SOIL MECHANICS

Terzaghi Institute

GATE | IES | TNPSC | PSU | 9994102301, 9994165522

SOIL MECHANICS

Terzaghi Institute

GATE | IES | TNPSC | PSU | 9994102301, 9994165522

S No Chapter Name Page No

01 Soil Properties

02 Consistency of soil

03 Soil classification & Structure

04 Effective stress

05 Permeability

06 Seepage Analysis

07 Stress distribution

08 Compaction

09 Consolidation

10 Shear strength

| GATE | ESE | TNPSC - AE | TRB | SOFTWARES | 9994102301, 8015512131 1

Unit I

PROPERTIES OF SOIL

Introduction

1) Disintegration of rock is called weathering

of rock. Weathering constitutes Soil.

Types of Weathering

a) Physical weathering

Due to change in temperature, running

water, roots of trees.

Constitutes cohesionless soil (like

sand, gravel, cobble, boulder)

b) Chemical weathering :

Due to Oxidation, Hydration,

Carbonation, Leaching.

Constitutes cohesive soil (like clay)

Types of Soil

a) Residual soil :

Rate of weathering is more than rate of

transportation. (Shallow depth)

b) Transported soil :

Rate of weathering is less than rate of

transportation.

It has great importance in geotechnical

engineering.

Table 1.1 Sources of Transportation

Source Term

Water :

Running Water

Lake (Still water)

Sea

Alluvial

Lacustrine

Marine/Marl

Gravity Colluvial, Talus

Air Aeoline, Dune, Loess

Ice (glacier) Drift, glacial drift

Cumulose – Organic Soil (Peat & Muck)

– Soil produced by decomposition of

vegetation under excess water

Bentonite – Soil formed from volcanic

origin (highly porous – Void ratio = 5)

Black cotton soil – It is the soil having

very high swelling & shrinkage property. The

most suitable foundation is under reamed pile.

Application of Soil Mechanics

a) Designing of foundation (shallow or

deep)

b) Designing of earthen dams

c) Designing of retaining walls

d) Designing of underground

structures

e) Designing of rigid pavements

f) Designing of canals

Table 1.2 Types of soil based on the size

Type of Soil Size

Fine grained Soil

Clay

Silt

Fine sand

< 2μ

2 μ to 75 μ

75 μ to 425 μ

Medium sand 425μ to 2mm

Coarse sand 2mm to 4.75mm

Gravel

Fine Gravel

Coarse Gravel

4.75mm to 20mm

20mm to 80mm

Cobble 80mm to 300mm

Boulder >300mm

Black cotton soils

These are residual deposits formed

from basalt or trap Rocks.

Suitable for growing cotton.

Black cotton soils are clays at high

plasticity.

Shear strength is very low.

They have high shrinkage and swelling

characteristics due to the clay mineral

(Montmorillonite)

They are highly compressible and have

very low bearing capacity.


Occurs in large part of central India

and a portion of south India.

Under Reamed piles are suitable

foundations for this type of soil.

Alluvial Soils:

Occurs in large parts of Northern India

(Indogangetic plains) and Brahmaputra

plains and some places in peninsular

India.

Alluvial deposits are generally of low

density and are liable to liquefaction in

Earthquake prone areas.

They are well graded and can be

compacted to a high dry density. These

have generally high shear strength.

Lateritic Soils

Formed by the decomposition of rock,

removal of bases and silica and

accumulation of Iron Oxide and

Aluminum Oxide.

These are residual soils formed from

basalt.

Lateritic soils are soft and can be cut with

a knife when wet, hardens with passage of

time.

Muck- It denotes a mixture of fine soil

particles and highly decomposed organic

matter. It is black in colour and of

extremely soft consistency. It cannot be

used for engineering works. The organic

matter is in an advanced stage of

decomposition.

Cumulose- peat + Muck

Hardpans are types of soil that after great

resistance to the drilling tools during Soil

Exploration.

They are dense, well graded, cohesive

aggregates of mineral.

Hardpans do not disintegrate when

submerged in water.

Humus→ Dark, Brown, Organic

amorphous (shapeless) earth of the top

soil. It consists of partly decomposed

vegetal matter.

Loam→ Mixture of sand, silt and clay.

Marl→ Stiff, marine calcareous clay of

Greenish colour.

Moorum→ The word moorum is derived

from a Tamil word meaning powdered

rock.

Consists of small pieces of disintegrated

rock (or) shale with (or) without Boulders.

Varved clays→ Sedimentary deposits.

