Method of Stress Determination:
This method is based on theory of elasticity and is basically developed for a semi-infinite,
isotropic, homogeneous half-space, however the method is used for all type of masses
(layered etc), Newmark presented numerical solution to the Boussinesq equation, which is
applicable beneath the corner of an area B x L.
Δσ = q0 m Iα
Where m is the number of corners contributing to is Δσ; m = 1 when Δσ under the corner
of a footing is required and m=4 if Δσ under the center of a footing is required.
To Determine the Specific Gravity of Soil

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ASTM Designation: C128
Apparatus
Sieve #4, balance, electric oven, pycnometer.
Theory
Specific gravity is defined as the ratio of the weight of given volume of material to the
weight of an equal volume of water.
G = density of soil/density of equal volume of water
G = mass of dry soil/mass of and equal volume of water.
Procedure

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 Take at least 25g of soil which has been passed through sieve#4 and place it in an
oven at fixed temperature of 105-110 °C for 24 hours to dry it completely.
 Clean and dry the pycnometer thoroughly and find its mass (M1).
 Find the mass (M2) of pycnometer by placing dried soil in it.
 Add sufficient quantity of water to fill the pycnometer up to the given mark and then
find mass of the pycnometer ( m3
) and its content.
 Empty the pycnometer then fill it with water up to the same level. Now find the
mass (M4) of the pycnometer having water in it.
 Determine the specific gravity of the given soil sample.
Precautions
 The graduated cylinder used should be cleaned.
 Dry the coarse aggregate so that it does not absorb moisture otherwise it will not
give the desired results.
 All the readings of mass should be noted carefully.
Practical applications
 The value of specific gravity helps us to some extent in identification
and classification of soil.
 It gives the idea about the suitability of a given soil as a construction material.
 It is utilized in calculating voids ratio, porosity, and degree of saturation if the
density or unit weight and water content are known.
Shear Strength of Pervious and Impervious Soils

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Soils derive their strength from contact between particles capable of transmitting normal
as well as shear forces. The contact between soil particles is mainly due to friction and
the corresponding stress between the soil grains is called the effective (or inter-granular)
stress s'.
Thus, the shear strength of a soil is mainly governed by the effective stress. Besides the
effective stress between soil grains, the pore water contained in the void spaces of the soil
also exerts pressure which is known as pore pressure, u. The sum of the effective stress
and pore pressure acting an any given surface within a compacted earth embankment is
called the total stress s. As the pore water cannot resist shear, all shear stresses are
resisted by the soil grains only. The effective stress is approximately equal to the average
intergranular force per unit area and cannot be measured directly.
The total stress is equal to the total force per unit area acting normal to the plane. The
pore water affects the physical interaction of soil particles. Soils with inactive surfaces do
not absorb water, but clay particles, formed of silicates which are frequently, charged
electrically, absorb water readily and exhibit plastic, shrinkage, and swelling
characteristics. Besides, a change in pore pressure can directly affect the effective stress.
The pore water thus influences the shear strength parameters of a soil to a considerable
extent.
The shear strength of a soil is fully mobilized when a soil element can only just support
the stresses exerted on it. For most soils, the shear strength of any surface, at failure, is
approximated by the following Mohr-Coulomb's linear relationship
s = c' + s' tan f'

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where, s = shear strength of soil (or shear stress at failure) on the surface under
consideration,
c' = cohesion,
f' = angle of internal friction (or angle of shearing resistance), and
s' = effective normal stress acting on the failure surface = s - u.
Here, c' and f' are determined using effective stresses.
While analyzing the stability of an embankment, the designer uses one of the two
methods. These are:
1. Total stress method,
2. Effective stress method
Laboratory tests (for determining shear strength parameters of a soil) for the total stress
method are performed such that the experimental conditions with regard to pore pressure
simulate the pore pressure conditions in the embankment at failure and the shear strength
of the soil sample is measured in terms of total stress without pore pressure
measurements. For the effective stress method, pore pressures on the potential failure
surface are estimated and the shear strength determined in terms of the effective stresses.
The total stress method is relatively simpler. But, the effective stress method is more
fundamental in nature and, hence, is recommended for the stability analysis of
embankment dams.
Shear Strength for Impervious Soils
There are three types of laboratory tests which are commonly used for the determination
of shear strength of compacted impervious soils. These methods differ in the method of
consolidating the soil sample before the sample is failed in shear. These methods are as
follows:
1. Undrained test,
2. Consolidated-undrained test, and
3. Drained test
In the undrained test (also known as the 'quick', 'unconsolidated-undrained' or 'Q' test),
drainage or dissipation of pore pressure is not allowed at any stage of the test. The
relationship between the shear strength and normal pressure, obtained in terms of total
stress, is used in the stability analysis of an embankment dam for 'during' and
'immediately after' construction condition. The soil samples are tested at a moisture

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content and density which would prevail during or immediately after construction. If the
moisture content corresponds to saturation level, it will be found that all soil samples
(saturated) have the same shear strength if no consolidation (i.e., drainage) is allowed.
In consolidated undrained test (also known as 'consolidated-quick' or 'R' test) the sample
is first allowed to consolidate (with full pore pressure dissipation) under a specified
consolidation pressure, and is then failed in shear without permitting drainage. To obtain
strength parameters in terms of effective stress (i.e., c' and f'), the pore pressure should be
measured. These values are used for effective stress method of stability analysis. If the
test values are to be used for the total stress method of stability analysis, the sample
should be tested (without pore pressure measurement) at a water content which is
anticipated in the dam during the period being analyzed. It should often be difficult to
determine the water content which would prevail in the dam.
Hence, it is usual practice to conduct the tests on saturated samples for total stress method
of analysis. The results of these tests would be useful for analyzing sudden draw-down
condition of impervious zones of embankment and foundation. The test results would
also be useful for analyzing the upstream slope during a partial pool condition and the
downstream slope during steady seepage. Drained test (also known as the 'slow' test or 'S'
test) permits drainage and complete dissipation of pore pressure at all stages of the test.
The strength parameters are determined in terms of effective stress. The results of this
test are to be used for freely-draining soils in which pore pressures do not develop.
Shear Strength of Pervious Soils
In embankment sections of clean sand and gravel, the pore pressures develop primarily
due to seepage flow. The changes in pore pressure on account of changes in the
embankment volume are short-lived and can be neglected. Consequently, the effective
stress method of stability analysis is used for sections of pervious sand and gravel. The
strength characteristics of sand and gravel are determined by drained tests. Because of the
limitations of the size of test equipment, it may not be possible to test gravels of larger
size. The strength characteristics of such soil can safely be assumed as those obtained for
finer fractions of the same soil.
Total Stress and Effective Stress Analysis in Soil

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Plastic saturated soils (silts and clays) usually have lower shear strength than non-plastic
cohesion less soil and are more susceptible to bearing capacity failure. For saturated
plastic soils, the bearing capacity often has to be calculated for different condition. Total
Stress Analysis (Short term condition) that uses the un-drained shear strength of the
plastic soil. Effective stress analysis (Long term condition that uses the drained shear
strength parameters (c' & F') of the plastic soil).
Total Stress Analysis
Total stress analysis uses the un-drained shear strength of the plastic soil. The un-drained
shear strength (sigma a) could be determined from field tests, such as the vane shear test
(VST), or in the laboratory from unconfined compression tests.
If the un-drained shear strength is approximately constant with depth, then sigma a=c and
F = 0 For F = 0, the bearing capacity factors are Nc = 5.5, N ? = 0 and Nq = 1 (Put these
values in Terzaghi's bearing capacity equation)
(For strip footing)
Q ult = 5.5c + ? Df

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Because of the use of total stress parameters the ground water table does not affect the
above equation. The ultimate bearing capacity of the above example, the ultimate bearing
capacity of plastic soil is often much less than the ultimate bearing capacity of cohesion
less soil. This is the reason that building codes allow higher allowable bearing pressure
for cohesion less soil (such as sand) than plastic soil (clay).
Also, because the ultimate bearing capacity does not increase with footing width for
saturated plastic soils, there is often no increase allowed for an increase in footing width.
In some cases, it may be appropriated to use total stress parameters "c" and "F" in order
to calculate the ultimate bearing capacity [For example, a structure such as an oil tank or
grain elevator could be constructed and the sufficient time elapses so that the saturated
plastic soil consolidates under this load. If an oil tank or grain elevator were then quickly
filled, the saturated plastic soil would be subjected to an un-drained loading.]
This condition can be modeled by performing consolidated un-drained tri-axial tests
(ASTM 4767-02, 2004) in order to determine the total stress parameter (c & F). Based on
F value, the bearing capacity factors would be obtained from figure; and then the ultimate
bearing capacity would be calculated from equation 1. If site consists of two layers of
cohesive soil having different shear strength parameters/properties; Calculate the ratio of
the un-drained shear strength of layer 2 to the un-drained shear strength of layer 1 i.e.
c2/c1 = su2/su1
Determine the ratio T/B, where T= vertical distance from the bottom of the foundation to
the top of the layer 2 and B = width of the foundation. Enter the values (c2/c1) in graph,
intersect appropriate T/B curve, and determine the value of Nc For strip footing; F = 0 (N
? = 0 Nq = 1).
Effective stress Analysis
The effective stress analysis uses the drained shear strength (c' & F') of the plastic soil.
The drained shear strength could be obtained from tri-axial compression tests. This
analysis is termed as long term analysis because the shear induced pore water pressure
from the loading have dissipated and the hydrostatic pore water conditions now prevail in
the field. Because an effective stress analysis is being performed the location of the
ground water table must be considered in the analysis.
The first step to perform the bearing capacity analysis would be to obtain the bearing
capacity factors (Nc, N ?, Nq) from Fig; using the value of F'. An adjustment to the total
unit weight may be required depending on the location of the ground water table. Then
Terzaghi's bearing capacity equation would be utilized (with c' substituted for c) to obtain
the ultimate bearing capacity, with a factor of safety of 3 applied in order to calculate the
allowable bearing capacity or pressure.

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Governing case:
Total stress analysis will provide a lower allowable bearing capacity for soft or very soft
saturated plastic soils. This is because load will consolidate the plastic soil leading to an
increase in the shear strength as the time passes. For long term case, the shear strength of
the plastic soil is higher with a resulting higher bearing capacity. Effective stress analysis
will provide a lower allowable bearing capacity for very stiff or hard saturated plastic
soils.
Firm to stiff plastic soils are intermediate condition. The ORC and the tendency of the
saturated plastic soil to consolidate (gain shear strength) will determine whether the short
term condition or the long term condition provides the lower bearing capacity. Bearing
capacity analysis for granular soils Granular soil does not liquefy, but rather there is a
reduction in shear strength due to an increase in pore water pressure.
Examples include sands and gravels that are below the ground water table and have a
factor of safety against liquefaction is greater then 2 the earthquake induced excess pore
water pressure will typically be small enough that its effect can be neglected. Using the
Terzaghi's bearing capacity equation and an effective stress analysis and recognizing that
sands and gravels are cohesion less (i.e. c' = 0) Terzaghi's bearing capacity equation
Qult = cNc + q'Nq + 1/2 t BN
For cohesion less soil
Qult = 1/2 t BN? + t Df Nq
For shallow foundations, it sis best to neglect the second term (?t Df Nq) in equation 2.
This is because the term represents the resistance of soil located above the bottom of the
footing, which may not be mobilized for punching shear failure. So, Qult = 1/2 ?t BN?
Cohesion less soils include gravel, sands Cohesion less soil develops its shear strength as
a result of frictional and interlocking resistance between the individual soil particles. This
is due to confining pressure. In case of cohesion less soil c = 0
Qult = cNc + q'Nq + 1/2 ?t BN?
First term c = 0
Qult = qNq + 1/2 ?t BN?
For cohesion less soil the location of the ground water table can effect the ultimate bearing
capacity. The depth of the bearing capacity failure is often assumed that the soil involved
in the bearing capacity failure extends to a depth equal to width of footing. Thus for a
ground water table located in this zone, change the third term in the above equation.

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Engineering Properties of Silt and Clay
Silt and Clay are considered to be smaller family members of soil group,
Even small amounts of fines can have significant effects on the engineering
properties of soils. If as little as 10 percent of the particles in sand and
gravel are smaller than the No.200 sieve size, the soil can be virtually
impervious, especially when the coarse grains are well graded.
Silica is decided to be acidic and magnesia bricks are strongly basic.
However, fire clay bricks are generally placed in the neutral group. Chemical
reaction may take place when the refractory comes in contact with fuel
ashes, slag, gases inside the furnace, and the products such as glass or
steel. Moreover, serious frost heaving in well graded sands and gravels can
be caused by fines making up less than 10 percent of the total soil weight.
The utility of coarse-grained materials for roads can be improved by the
addition of a small amount of clay to act as a binder for the sand and gravel
particles. Soils containing large quantities of silt and clay are the most
troublesome to the engineer. These materials exhibit marked changes in
physical properties with changes in water content.
A hard, dry clay, for example, may be suitable as a foundation for heavy
loads so long as it remains dry, but it may become unstable when wet.

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Many of the fine soils shrink on drying and expand on wetting, which may
adversely affect structures founded upon them or constructed of them.
Even when the water content does not change, the properties of fine soils
may vary considerably between their natural condition in the ground and
their state after being disturbed.
Deposits of fine particles that have been subjected to loading in geologic
time frequently have a structure that gives the material unique properties in
the undisturbed state. When the soil is excavated for use as a construction
material or when the natural deposit is disturbed, for example by driving
piles, the soil structure is destroyed and the properties of the soil are