Alternate thin layers of silt and clay

(layers less than 1cm each)

Peat→ It is an organic soil having fibrous

aggregates of macroscopic and

microscopic particles. It is formed from

vegetal matter under conditions of excess

moisture, such as in swamps. It is highly

compressible and not suitable for

foundations.

Three Phase Diagram

1.2 Technical Terms

1. Void ratio (e)

• e = 𝑽𝒐𝒍.𝒐𝒇 𝒗𝒐𝒊𝒅𝒔

𝑽𝒐𝒍.𝒐𝒇𝒔𝒐𝒍𝒊𝒅𝒔 =

e = 𝑽𝒗

𝑽𝒔

• Range : Can have any value greater than

0

Table 1.3 Range of void ratio

Void ratio Range

e > 1

e < 1

e = 1

Fig : 1.1 Three phase diagram


2. Porosity (n):

• n = 𝑽𝒐𝒍. 𝒐𝒇 𝒗𝒐𝒊𝒅𝒔

𝑽𝒐𝒍.𝒐𝒇 𝒔𝒐𝒊𝒍 𝒎𝒂𝒔𝒔 x 100

n = 𝑽𝒗

𝑽 x 100

Range: 0 < n < 100%.

• Relation between 'n' and 'e' is

en =

1+ e

3. Degree of saturation (Sr):

• Sr = 𝐕𝐨𝐥𝐮𝐦𝐞 𝐨𝐟 𝐰𝐚𝐭𝐞𝐫

𝐕𝐨𝐥𝐮𝐦𝐞 𝐨𝐟 𝐯𝐨𝐢𝐝𝐬 𝑿 100

Sr = 𝐕𝐰

𝐕𝐯 𝐗 𝟏𝟎𝟎

• Range : 0 ≤ Sr ≤ 1

Table 1.4 Sr Range

Soil type Degree of

saturation

Fully saturated soil

mass 1

Dry soil mass 0

4. Air content (ac)

• ac= 𝑉𝑎

𝑉𝑣

• Range : 0 ≤ ac ≤ 1

• ac + sr = 1

Table 1.5

Soil type Air content

Fully saturated soil mass 0

Dry soil mass 1

5. Percentage of air voids (ηa)

ηa= 𝑉𝑎

𝑉x 100 Range: 0 ≤ ηa< 1

Table 1.6

Soil type Percentage of air

voids

Fully saturated soil

mass 0

Dry soil mass < 1 (ηa = ac x n)

6. Water content or moisture content:

• Wt.of water

w= ×100Wt.of solids

W

S

Ww = ×100

W | Range: w ≥ 0

7. Bulk unit weight of soil (𝜸𝒃) (Partially

saturated state)

γb = Total weight

Volume of soil mass =

WS+ Ww

V

8. Dry unit weight of soil (𝜸𝒅)

γd = Weight of solids

Volume of soil mass =

Wd(or)Ws

V

9. Unit weight of solids (𝜸𝒔)

γs = Weight of solids

Volume of Solids =

Ws

Vs

10. Saturated unit weight: (sat) (Fully

saturated state)

• It is bulk unit weight when soil is fully

saturated

• It is also equal to total weight of soil

when fully saturated per unit of total

volume.

sat = 𝑾𝒔𝒂𝒕

𝑽

11. Submerged unit weight (sub or )

• It is the submerged weight of soil

solids per unit of total volume of soil.

sub = sat - w

• It is based on Archimedes’ principle.

γs > γsat > γbulk > γd > γsub

12. Specific Gravity of soil solids or soil (G or

Gs)

S

W

γG =

γ

• For a given soil, G remains constant

• G = 2.65 (sand) and G = 2.7 (Clay)


• G (true / absolute / specific gravity) if

voids are not considered at all.

Table 1.7 Apparent / Mass / Bulk specific

gravity of soil

Type of Soil (Gm) Bulk Specific gravity

Partially Saturated

soil

γb

γw

Dry soil γd

γw

Saturated Soil γsat

γw

• Gm is not constant and Gm< G

15. Relative density or Density index:

R.D = I.D = 𝒆𝐦𝐚𝐱 − 𝒆𝒏𝒂𝒕

𝒆𝐦𝐚𝐱 − 𝒆𝒎𝒊𝒏 x 100%

emax = soil in loosest state

emin = soil in densest state

enat = voids in natural state

min

min mad x

d d

d

1 1

1 1

γ γ

γ γ

R.D = ×100

Or

dmax d dmin

d dmax dmin

γ γ - γRD= ×

γ γ - γ

Table 1.7

Soil type Relative Density

Very loose 0 to 15%

Loose 15 to 35%

Medium 35 to 65%

Dense 65 to 85%

Very dense 85 to 100%

1.3. Some Important Relations

1) Gs.w = e.Sr

2) e = 𝑛

1−𝑛

3) n = 𝑒

1+𝑒

4) γb = γd(1 + w)