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changed radically. Silts are different from clays in many important respects,
but because of their similar appearance, they are often mistaken for each
other, sometimes with unfortunate results. Dry, powdered silt and clay are
indistinguishable, but they are easily identified by their behavior in the
presence of water. Recognition of fines as either silt or clay is an essential
part of the USCS.
Silts are the nonplastic fines, they are inherently unstable in the presence
of water and have a tendency to become "quick" when saturated that is,
they assume the character of a viscous fluid and can flow. Silts are fairly
impervious, difficult to compact, and highly susceptible to frost heaving. Silt
masses undergo change of volume with change of shape (the property of
dilatancy), in contrast with clays, which retain their volume with change of
shape (the property of plasticity). The dilatancy of silt together with its quick
reaction to vibration affords a means of identifying typical silt in the loose,
wet state. When dry, silt can be pulverized easily under finger pressure
(indicative of very slight dry strength), and has a smooth feel between the
fingers unlike the grittiness of fine sand.
Silts differ among themselves in size and shape of grains. This is reflected
mainly in the property of compressibility. Generally, the higher the liquid
limit of a silt, the more compressible it is. The liquid limit of a typical bulky-
grained, inorganic silt is about 30 percent; whereas, highly micaceous or
diatomaceous silts (elastic silts), consisting mainly of flaky grains, may
have liquid limits as high as 100 percent. Clays are the plastic fines. They
have low resistance to deformation when wet, but they dry to hard,
cohesive masses. Clays are virtually impervious, difficult to compact when
wet, and impossible to drain by ordinary means. Large expansion and
contraction with changes in water content are characteristics of clays. The
small size, flat shape, and mineral composition of clay particles combine to
produce a material that is both compressible and plastic. Generally, the
higher the liquid limit of a clay, the more compressible it will be. At the
same liquid limit, the higher the plasticity index, the more cohesive the clay.
Field differentiation among clays is accomplished by the toughness test, in
which the moist soil is molded and rolled into threads until crumbling
occurs, and by the dry strength test, which measures the resistance of the
clay to breaking and pulverizing. The clays of low compressibility and low
plasticity, lean clays, can be readily differentiated from the highly plastic,
highly compressible fat clays.

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To Determine Moisture Content of Soil By Oven
Drying Method
(AASHTO DESIGNATION: T-265
ASTM D-2216-90)
The water-content determination is a routine laboratory procedure. ASTM has designeated
it with a Standard, ASTM D-2216-90 which can be found in “ASTM Standards vol. 4.08”,
and also AASHTO T-265, found under “AASHTO Materials: Part II: Tests”. This is a
laboratory procedure to determine the amount of water Ww present in a quantity of soil in
terms of its dry weight Ws. The water content w is usually expressed in percent.
Although it ia a simple experiment to perform, there are several sources of error that can
occur. The most significant is the oven temperature. Many soil-forming minerals are

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hydrous, meaning they contain water within their crystal structures. Normally, the water
content of a soil is measured by oven drying the soil at 110º C. This temperature is used
because it is high enough to evaporate all the water present in the pore spaces of the soil
but is not so large that it drives water out of the structure of most minerals.
Other sources of error include: the time period used for drying the soil, the sample
size, and weighing errors.
Apparatus:
1. Three to five moisture cans (tin or aluminum) with their lids;
2. Temperature controlled oven (a forced-draft type). The oven should be kept at a
temperature of 110 ± 5°C;
3. An electronic scale.
PROCEDURE:
1. Weigh each of the empty moisture cans with their lids and record their weight W1
and its number; you may have to mark it with a felt tip pen
2. Take the sample of soil (under 100 g) collected from the field and place a sample of
it into a can. If you are not testing a field sample, then moisten the sample given to
you (20 to 40 g) with a small amount of water and thoroughly mix it with a spatula.
Place the cap on the can and, weigh and record the can with the lid and the moist
soil weight W2;
3. Always use the same scale, and always check to see that they read zero;
4. Remove the lid, place it underneath the can, and put the can into the drying oven
5. Repeat these steps for the two other cans. There should be three moisture cans in
the oven. The temperature of the drying oven should be kept between 105º and 110º
C, and the cans should remain in the oven for at least 24 hours;
6. After 12 to 18 hours (or overnight), weigh and record the new weight of the moisture
can with the dried soil and its lid W3. This procedure is adequate for small amounts
of soil (10 to 200 grams). Much larger soil samples may require occasional stirring
so that a uniform drying takes place;
7. Remove from the oven with tongs or heat-treated loves and weigh immediately;
some manuals claim that convection currents affect the result, but this writer has
never found this to be true;
8. The total weight difference between W3 and W2 is the weight of the water that was
evaporated from the soil. This weight loss will be then used to calculate the
percentage of water content w in the soil.
9. Report the water content to the nearest 0.1 percent, but in computations w is used as
a decimal quantity.

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TEST SAMPLE:
Sample shall be washed and oven-dried at a temperature of 105 °C-110 °C and should
conform to one of the grading in observation.
Ca
n
(#)
Weig
ht of
Can
(W1l
b)
(Lb)
Weigh
t of
Can +
Moist
Soil
(W2lb)
(lb)
Weig
ht of
Can
(W1g
)
(g)
(1)
Weigh
t of
Can +
Moist
Soil
(W2g)
(g)
Weig
ht of
Can
+ Dry
Soil
(W3)
(g)
(2)
Weig
ht of
Wate
r
(WW
)
(g)
(3)
Weig
ht of
Dry
Soil
(WS)
(g)
(4)
Water
Conte
nt
(W)
(%)
(5)
Erro
r
(%)
1 0.034
5
0.0690 15.65 31.30 28.9 2.398 13.25
1
18.10 4.97
2 0.034
5
0.0625 15.65 28.35 26.4 1.950 10.75
1
18.13 4.77
3 0.035
5
0.0695 16.10 31.52 28.9 2.625 12.79
7
20.51 7.71
4 0.035
0
0.0635 15.88 28.80 26.7 2.103 10.82
4
19.43 2.04
Average 19.04 4.8
7
Standard
Deviation
1.16
Sample Calculations:
Conversion of pounds to
grams =
Weight of water in
Sample =
Precautions:
 The soil sample should be loosely placed in the container.
 Over heating should be avoided.
 Mass should be found carefully.

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USES AND SIGNIFICANCE:
1. Moisture content plays an important role in understanding the behavior of soil.
2. It shows the degree of compaction of soil in the field.
Standard Test Methods are:
 AASHTO T 96 and ASTM C 131: Resistance to Degradation of Small-Size
Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine
 ASTM C 535: Resistance to Degradation of Large-Size Coarse Aggregate by
Abrasion and Impact in the Los Angeles Machine
Modes of Shear Failure of Soil | General, Local,
Punching Shear Failure

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Shear failure of Soil

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There are three modes of shear failure, i.e. General, Local and Punching shear failures
depending upon the compressibility of soil and depth of footing with respect to its
breadth (i.e D/B Ratio). When the ultimate bearing capacity of the soil is reached, it may
fail in one of the following three failure mode depending upon the type of soil and depth
to width ratio of the footing (i.e. D/B)
 General Shear Failure
 Local Shear Failure
 Punching Shear Failure
1. General Shear Failure
 In this mode a slight downward movement of the footing develops fully plastic zones and
a sudden failure takes place with a considerable bulging of the ground surface adjacent to
the footing
 Characterized by well defined failure pattern, consisting of a wedge and slip surface and
bulging (heaving) of soil surface adjacent to the footing
 Sudden collapse occurs, accompanied by tilting of the footing
 This type of failure occurs in case of dense sand or stiff cohesive soil supporting the footing
 Failure load is well defined
 The load-settlement diagram is similar to stress-strain for dense sand or over-consolidated
clay as shown
 The ultimate load is well defined on this curve as shown typically in figure given below
2. Local shear failure
Failure pattern consists of wedge and slip surface but is well defined only under the footing.
Slight bulging of soil surface occurs. Tilting of footing is not expected.
 In this mode a large deformation takes place under the footing before the development of
failure zones, i.e. large vertical settlement takes place before slight bulging of the ground
surface
 Tilting of footing is not expected
 Ultimate load is not well defined
 It takes place in moderately compressible soils or loose sand i.e Occurs in soil of high
compressibility
 Yielding takes place close to the lower edges of the footing
 Several yield developments may occur accompanied by settlement in a series of jerks
 The bearing pressure at which the first yield takes place is referred to as the first-failure
pressure or first failure load