5) ηa = ac x n

6) ac + Sr= 1

7) ηa = ( 1 - Sr)n

8) γd = 𝐺𝑠 .γ𝑤

1+𝑒

9) γd = (1− 𝜂𝑎)𝛾𝑤𝐺

1+𝑤.𝐺

10) γb= γd+ Sr (γsat- γd)

11) γb = (𝐺𝑠+𝑆𝑟𝑒

1+𝑒)γw

12) γsat = (𝐺𝑠+𝑒

1+𝑒)γw

13) γsub = γsat - γw

14) γsub = (𝐺𝑠−1

1+𝑒)γw

15) Critical hydraulic gradient

ic = 𝑮𝒔−𝟏

𝟏+𝒆 (i.e) ic =

𝜸𝒔𝒖𝒃

𝜸𝒘

16) Actual hydraulic gradient,

ia = ∆𝑯

𝑳𝒔 =

𝑯𝟏− 𝑯𝟐

𝑳𝒔

• ia should not be less than ic

• If ia≥ ic (Sand boiling occurs)

• Safe Hydraulic gradient = 𝑖𝑐

𝐹.𝑂.𝑆

Sand Boiling (Quick Condition)

It is not a type of sand but a hydraulic

condition in which human beings and other

objects are not sucked in.

It occurs only in cohesion less soil,

especially in fine sand and not in clay soil.

Sand boiling does not takes place in soils

having high permeability (eg. Coarse sand

and gravel)

Sand boiling occurs only when the effective

pressure in the soil becomes zero. σ' = 0

Downward soil pressure due to self-weight –

upward seepage pressure = 0

Fig : 1.2


Actual hydraulic gradient is dependent on

head of water in upstream, downstream and

length of seepage.

Critical hydraulic gradient is dependent

only on soil properties.

To prevent Sand Boiling

Increase the seepage length by providing

sheet piles.

Provide extra head of water on

downstream side.

WATER CONTENT DETERMINATION

Water content of soil is an important

soil parameter which significantly

influences the behavior of soil,

particularly of cohesive soils.

It is important to quantify the state of

soil immediately after it is received in

the testing laboratory and just prior to

commencing any other tests (example,

shear strength test compression test

etc.)

Water content and unit weight changes

during transportation and storage.

Hence it is important to determine it

before carrying out any other tests.

The water content of a soil sample can

be determined by any one of the

following methods:

(1) Oven Drying method

(2) Torsion Balance method

(3) Pycnometer ·method

(4) Sand Bath method

(5) Alcohol method

(6) Calcium Carbide method

(7) Radiation method

(1) Oven drying method

The oven drying method is a standard

laboratory method. This is a very

accurate method.

The soil sample is taken in a small,

non-corrodible, airtight container.

The soil sample in container is then

dried at a temperature of 105 - 110°C

for 24 hrs in laboratory. Above 110°C,

water of crystallization may be lost

Water of crystallization is the water in

molecular structure.

For soils containing significant amount

of organic matter a temperature of 60°

to 80°C is recommended. At higher

temperature, gypsum loses its water of

crystallization and the organic soils

tend to decompose and get oxidized.

w 2 3

w 3 1

W W WWater content

W W W

W1 = wt of container

W2 = wt of container + wt of moist soil

W3 = wt of container + wt of dry soil

(2) Torsion Balance Method

The infra-red lamp and torsion balance

moisture meter is used for rapid and

accurate determination of the water

content. The equipment has two main

parts:

(i) The infra-red lamp, and

(ii) The torsion balance.

Since drying and weighing occur

simultaneously, the method is useful

for soils which quickly re-absorb

moisture after drying.

(3) Pycnometer method

A pycnometer is a glass jar of about 1

litre capacity and fitted with a brass

conical cap by means of a screw-type

cover. The cap has a small hole of 6

mm-diameter at its apex.

The pycnometer method for the

determination of water content can be

used only if the specific gravity of

solid (G) particles is known.

First, the weight of the empty

pycnometer is determined (W1) in the

dry condition. Then the sample of


moist soil is placed in the pycnometer

and its weight with the soil is

determined (W2).

The remaining volume of the

pycnometer is then gradually filled

with distilled water or kerosene. The

entrapped air should be removed either

by gentle heating and vigorous shaking

or by applying vacuum.

The weight of the pycnometer,

soil and water is obtained (W3)

carefully. Lastly, the bottle is emptied,

thoroughly cleaned and filled with

distilled water or kerosene, and its

weight taken (W1).

Ww = W2 – Ws

w 2 1

s s

W W WW 1

W W

s w

4 1 3 1 s

s w

W γW W W W W

G γ

3 4

s

s

W WW

G 1

2 1

3 4 s

s

W Ww 1

W W G

G 1

s2 1

3 4 s

G 1W Ww = 1

W W G

Removal of entrapped air is difficult

from cohesive soil. Hence this method

is more suited for cohesionless soil

(4) Sand Bath Method

Sand bath method is a field method for

the determination of water content.

This method is rapid, but not very

accurate.

A sand bath is a large, open vessel

containing sand filled to a depth of 3

cm or more.

The soil sample is taken in a tray. The

sample is crumbled and placed loosely

in the tray. A few pieces of white

paper are also placed on the sample.

The tray is weighed and the mass of

wet sample is obtained.

The tray is ·then placed on the sand-

bath. The sand bath is heated over a

stove. Drying takes about 20 to 60

minutes, depending upon the type of

soil.

During heating, the specimen is turned

with a palette knife. Overheating of

soil should be avoided. The white

paper turns brown when overheating

occurs.

The drying should be continued till the

sample attains a constant mass.

When drying is complete, the tray is

removed from the sand bath, cooled

and weighed. The water content is

determined as

w Final initial

w Final trav

W W W

W W W

(5) Calcium Carbide Method

This method of the determination of

water content makes use of the fact

that when water reacts with calcium

carbide (Ca C2) acetylene gas (C2 H2)

is produced.

CaC2 + H2O = C2 H2 + Ca (OH)2

The water content of the soil is

determined indirectly from the

pressure of the acetylene gas formed.

The instrument used is known as

moisture tester.

Fig : 1.3


The pressure of the acetylene gas

produced acts on the diaphragm of the

moisture tester. The quantity of gas is

indicated on a pressure gauge. From

the calibrated scale-of the pressure

gauge, the water content (wt) based on

the total mass is determined. The water

content (w) based on the dry mass is

determined using the following

formula.

w wt

s t

W Ww

W W W

Ww = Wt × Ws + wt ×Ww

W w (1 – wt) = Ws × wt

w t

s t

W w

W 1 w

t

t

ww

1 w

= water content based on weight

of solid.

(6) Radiation Method

Radio-active isotopes are used for the

determination of water content of

soils. A device containing a radio-

active isotopes material, such as cobalt

60, is placed in a capsule It is then

lowered in a steel casing A, placed in a

bore hole as shown in Fig.

The steel casing has a small

opening on its one side through which

rays can come out. A detector is

placed inside another steel casing B,

which also has an opening facing that

in casing A.

Neutrons are emitted by the radio-

active material. The hydrogen atoms in

water of the soil cause scattering of

neutrons. As these neutrons strike with

the hydrogen atoms, they lose energy.

The loss of energy is

proportional to the quantity of water

present in the soil. The detector is

calibrated to give directly the water

content.

The method is extremely useful for the

determination of a soil in the in-situ

conditions. The method very carefully

used, as it may lead to radioactive

problem if proper shielding

precautions are not taken.

SPECIFIC GRAVITY OF SOLID

DETERMINATION (Gg)

Pycnometer Method

This method is same as pycnometer

method of water content determination

with the difference that here dry soil

sample is taken instead of moist soil

sample as was taken in water content

determination.

Fig 1.5

Wt of solid = (W2 – W1)

Wt of equivalent volume of water =

(W4 – W1) – (W3 – W2)

Fig : 1.4


s

wt of solidG

wt of equivalent volume of water

2 1

s

4 1 3 2

W WG

W W W W

Sp. gravity values are generally

reported at 27°C.

If Gs T°C = Sp gravity at T°C [if test

temperature = T°]

Then o

sT C

os27

G unit weight of water at T CG

unit weight of water at 27 CC

MEASUREMENT OF UNIT WEIGHT

The Bulk Unit weight of the sample is

total weight per Unit Volume of soil

mass.

The weight of soil sample can be

determined to a high degree of

precision in comparison to the volume

of sample

The methods basically differ in

procedure for the measurement of the

Volume.

Dry unit weight can be determined

from the bulk unit weight by using the

formula.

1d

γγ

1 w

The following methods are generally used for

determination of Bulk unit weight.

(1) Core Cutter Method.

(2) Water Displacement Method.

(3) Sand Replacement Method

(4) Water Balloon Method.

(5) Radiation Method.

1. Core Cutter Method

It is a field method.

A core cutter of known volume 1000

cm3 is punched into ground and then

core containing soil is taken out of the

ground.

Weight of core with soil sample and

empty core is measured

A representative soil sample is taken

for water content measurement.