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3. Punching shear failure
 Failure pattern is not well defined
 No bulging of ground surface and no tilting of footing occurs
 The yield surfaces are vertical planes immediately adjacent to the sides of the foundation
 The ground surface may be dragged down thus, no bulging of the surface takes place
 Failure take place immediately below footing and surrounding soil remains relatively
unaffected
 Large settlements-ultimate load is not well defined
 Punching Shear Failure takes place in weak compressible soils with considerable vertical
settlement i.e Occurs in soil of very high compressibility
 It also occurs in the soil of low compressibility, if the foundation is located at considerable
depth.
 After the first yield the load-settlement curve will steepen slightly, but remain fairly flat
 Punching Shear Failure may also take place in soil of low compressibility, if the foundation
is located at a considerable depth
Modes of Shear Failure (Summary)
General
Local
Punching
Relative Settlement Less Large Large
Bulging Significant Less No
Tilting of Footing Expected Not expected Not expected
Ultimate Load Well defined Not well defined Not well defined
Failure Pattern
Wedge +
Slip Surface +
Bulging
Wedge +
Slip Surface +
Bulging(no or less)
Not well defined
Occurs in (Soil Type) Dense Less compressible Highly Compressible
How to find Bearing Capacity of Soil

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Methods of bearing capacity determination
 Analytic method i.e. through bearing capacity equations like using Terzaghi
equation, Meyerhof equation, Hansen equation etc
 Correlation with field test data e.g. Standard penetration test (SPT), Cone
penetration test (CPT) etc
 On site determination of bearing capacity e.g Plate load test, Pile load test
 Presumptive bearing capacity (recommended bearing capacity, in various codes)
Following are the methods:
1. Analytical Method of Bearing capacity determination
Analytical Method
Lower Bound Failure
Lower bound failure states that “If an equilibrium distribution of stress can be found
which balances the applied load and nowhere violates the yield criterion, the soil mass
will not fail or will just be at a point of failure i.e. it will be a lower bound estimate of
capacity. Consider the equilibrium conditions in soil under the footing load. When the
foundation pushes into the ground, stress block 1 has principal stresses, as shown. The
push into the ground however, displaces the soil on the right side of the line OY laterally,

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resulting in the major principal stress on block 2 being horizontal as shown. When the
two blocks are adjacent to each other at the vertical line OY, then
Some Formulae
Upper Bound theorem
Upper bound theorem states that “If a solution is kinematically admissible and
simultaneously satisfies equilibrium failure must result” i.e. it will be an upper bound
estimate of capacity. For a possible upper bound, consider failure surface as semicircle.
Taking moment about O
Terzaghi’s bearing capacity equation (1943)
Terzaghi developed a general formula for ultimate bearing capacity of spread footing
foundation under the following assumptions:
 The depth of the footing is less than or equal to its width (D, B)
 The foundation is rigid and has a rough bottom
 The soil beneath the foundation is homogeneous semi-infinite mass
 Strip foundation with a horizontal base and level ground surface under vertical loads.
 The general shear mode of failure governs and no consolidation if the soil occurs
(settlement is due only to shearing and lateral movements of the soil)
 The shear strength of the soil is described by s = c + σ tan
Presumptive Soil Bearing Values by Soil Description
Presumptive Soil Bearing Values by Soil Description
Presumptive Load Bearing Value
(psf)
Soil Description
1500 Clay, Sandy Clay, Silty Clay, Clayey Silt, Silt and Sandy Silt
2000 Sand, Silty Sand, Clayey Sand, Silty Gravel and Clayey Gravel
3000 Gravel and Sandy Gravel
4000 Sedimentary Rock
12000 Crystalline Bedrock
Index Properties of Soil

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The soil properties on which their classification and identification are based are known as
index properties. The index properties which are used are:
 Grain Size Distribution
 Consistency Limits
 Plasticity Index
Grain Size Distribution (Sieve Analysis)
The sieves are arranged, keeping the finest below and the coarser above it. A definite
quantity of soil is dried in an electric oven (for 24 hrs at 105°C) and put in the top sieve.
The cover is placed over the top sieve and a pan below the lowest sieve. Then the sieves
are placed in the sieve shaker and are shaken for few minutes. The weight of soil thus
retained on each sieve is determined and is converted into a percentage.
Consistency of Soil
Consistency of soil is the physical state of soil with respect to moisture content present at
that time. By consistency means the relative ease with which soil can be deformed.
Consistency is related to the fine grained soil.
Consistency Limits or Atterberg Limits
The moisture content at which the soil change from one state to another State is called
consistency limits or Atterberg limits There are four different states of soil:

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1. Solid State
2. Semi Solid State
3. Plastic State
4. Liquid State
When water is added to the dry soil mass its change from solid state to liquid state
passing through semi solid and plastic state. Atterberg identify some other limits which
are most useful for engineering purpose.
Why Study Soil Mechanics in Civil Engg?
Definition of Soil:
Soil is a natural agglomerate used as a construction as well as supporting material and is
composed of particles of broken rocks which keep on changing in their texture, structure,
consistency, color, chemical, biological and other characteristics from their parent
material (rock) by chemical and mechanical processes like disintegration erosion,
freezing and thawing.
Definition of Mechanics:

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Mechanics is the branch of science dealing with the behavior or response of bodies at rest
or with negligible velocity subjected to forces, pressures or displacements.
Soil Mechanics
Soil Mechanics is the branch of civil engineering that is concerned with behavior of soil
at rest under different conditions of direct (Structural Loads) and indirect (Thermal
Stresses) loading.
According to Terzaghi
Soil Mechanics is the application of the laws of mechanics and hydraulics to engineering
problems dealing with sediments and other unconsolidated accumulations of solid
particles produced by chemical and mechanical disintegration of rocks regardless of
whether or not they contain an admixture of organic constituents.
Why do we Study Soil Mechanics?
 Every man made structure rests on earth and thus need a foundation to transfer the
ultimate load without failure. To determine the behavior of soil and to resist these
ultimate loads we need to study its mechanics.
 To make new researches in the relevant field
 To replace the old (unscientific) methods of construction with the new and advanced ones
Uses of Soil in Civil Engineering
In civil engineering the soil can be used in different ways. The uses of soil can be divided
into three categories:
1) As a Supporting Material
Soil is used as a supporting material to bear the load of structures e.g.
 Bridges,
 Roads
 Buildings
2) As a Raw Material (Unprocessed Material)
In natural form soil can be used for the following purposes
 Flood Control Embankments
 Road Embankments
 Earth-Filled Dam