Bulk unit weight is calculated as

below.

2 1t

W Wγ

V

W2: Weight of core filled with soil.

W1: Weight of Empty core.

V: Volume of Core.

Further d is calculated from the t &

w (water content) value as:

td

γγ

1+w

This method is quite suitable for soft,

fine grained soils.

It cannot be used for stoney, gravelly

soils & dry soil.

This method is practicable only at the

places where the surface of the soil is

exposed and the cutter can be easily

driven.

2. Water Displacement Method

The volume of the specimen is

determined in this method by water

displacement. As the soil mass

disintegrates when it comes in contact

with water, the sample is coated with

paraffin wax to make it impervious.

A test specimen is trimmed to more or

less a regular shape and weighed. It is

then coated with a thin of paraffin wax

by dipping it in molten wax.

The specimen is allowed to cool and

weighed. The difference between the


two observations is equal to the mass

of the paraffin.

The waxed specimen is then immersed

in a water displacement container. The

volume of the specimen is equal to the

volume of water which comes out of

the outflow tube.

The actual volume of the soil

specimen is less than the volume of the

waxed specimen.

The volume of the wax is determined

from the mass of the wax peeled off

from the specimen after the test and

the mass density of wax.

Now, 1

1

p

M MV V

Where,

V = volume of specimen

V1 = Volume of waxed specimen

M1 = mass of waxed specimen

M = mass of specimen

ρp = mass density of paraffin

(approximately 0.998 gm/m2)

A representative sample of the soil is

taken from the middle of specimen

for· the water content determination.

Once the mass, volume and the water

content of the specimen have been

determined, the bulk density and the

dry density are found.

3. Sand Replacement Method

This is a field method.

A small area is excavated, and the

excavated soil sample, is precisely

weighed.

A calibrated cylinder containing sand

is placed over the excavated area and

the pit is filled with the sand.

Volume of the pit is obtained from the

calibrated cylinder

The bulk unit weight is calculated.

A representative sample is tested for

water content.

γd is obtained from t & w.

It is suitable for gravelly sandy and dry

soil.

4. Water Balloon method

In this method, volume of the

excavated pit is obtained by covering

the hole with a plastic sheet and filling

it with water. Weight of excavated

sample is measured and γt, is

calculated.

5. Radiation Method

The bulk density of insitu soil can also

be determined using the radiation

method.

Radiation methods for determination

of bulk density of soil are quick and

convenient and are gaining popularity.

Note :

Oven drying method : Most accurate

method

Calcium carbide method: Quickest

method

Fig : 1.6


SEDIMENTATION ANALYSIS

The soil particles less than 75 – μ size

can be further analyzed for the

distribution of the various grain-sizes

of the order of silt and clay by

'sedimentation analysis' or 'wet

analysis'.

The soil fraction is kept in suspension

in a liquid medium, usually water.

The particles descend at velocities

related to their sizes, among other

things.

The analysis is based on 'Stokes Law'.

As per this law, if a single sphere is

allowed to fall in an infinite liquid

medium without interference, its

velocity first increases under the

influence of gravity, but soon attains a

constant value.

This constant velocity, which is

maintained indefinitely unless the

boundary conditions change, is known

as the 'terminal velocity'.

The principle is obvious, coarser

particles tend to settle faster than finer

ones.

By Stokes' law the terminal velocity of

the spherical particle is given by

t 1γ γu

18μ

Stokes' Law is considered valid for

particle diameters ranging from 0.2 to

0 .002 mm.

For particle sizes greater than 0.2 mm

turbulent motion is set up and for

particle sizes smaller than 0.002 mm.

Brownian motion is set up. In both

these cases, Stokes’ law is not valid.

The limitations of sedimentation analysis,

based on Stokes’ law are as follows:

(i) The finer soil particles are never

perfectly spherical. Their shape is

plate-like or needle-like. However, the

particles are assumed to be spheres,

with equivalent diameters (the basis of

equivalence being the attainment of

the same terminal velocity as that in

the case of a perfect sphere.)

(ii) Stokes' law is applicable to a sphere

falling freely without any interference

in an infinite liquid medium.

The sedimentation analysis is

conducted in a one-litre jar, the depth

being finite the walls of the jar could

provide a source of interference to the

free fall of particles near it.

The fall of any particle may be

affected by the presence of adjacent

particles, thus the fall may not be

really free.

However, it is assumed that

the effect of these sources of

interference is insignificant if

suspension is prepared with about 50 g

of soil per litre of water.

(iii) All the soil grains may not have the

same specific gravity. However, an

average value is considered all right,

since the variation may be

insignificant in .the particles

constituting the fine fraction.