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3) As a Processed Material
Passed through some process to get
 Burnt Bricks
 Concrete Mixes
Formation of Soil
 The formation of soil starts with the parent material and continues for a very long period
of time taking 1000 years or more.
 As the parent material is weathered and / or transported, deposited and precipitated it is
transformed into a soil.
 The parent material may be in the form of bedrock, glacial deposits, and loose deposits
under water or material moving down sloping land.
Factors Affecting Formation of Soil
Soil formation process is influenced by the following factors
 Composition of Parent Material
 Climate
 Topography
 Organisms
 Time
Bare rocks exposed to warm climate, frequent and heavy annual rainfalls causes faster
development of soil.
Standard Values for Liquid Limit of Soil and
Limitations of L.L Test

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The liquid limit of a soil is the moisture content, expressed as a percentage of the mass
of the oven-dried soil, at the boundary between the liquid and plastic states The moisture
content at this boundary is arbitrarily defined as the liquid limit and is the moisture
content at a consistency as determined by means of the standard liquid limit apparatus.
Introduction to Liquid Limit Test
The liquid limit test is one of the most widely used tests in the soil engineering practice.
Several properties, including mechanical properties (for example, compressive index),
have correlations with the liquid limit.
In this paper detailed investigations of the liquid limit of soil mixtures have been carried
out using bentonite, kaolinite, sand (coarse grained, fine grained, rounded and angular

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shaped), and silts. Based on the results obtained, it has been shown that the liquid limits
of soil mixtures are not governed by the linear law of mixtures. While the shape of the
sand was not found to influence the liquid limit, the size of the sand particles had a
definite influence.
First of all a grooved is made in the soil sample by using a standard grooving tool along
the diameter through the center line of the cam follower so that a clean, sharp groove of
proper dimension is formed, the cup shall be dropped by turning the crank at the rate of
two revolutions per seconds and the number of blows counted until the two halves of the
soil cake come into contact with the bottom of the groove along a distance of about 12
mm. A representative soul sample nearer the groove is taken for moisture content
determination. The moisture content is reported along with number of blows required to
close a groove. The operations specified above shall be repeated for at least three trails at
different moisture content. The specimens shall be not less than 15 and more than 35/ the
test should proceed from the drier (more drops) to wetter (less drops) condition of the
soil. It has been found that the liquid limit of certain materials is influenced by the time of
mixing. There is a tendency, particularly noticeable in the case of decomposed dolerites
and certain pedogenic materials, for the liquid limit to increase as the time of mixing is
increased, although this increase will, of course, not continue indefinitely.
Hence it was considered necessary to stipulate a mixing time and a period of ten minutes
was decided on. Some times soil tends to slide on the surface of the cup instead of
flowing. If this occurs, the results should be discarded and the test repeated until flowing
does occur. If sliding still occurs, the test is not applicable and it should be reported that
the liquid limit could not be obtained.
Standard Values for Liquid Limit Test
Liquid limit is the water content at which a part of soil, cut by a groove of standard
dimensions, will flow together for a distance of 1.25 cm under an impact of 25 blows in a
standard liquid limit apparatus. The soil at the water content has some strength which is
about 0.17 N/cm.sq. (17 gms/sq.cm.) .

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Limitations of Liquid Limit Test
The operations specified above shall be repeated for at least three trails at different
moisture content. The specimens shall be of such consistency that the number of drops
required to close the groove shall not be less than 15 and more than 35 the test should
proceed from the drier (more drops) to wetter (less drops) condition of the soil.
Some times soil tends to slide on the surface of the cup instead of flowing. If this occurs,
the results should be discarded and the test repeated until flowing does occur. If sliding
still occurs, the test is not applicable and it should be reported that the liquid limit could
not be obtained.
Liquid limit depends on the type of plant detritus contained, on the degree of
humification, and on the proportion of clay soil present. Generally the liquid limit of fen
peat according to Hobbs, ranges from 200 to 600% and bog peat from 800 to 1500% with
transition peats between. The liquid limit in other words is reduced by increasing degree
of humification. In addition as the organic content declines to lower values of liquid limit
are obtained. Usually fen peats have water content at or somewhat below their liquid
limits. This is because partially decomposed plant material has a higher cation exchange
capacity than any clay which occupies the pores, bog peats contain less mineral matter
and so their water contents exceed their liquid limits.
Sieve Analysis of Coarse Grained Soil

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Apparatus
A set of various sizes of sieves, balance.
Procedure
1. Arrange different types of sieves in order of there decreasing size of opening.
2. Find the total weight of the given soil sample and pour it in the top sieve.
3. Place the set of sieves on mechanical shakers and shake it properly.
4. Find the weight of soil retained on each sieve.
5. Calculate percentage weight of soil passing through each sieve.
6. Draw a grain size distribution/gradation curve.
Precautions:
 During shaking soil sample should not b allowed to spell out.
 All the readings should be noted carefully.
Practical applications
 Grain size analysis gives an idea regarding the gradation of soil.
 It is used to proportion the selected soil in order to obtain the desired soil mix.
 It is also utilized in part of the specification of soil for air field’s roads, earth dams
and other soil embankment construction.
Observations & Calculations:

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Sieve no. Weight of soil
retained on
each sieve (gm)
Percent weight
retained
Cumulative
percent weight
retained
Cumulative
percent
passing
04 181.8 36.36 36.36 63.64
08 91 18.2 54.65 45.44
16 99.6 19.92 74.48 25.52
30 55.33 11.066 85.55 14.45
50 46.8 9.36 94.91 5.09
100 10.3 2.06 96.97 3.03
200 9.6 1.92 98.89 1.11
pan 4.8 0.96 99.85 0.15
5 Factors Affecting Formation of Soil

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The formation of soil starts with the parent material and continues for a very long period
of time taking 1000 years or more. As the parent material is weathered and / or
transported, deposited and precipitated it is transformed into a soil. The parent material
may be in the form of bedrock, glacial deposits, and loose deposits under water or
material moving down sloping land.
Factors Affecting Formation of Soil
Soil formation process is influenced by the following factors
 Composition of Parent Material
 Climate
 Topography
 Organisms
 Time
Bare rocks exposed to warm climate, frequent and heavy annual rainfalls causes faster
development of soil.
Weathering
It is a natural process of breaking down of rocks, and minerals as well as artificial
materials due to its prolonged exposure to the environment. These particles when moved
away by gravity, water, ice, wind etc is then known as erosion. In short, when a particle is

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loosened by a chemical or physical process, but does not move, it is called Weathering.
When the particle moves, it is called Erosion.
Types of Weathering
Depending upon the mechanism and causes, weathering can be divided into three broad
categories
1. Physical Weathering
2. Chemical Weathering
3. Biological Weathering
Physical Weathering
Physical Weathering also known as Mechanical Weathering may occur due to the
following reasons
1. High Speed Winds
High Speed winds causes the erosion of surface material
2. Thermal Stress
Thermal stresses are caused by repeated heating and cooling. In deserts the temperature at
day time is more causing expansion of bodies while at night the temperature decreases
causing contraction eventually leading to breaking of rocks
3. Freezing and Thawing
The moisture or water penetrated into cracks when freezes, it expands causing stresses
and breaking of rock
4. Pressure Releases
When some of the material is removed from the surface, the load is reduced and the
underlying body expands up causing cracking.
5. Hydraulic Actions
Water from power full waves when strike against the rock causes erosion.