(iv) Particles constituting to fine soil

fraction may carry surface electric

charges, which have a tendency to

create ‘flocs’. Unless these flocs are

broken, the sizes calculated may be

those of the flocs. Flocs can be a

source of erroneous results.

A deflocculating agent, such

as sodium silicate, sodium oxalate, or

sodium hexametaphosphate, is used to

get over this difficulty.

The general procedure for sedimentation

analysis, which may be performed either with

the aid of a pipette or a hydrometer is as

follows:


An appropriate quantity of' an oven-

dried soil sample, finer than 75–μ size,

is mixed with a known volume (V) of

distilled water in jar.

The sample is pretreated with an

oxidizing agent and an acid to remove

organic matter and calcium

compounds.

Addition of hydrogen peroxide on

heating would remove organic matter.

Treatment with 0.2 N hydrochloric

acid would remove· calcium

compounds.

Later, a deflocculating or a dispersing

agent such as sodium

hexametaphosphate is added to the

solution. The mixture is shaken

thoroughly by means of a mechanical

stirrer and the test is started keeping

the jar vertical.

The soil particles assumed to be

uniformly distributed throughout the

suspension. at the instant of

commencement of the test

PIPETTE METHOD

It is a laboratory method. A pipette

sedimentation jar and a number of

sampling bottles are necessary for the

test.

The method consists in drawing off 10

ml samples of soil suspension by

means of the sampling pipette from a

standard depth of 10 cm at various

time intervals after the start of

sedimentation.

The usual total time intervals at which

the samples are drawn are 30 s, 1 min,

2 min, 4 min, 8 min, 15 min, 30 min, 1

h, 2h, and 4 h from the start of

sedimentation.

The pipette should be inserted about

20 seconds prior to the chosen instant

and the process of sucking should not

take more than 20 seconds.

Each of the samples taken is

transferred ton sampling bottle and

dried in an oven.

The concentration of all sized particles are

present in a particular volume at time t = 0.

This concentration is equal to weight

of solid per cc in original soil

suspension = x.

If after time t sample is taken out in

pipette from height He, then all

particles having settling velocity

greater than He will have settled below

height He.

At this height at time t sample

collected will have the same

concentration of particles of settling

velocity less than eH

t as was there in

the original soil suspension.

If settling velocity of particles size (D)

= He/t then

% finer than D =

wt of solid per cc at

depth Heafter time t

wt of solid per cc in the

original suspension

As,

2

s w eγ γ D H

18μ t

Find ‘D’ from this formula corresponding to

various time ‘t’ and plot grain size distribution

curve.

Fig : 1.7


In pipette method, sample is collected

from height He at various time intervals i.e. He

is fixed.

Note: Let there be 4-types of particles in the

sample. Let in the original soil suspension

before the start of sedimentation the

concentration of various particles be as shown

below.

Fig : 1.9

If particle has settling velocity greater

than He/t, then concentration of soil

suspension collected at height He will

be 600 g/l.

e

Thus,

%finer than size

concentration of sample

collected at time t from height H 100

concentration of soil from

original soil suspension

600100

650

If particle has size D. then % finer than size D

is 600

100650

.

Where D is given by 2

s weγ γ DH

t 18μ

HYDROMETER ANALYSIS

The hydrometer method differs from the

pipette analysis in that the weight is of solids

per ml in the suspension at the chosen depth at

chosen instants of time are obtained indirectly

by reading the specific gravity of the soil

suspension with the aid of a hydrometer.

Hydrometer is a device which is used

to measure the specific gravity of

liquids. However, for a soil suspension

the particles start settling down right

from the start and hence the unit

weight of the suspension varies from

top to bottom.

As time passes the hydrometer goes

down because weight of hydrometer is

balanced by the weight of liquid

displaced by hydrometer. As solid

concentration goes on reducing in

water, more amount of liquid has to be

displaced to .balance the weight of

hydrometer.

We have already discussed that % of finer than

D

ewt.of solid per cc at depth H

after time t

wt.of solid per cc in original

soil suspension

Where D is given by

Fig : 1.10 Hydrometer

Fig : 1.8


Fig : 1.12

2

s w eγ γ D H

18μ t

Thus using hydrometer we have to measure

density of soil suspension at a depth He after

time t. Fig. (a) Shows the height He But due to

its own volume, insertion of hydrometer will

lead to displacement of liquid.

Fig 1.11

Thus, Point at A1 occupies the position A2. A2

is the centre of volume of hydrometer.

Vh = Volume of hydrometer

AJ = Area of Jar

h = length of bulb in hydrometer

Volume of bulb can be thought of as

the total volume of hydrometer.

Hydrometer measures density of soil

suspension at depth He which is given

by

h h1

1 j

V VhH H

2 2A A

h1

j

V1H H h

2 A

H will correspond to hydrometer reading Rh.

Thus He will be related to Rh . Hydrometer is

calibrated such that hR1 Sp

1000 gravity of

soil suspension at depth He

h

density of soil

R suspension at depth He1+ =

1000 density of water

h

density of immersed wt of solid per+

R water unit volume1+ =

1000 density of water

Note:

s s w w

s w

s w w s w s

s w

V γ V γDensity of soil suspension

V V

V V γ γ γ V

V V

s w s

w

s w

s s ww

s v s

w d

s

γ γ Vγ

V V

V γ γ= γ 1

V V γ

1γ W 1

G

Wd = wt of ·solid per unit volume of solution

s s w s s

s

1V γ γ immersed wt V γ 1

G

Immersed wt per unit volume

s s

s w s

V γ 11

V V G

hR immersed wt of solid/cc of solution1 1

1000 wt density of water

d

sh

w

1W 1

GR1 1

1000 γ


h w s

d

s

R γ GW

1000 G 1

% finer than dWD 100

W

Where,

W = weight of solid per unit volume in original

soil suspension

% finer than

h w s

s

R γ G 100D

G 1 1000 W

h s w

s

R G γ%finer than D

G 1 10W


Unit II

CONSISTENCY OF SOIL

2.1 Introduction

The term consistency denotes the degree of

firmness.

1) It represents the different physical states

such as

a) Liquid state

b) Very soft

c) Soft

d) Stiff

e) Very stiff

f) Semi solid or solid

2) The term consistency is mainly meant for

fine grained soils (less than 425 micron)

3) It has been developed by Mr.Atterberg in

1911.

4) Consistency of soil is expressed in

a) Atterberg Limit

b) Unconfined compressive strength of

clay (qu)

5) Atterberg limits

a) Liquid limit (LL)

b) Plastic Limit (PL)

c) Shrinkage Limit (SL)

2.2 Liquid Limit

a) It is the min. water content of fully

saturated soil mass on which the soil is in

liquid state having very low shear strength

(2.7 kPa) which is measurable.

Table 2.1 Dimensions of Groove

Apparatus Bottom

width

Top

width Depth

Casagrande

groove 2mm 11mm 8mm

ASTM

groove 2mm 13.6mm 10mm

b) There are three methods to determine

Liquid Limit in the lab

Casagrande L.L method

One point Method

Static cone penetrometer method (IS

2720 – 1970)

2.2.1 Casagrande Method Procedure

• Fine grained soil (dry state) passing

through IS 425 micron sieve is taken into

account

• The dry mass is about 120 gm

• A certain amount of water (say about 25%)

is added to dry soil.

• Soil paste is made and placed on the brass

cup of Casagrande apparatus.

• The Casagrande cup is lifted (1cm) and

dropped on the rubber pad with the help of

handle (Blow).

• The number of blows is counted till the

groove is closed (N1).

• The water content of the soil mass is

determined accurately (Say w1).

Fig : 2.2

Fig : 2.1


Fig : 2.5

• The volume of the water content is

changed and the test is repeated.

• A number of test results are obtained.

• A graph is plotted in between number of

blow (log scale) on x axis and

corresponding water content on y axis

(ordinate).

• The graph such plotted is known as Flow

curve (st. line).

• The water content corresponding to 25

blows on the flow curve is known as liquid

limit.

• The slope of the flow curve is known as

flow index (If)

𝑰𝒇= tanθ = 𝒘𝟏− 𝒘𝟐

𝒍𝒐𝒈𝟏𝟎(𝑵𝟐𝑵𝟏

)

• Flow index is a measure of shear strength

of soil.

• If flow index is higher, shear strength will

be lesser.

• Both are indirectly proportional to each

other.

Note: Ifb > IfA (θ2 > θ1) then shear strength of B

is less than shear strength of A.

2.2.2 One Point Method

In one point method, only one trial is

conducted using casagrande apparatus and then

by mathematical relations.

Liquid limit (LL) = w [𝑵

𝟐𝟓]

𝟎.𝟏

2.2.3 Static Cone Penetrometer Method

In static cone penetrometer method, the water

content corresponding to 25mm penetration of

cone is called liquid limit.

2.3 Plastic Limit

• It is the water content of fully saturated

soil mass on which a thread of 3mm

diameter of soil starts crumbling while

rolling into a thread.

• The size of the soil grain should be less

than 425 micron

2.4 Shrinkage Limit

• It is an arbitrary limit (water content)

between semi solid and solid state of the

saturated soil mass.