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6. Salt-Crystals Growth
When saline solutions seep into cracks and joints in the rocks and evaporate it leaves salt
crystals behind. These salt crystals expand as they are heated up thus exerting pressure on
the confining rock. In Physical Weathering the broken down particles have same properties
as the parent material
Chemical Weathering
Chemical Weathering may occur due to the following processes and reactions
1) Dissolution and Carbonation
2) Hydration
3) Hydrolysis
4) Oxidation
In Chemical Weathering the broken particles have different composition from the parent
material and change into different substances
1) Dissolution and Carbonation
 In unpolluted environments, the rainfall pH is around 5.6.
 Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the
atmosphere.
 These oxides react with the rain water to produce stronger acids and lower the pH to 4.5
or even 3.0 causing solution weathering to the rocks on which it falls.
 When rain combines with carbon dioxide or an organic acid it forms a weak carbonic
acid which reacts with calcium carbonate (Rock) and forms calcium bicarbonate.
 The process speeds up with a decrease in temperature because colder water holds more
dissolved carbon dioxide gas
2) Hydration
In this process rock minerals absorb water and expand, creating stress which causes the
disintegration of rocks.
Iron Oxide + Water ---> Iron Hydroxide (Increased Volume)
Anhydrite + Water ---> Gypsum (Increased Volume)
Anhydrite is Unhydrated Calcium Sulphate.

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Gypsum is Hydrated Calcium Sulphate
3) Hydrolysis
In this process a chemical reaction takes place between the minerals (Silicates and
Carbonates) in the rock and Hydrogen (H+) in rain water.
4) Oxidation
In this process oxygen combine with water and minerals in the rock such as iron, calcium
and magnesium to form their oxides
When iron reacts with oxygen, reddish-brown iron oxide is formed. The iron-oxide crust
crumbles easily and weakens the rock. This process is commonly known as Rusting
Iron + Oxygen ---> Iron Oxide (Crumbles)
Common Types of Soil
Gravel
 Course grained soils that possess little or no cohesion.
 Distinguished by their relative stability under wheel loads when confined, by their high
permeability and by their low shrinkage and expansion in deter mental amount with
change in moisture content.
 Size range from 2 mm to 60 mm

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Sand:
 Fine grained soils of low to medium plasticity intermediate in size bw sand & clay.
 Generally possess little cohesion, undergo considerable shrinkage and expansion with.
Change in moisture content and possess a variable amount of stability under wheel load.
 Organic soils contain appreciable amount of decomposed organic matter and is generally
highly compressible & unstable.
 Size of sand ranges from 0.06 mm to 2 mm
Silt
 The size of silt particles range from 0.002 mm to 0.06 mm
Clay:
 Fine grain soil of size 0.002 mm or finer.
 Possess medium to high plasticity, have considerable strength when dry, undergo extreme
changes in volume with change in moisture content and practically impervious to flow of
water.
Types:
 Lean clay: Silty clay.
 Flat clay: Finer colloidal clay with high plasticity
 Clay may possess considerable strength in their natural state; this strength is sharply
reduced and sometimes completely destroyed when their natural structure is disturbed,
that is when they are remolded.
Loam:
Agricultural term, fairly well graded from course to fine that is easily worked & that is the
reason of plant life.
 Sandy loam
 Silty loam &
 Clay loam
Depending on the size of predominant soil fraction
Loess:
 Uniform grain size predominantly silts with low density.
Muck:
 Soft silt or clay very high in organic content
 Found in swampy areas and rivers or lake bottoms.
Peat:
 Composed of practically decomposed vegetable matter
 Due to high water content, woody natural and high compressibility it is extremely
undesirable for use in roads.

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Compaction of Soil and Factors Affecting
Compaction
Compaction is the application of mechanical energy to a soil to rearrange the particles and
reduce the void ratio.
Purpose of Compaction of Soil
The principal reason for compacting soil is to reduce subsequent settlement under working
loads.
 Compaction increases the shear strength of the soil.
 Compaction reduces the voids ratio making it more difficult for water to flow through soil.
This is important if the soil is being used to retain water such as would be required for an
earth dam.
 Compaction can prevent the build up of large water pressures that cause soil to liquefy
during earthquakes.

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Factors affecting Soil Compaction
 Water content of the soil
 The type of soil being compacted
 The amount of compactive energy used
Effects of different factors on compaction of soil
Water content:
As water is added to a soil ( at low moisture content) it becomes easier for the particles to
move past one another during the application of the compacting forces. As the soil
compacts the voids are reduced and this causes the dry unit weight ( or dry density) to
increase. Initially then, as the moisture content increases so does the dry unit weight.
However, the increase cannot occur indefinitely because the soil state approaches the
zero air voids line which gives the maximum dry unit weight for a given moisture
content. Thus as the state approaches the no air voidsline further moisture content
increases must result in a reduction in dry unit weight. As the state approaches the no air
voids line a maximum dry unit weight is reached and the moisture content at this
maximum is called the optimum moisture content.
Increased compactive effort
Increased compactive effort enables greater dry unit weights to be achieved which
because of the shape of the no air voids line must occur at lower optimum moisture
contents. It should be noted that for moisture contents greater than the optimum the use of
heavier compaction machinery will have only a small effect on increasing dry unit
weights. For this reason it is important to have good control over moisture content during
compaction of soil layers in the field.
Effects of soil type
The table below contains typical values for the different soil types obtained from the
Standard Compaction Test.
Typical Compaction Values
Type of Soil (gdry )max (kN/ m3
) mopt (%)
Well graded sand SW 22 7
Sandy clay SC 19 12
Poorly graded sand SP 18 15
Low plasticity clay CL 18 15
Non plastic silt ML 17 17

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High plasticity clay CH 15 25
Note that these are typical values. Because of the variability of soils it is not appropriate
to use typical values in design, tests are always required.
Atterberg's Limits of Soil Classification -
Atterberg Test
A fine-gained soil can exist in any of several states; which state depends on the amount of
water in the soil system. When water is added to a dry soil, each particle is covered with a
film of adsorbed water. If the addition of water is continued, the thickness of the water film
on a particle increases. Increasing the thickness of the water films permits the particles to
slide past one another more easily. The behavior of the soil, therefore, is related to the
amount of water in the system. Approximately sixty years ago, A. Atterberg defined the
boundaries of four states in terms of "limits" as follows:
Atterberg's Definitions

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 Liquid limit: The boundary between the liquid and plastic states;
 Plastic limit: The boundary between the plastic and semi-solid states;
 Shrinkage limit: The boundary between the semi-solid and solid states.
Casagrande's Definition
These limits have since been more definitely defined by A. Casagrande as the water
contents which exist under the following conditions:
 Liquid limit: The water content at which the soil has such a small shear strength
that it flows to close a groove of standard width when jarred in a specified manner.
 Plastic limit: The water content at which the soil begins to crumble when rolled
into threads of specified size.
 Shrinkage limit: The water content that is just sufficient to fill the pores when the
Liquid Limit Definition
The Liquid Limit, also known as the upper plastic limit, is the water content at which soil
changes from the liquid state to a plastic state. OR
It is the minimum moisture content at which a soil flows upon application of very small
shear force. OR
The moisture content at which any increase in the moisture content will cause a plastic soil
to behave as a liquid. The limit is defined as the moisture content, in percent, required to
close a distance of 0.5 inches along the bottom of a groove after 25 blows in a liquid limit
device.
Liquid Limit (LL or wL) - the water content, in percent, of a soil at the arbitrarily defined
boundary between the semi-liquid and plastic states.

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Atterberg's Limits
Plastic Limit Definition
The Plastic Limit, also known as the lower plastic limit, is the water content at which a soil
changes from the plastic state to a semisolid state. OR
The moisture content at which any increase in the moisture content will cause a semi-solid
soil to become plastic. The limit is defined as the moisture content at which a thread of soil
just crumbles when it is carefully rolled out to a diameter of 1/8 inch.
Plastic Limit (PL or wP) - the water content, in percent, of a soil at the boundary between
the plastic and semi-solid states.
Plasticity
It is defined as the property of cohesive soil which posses the ability to undergo changes
of shape without rupture or a change in volume.
Non plastic soils
Soil that do not have plastic limit are reported as being non-plastic e.g fairly clean sand,
rock dust etc. Non plastic soils make excellent road materials when properly confined
under wearing course .e.g. Rock dust. Even when wet they form hard, durable surface
whereas clean sand displaces easily under the load. However, their use for base course or
for fill brings difficult construction problems.
Plasticity Index (PI)
Plasticity Index is the range of water content over which a soil behaves plastically. It
indicates the degree of plasticity of the soil. PI is the difference between the liquid limit

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and the plastic limit. Greater the difference, greater is the plasticity of the soil. Soils with a
high PI tend to be predominantly clay, while those with a lower PI tend to be predominantly
silt. Soils with high plasticity index are highly compressible. Plasticity index in also a
measure of cohesiveness with high value of PI indicating high degree of cohesion.
Experience shows that soils with high P.I are much less desirable for sub grade or base
course than those having less indexes. For base course common specification is P.I < 3.
Shrinkage Limit
It is the maximum water content at which a reduction in water content will not cause a
decrease in volume of the soil mass. It is the lowest water content at which soil can still be
saturated.
Liquidity Index / Water Plasticity Ratio
The Atterberg's limits are found for remoulded soil samples. These limits as such do not
indicate the consistency of undisturbed soil. The index to indicate consistency of
undisturbed soils is called liquidity index of the soil or Water Plasticity Ration of the soil.
Its advantage is that The liquidity index (LI) is used for scaling the natural water content
of a soil sample to the limits.
Flow Index
The curve obtained from the graph of water content against the log of blows while
determining the liquid limit lies almost on a straight line and is known as the flow curve.
The equation for flow curve is:
W = - If Log N + C
Where 'If is the slope of flow curve and is termed as "Flow Index"
Toughness Index
The shearing strength of clay at plastic limit is a measure of its toughness. It is the ratio
of plasticity index to the flow index. It gives us an idea of the shear strength of soil.
Physical Properties of Soil

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By: Haseeb Jamal / On: Mar 21, 2017 / NOTES, DEFINITION, MATERIAL
PROPERTIES
1. Voids ratio & Porosity
Using volumes is not very convenient in most calculations. An alternative measure that is
used is the voids ratio, e. This is defined as the ratio of the volume of voids, Vv to the
volume of solids, Vs, that is A related quantity is the porosity, n, which is defined as ratio
of the volume of voids to the total volume.
2. Degree of saturation
The degree of saturation, S, has an important influence on the soil behavior. It is defined
as the ratio of the volume of water to the volume of voids.
3. Air content
4. Water content:
The moisture content, m, is a very useful quantity because it is simple to measure. It is
defined as the ratio of the weight of water to the weight of solid material. The water content
(also known as moisture content) test is probably the most common and simplest type of
laboratory test. This test can be performed on disturbed or undisturbed soil specimens. The
water content test consists of determining the mass of the wet soil specimen and then drying
the soil in an oven overnight (12 to 16 hr) at a temperature of 110 °C (ASTM D 2216-92,
1998). The water content (w) of a soil is defined as the mass of water in the soil (Mw)
divided by the dry mass of the soil (Ms), expressed as a percentage (i.e., w _ 100 Mw/Ms).
Values of water content (w) can vary from essentially 0% up to 1200%. A water content
of 0% indicates a dry soil. An example of a dry soil would be near-surface rubble, gravel,
or clean sand located in a hot and dry climate. Soil having the highest water content is
organic soil, such as fibrous peat, which has been reported to have water content as high
as 1200%.
5. Unit weight:
Several unit weights are used in Soil Mechanics. These are the bulk, saturated, dry, and
submerged unit weights. The bulk unit weight is simply defined as the weight per unit
volume.

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6. Bulk unit weight of soil:
The weight of the aggregate that fills a 1-cubic-foot container. This term is used because
the volume contains both aggregate and voids air spaces. The total unit weight (also known
as the wet unit weight) should only be obtained from undisturbed soil specimens, such as
those extruded from Shelby tubes or on undisturbed block samples obtained from test pits
and trenches. The first step in the laboratory testing is to determine the wet density, defined
as _t _ M/V, where M _ total mass of the soil, which is the sum of the mass of water (Mw)
and mass of solids (Ms), and V _ total volume of the soil
Unit weight of soil solids
Dry unit weight
Saturated unit weight
Saturated unit weight (γsat) is the saturated weight of soil per unit volume. A saturated
soil has a degree of saturation (S) equal to 100%. The saturated unit weight is usually
expressed as kN/m3
.
Submerged unit weight
Buoyant unit weight or submerged unit weight is the effective mass per unit volume
when the soil is submerged below standing water or below the ground water table.
Specific gravity:
The specific gravity (G) is a dimensionless parameter that is defined as the density of solids
(_s) divided by the density of water (_w), or G _ _s / _w. The density of solids (_s) is
defined as the mass of solids (Ms) divided by the volume of solids (Vs). The density of
water (_w) is equal to 1 g/cm3 (or 1 Mg/m3) and 62.4 pcf. For soil, the specific gravity is
obtained by measuring the dry mass of the soil and then using a pycnometer to obtain the
volume of the soil. Because quartz is the most abundant type of soil mineral, the specific
gravity for inorganic soil is often assumed to be 2.65. For clays, the specific gravity is often
assumed to be 2.70 because common clay particles, such as montmorillonite and illite, have
slightly higher specific gravity value.
Mass specific gravity
It is the ratio of the mass of a substance to the mass of a reference substance for the same
given volume.

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Classification of Soil on Particle Size & Moisture
Content Basis
Classification of Soil (Moisture Content Based)
1. Dry
2. Saturated
A saturated soil is a two phase material consisting of a soil skeleton and voids
which are saturated with water. It is reasonable to expect that the behavior of
an element of such a material will be influenced not only by the forces
applied to its surface but also by the water pressure of the fluid in the pores.
Suppose that a soil sample having a uniform cross sectional area A is
subjected to an applied load W, then it is found that the soil will deform. If
however, the sample is loaded by increasing the height of water in the
containing vessel, as shown in Fig lb, then no deformation occurs.
3. Partially saturated
2. Types (on the basis of particle size)
Gravel Sand Silt Clay
Coarse Medium Fine Coarse Medium Fine Coarse Medium Fine Coarse Medium Fine
60 20 6 2 0.6 0.2 0.06 0.02 0.006 0.002 0.0006 0.0002
Table 1. Particle size boundaries
1. Boulders
2. Cobbles
3. Gravel
4. Sand
5. Silt
6. Clay
7. Organic Soil
Particle Size Classification of Soil
There are two soil classification systems in common use for engineering purposes.
The Unified Soil Classification System is used for virtually all geotechnical engineering
work except highway and road construction, where the AASHTO soil classification
system is used. Both systems use the results of grain size analysis and determinations
of Atterberg limits to determine soil’s classification. Soil components may be described
as gravel, sand, silt, or clay. A soil comprising one or more of these components is given a
descriptive name and a designation consisting of letters or letters and numbers which

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depend on the relative proportions of the components and the plasticity characteristics of
the soil.
1. MIT System of soil classification:
2. AASHTO classifications of soils:
The AASHTO system classifies soils into seven primary groups, named A-1 through A-7,
based on their relative expected quality for road embankments, sub-grades, sub-bases,
and bases. Some of the groups are in turn divided into subgroups, such as A-1-a and A-1-
b. Furthermore, a Group Index may be calculated to quantify a soil’s expected
performance within a group. To determine a soil’s classification in the AASHTO system,
one first determines the relative proportions of gravel, coarse sand, fine sand, and silt-clay.
AASHTO Classification Chart
General
Classification
Granular Materials (35% or less passing the
0.075 mm sieve)
Silt-Clay Materials
(>35% passing the 0.075
mm sieve)
Group
Classification
A-1 A-3 A-2 A-4 A-5 A-6 A-7
A-1-
a
A-1-
b
A-2-
4
A-2-
5
A-2-
6
A-2-
7
A-7-
5 A-
7-6
Sieve Analysis, %
passing
2.00 mm (No.
10)
50
max
… … … … … … … … … …
0.425 (No. 40) 30
max
50
max
51
min
… … … … … … … …
0.075 (No. 200) 15
max
25
max
10
max
35
max
35
max
35
max
35
max
36
min
36
min
36
min
36
min
Characteristics of
fraction passing
0.425 mm (No.
40)
Liquid Limit … … 40
max
41
min
40
max
41
min
40
max
41
min
40
max
41
min
Plasticity Index 6 max N.P. 10
max
10
max
11
min
11
min
10
max
10
max
11
min
11
min
Usual types of
significant
constituent
materials
stone
fragments,
gravel and
sand
fine
sand
silty or clayey gravel
and sand
silty soils clayey soils

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General rating as
a subgrade
excellent to good fair to poor
Note: Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity
index of A-7-6 subgroup is greater than LL - 30
In the AASHTO system:
 gravel is material smaller than 75 mm (3 in.) but retained on a No. 10 sieve;
 coarse sand is material passing a No 10 sieve but retained on a No. 40 sieve; and fine
sand is material passing a No. 40 sieve but retained on a No. 200 sieve.
 Material passing the No. 200 sieve is silt-clay and is classified based on Atterberg limits.
 It should be noted that the division between gravel and sand is made at a smaller size (No.
10 sieve) in the AASHTO system than in the unified system (No. 4 sieve).
Secondly, if any fines are present, Atterberg limits are determined and the plasticity index
is calculated. A soil is a granular material if less than 35% of the soil by weight passes the
No. 200 sieve (#200). Granular materials are classified into groups A-1 through A-3. Soils
having more than 35% passing the No. 200 sieve are silt-clay and fall in groups A-
4 through A-7. Having the proportions of the components and the plasticity data, one enters
one of the two alternatives AASHTO classification tables and checks from left to right
until a classification is found for which the soil meets the criteria. It should be noted that,
in this scheme, group A-3 is checked before A-2. Soils classified as A-1 are typically well-
graded mixtures of gravel, coarse sand, and fine sand. Soils in subgroup A-1-a contain
more gravel whereas those in A-1-b contain more sand.
Soils in group A-3 are typically fine sands that may contain small amounts of non-plastic
silt. Group A-2 contains a wide variety of “borderline” granular materials that do not meet
the criteria for groups A-1 or A-3. Soils in group A-4 are silty soils, whereas those in
group A-5 are high-plasticity elastic silt. Soils in group A-6 are typically lean clays, and
those in group A-7 are typically highly plastic clays. Within groups containing fines, one
may calculate a group index to further evaluate relative quality and supporting value of a
material as sub-grade. The group index is calculated according to the following empirical
formula:
Group index F 35 – ( )0.2 0.005 LL 40 – ( ) + [ ] +
0.01 F 15 – ( )PI 10 – ( )
3. Unified soil classification system (USCS):
The Unified Soil Classification System is based on the airfield classification system
developed by Casagrande during World War II. With some modification it was jointly
adopted by several U.S. government agencies in 1952. Additional refinements were made
and it is currently standardized as ASTM D 2487-93. It is used in the U.S. and much of the

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world for geotechnical work other than roads and highways. In the unified system soils are
designated by a two-letter symbol: the first identifies the primary component of the soil,
and the second describes its grain size or plasticity characteristics. For example, poorly
graded sand is designated SP and low plasticity clay is CL. Five first-letter symbols are
used:
1. G for gravel
2. S for sand
3. M for silt
4. C for clay
5. O for organic soil
Clean sands and gravels (having less than 5% passing the No. 200 sieve) are given a second
letter P if poorly graded or W if well graded. Sands and gravels with more than 12% by
weight passing the No. 200 sieve are given a second letter M if the fines are silty or C if
fines are clayey. Sands and gravels having between 5 and 12% are given dual
classifications such as SP-SM. Silts, clays, and organic soils are given the second letter H
or L to designate high or low plasticity. The specific rules for classification are summarized
as follows and described in detail in ASTM D 2487.
1. Organic soils are distinguished by a dark-brown to black color, an organic odor, and
visible fibrous matter.
2. For soils that are not notably organic the first step in classification is to consider the
percentage passing the No. 200 sieve.
3. If less than 50% of the soil passes the No. 200 sieve, the soil is coarse grained, and the first
letter will be G or S;
4. if more than 50% passes the No. 200 sieve, the soil is fine grained and the first letter will
be M or C.
For coarse-grained soils, the proportions of sand and gravel in the coarse fraction
(not the total sample) determine the first letter of the classification symbol. The coarse
fraction is that portion of the total sample retained on a No. 200 sieve. If more than half of
the coarse fraction is gravel (retained on the No. 4 sieve), the soil is gravel and the first
letter symbol is G. If more than half of the coarse fraction is sand, the soil is sand and the
first letter symbol is S. For sands and gravels the second letter of the classification is based
on gradation for clean sands and gravels and plasticity of the fines for sands and gravels
with fines.
For clean sands (less than 5% passing the No. 200 sieve), the classification is well-graded
sand (SW) if C ≥ 6 and 1 £ Cc £ 3. Both of these criteria must be met for the soil to be SW,
otherwise the classification is poorly graded sand (SP). Clean gravels (less than 5% passing
the No. 200 sieve) are classified as well-graded gravel (GW) if Cu ≥ 4 and 1 £ Cc £ 3. If
both criteria are not met, the soil is poorly graded gravel (GP). For sands and gravels where

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