• It is the lowest level of full saturation.

• It is the max. water content of the saturated

soil mass below which the volume of soil

mass does not change even though water

content of saturation is lowered.

• If saturated soil sample is taken and

allowed to dry up gradually, its volume

will go on reducing till a stage will come

after which the reduction in the soil water

will not result in further reduction in the

total volume of the soil sample.

• The water content corresponding to that

stage is known as shrinkage limit.

Fig : 2.6

Fig : 2.3

Fig : 2.4


• The oven dried sample having volume Vd

and mass Md. Evidently, according to the

definition, volumes V2 and Vd are equal.

Now, mass of water in (a) = M1 –Md

Loss of water from (a) to (b) = (V1 − V2)ρw

Mass of water in (b) = (𝑀1 –𝑀𝑑)-(𝑉1 − 𝑉2)𝜌𝑤

ws = (M1 –Md)−(V1−V2)ρw

Mdx 100

ws = [w1 −(V1−V2)ρw

Md]x 100

𝒘𝒔 = [𝒘𝟏 −( 𝑽𝟏−𝑽𝟐)𝝆𝒘

𝑾𝒅] x 100

w1 = Water content of the original saturated

sample of volume V1

𝑉𝑑 = dry volume of the soil sample

𝑤𝑑 = dry weight of the soil sample

𝑀𝑑 = dry mass of the soil sample

2.5 Formulas

1. Degree Of Shrinkage

D.S = 𝑽𝟎−𝑽𝒅

𝑽𝟎 x 100%

Vo = Initial volume of soil mass

Vd = Dried volume of soil mass

Table 2.2 Quality of soil

D.S Quality of Soil

< 5 Good

5 – 10 Moderate

10 – 15 Poor

> 15 V. poor

2. Volumetric Shrinkage

V.S = 𝑽𝟎−𝑽𝒅

𝑽𝒅 x 100%

3. Shrinkage Ratio

S.R =

𝑽𝟎−𝑽𝒅𝑽𝒅

𝑳.𝑳 −𝑺.𝑳 (or) S.R =

𝜸𝒅

𝜸𝒘 = Gm

4. Plasticity Index

It indicates the plasticity of the soil

IP = L.L – P.L (or) WL - WP

For organic soil L.L and P.L both are

high, hence 𝐼𝑃 will be lower.

For clay if IP is high, then it is called

fat clay and if IP is low it is called lean

clay.

Table 2.3 Degree of Plasticity

IP Degree of Plasticity

-ve Non plastic

< 7 Low plastic

7 to 17 Medium plastic

> 17 Highly plastic

5. Liquidity Index

IL = 𝑤𝑛−𝑊𝑃

𝑤𝐿−𝑊𝑃 x100 % (or) IL =

𝒘𝒏−𝑾𝑷

𝑰𝑷 x100%

Wn - water content at natural state

Wp - water content at plastic limit

WL - water content at liquid limit

6. Consistency Index

IC = 𝒘𝑳−𝑾𝒏

𝑰𝑷

IL + IC = 1

Wn - Water content at natural state

Table 2.4 Range of IL and IC

IL IC Consistency

> 1 0 Liquid state

1 – 0.75 0 – 0.25 Very soft

0.75 - 0.5 0.25 – 0.5 Soft

0.5 – 0.25 0.5 – 0.75 Medium

0.25 – 0 0.75 - 1 Stiff

< 0 > 1 Semi solid (or) Solid

7. Toughness Index

IT = 𝑰𝑷

𝑰𝒇

Table 2.5

IT Soil type

0 – 3 For general soil

< 1 Crushable / friable soil at plastic limit

8. Activity Number

AC = 𝑰𝑷

% 𝑭𝒊𝒏𝒆𝒓 𝒕𝒉𝒂𝒏 𝟐 𝒎𝒊𝒄𝒓𝒐𝒏


Table 2.6 Nature of soil

AC Nature of soil

< 0.75 Inactive

0.75 – 1.25 Normal

> 1.25 Active

9. Sensitivity of Clay

It indicates the loss of shear strength

without change in the water content.

St = 𝒒𝒖 (𝒖𝒏𝒅𝒊𝒔𝒕𝒖𝒓𝒃𝒆𝒅)

𝒒𝒖 (𝒓𝒆𝒎𝒐𝒖𝒍𝒅𝒆𝒅)

Table 2.7 Sensitivities of clay

St Nature

1 – 4 Normal

4 – 8 Sensitive

8 – 16 Extra sensitive

> 16 Quick condition

10. Shrinkage limit

s

m s

1 1W = -

G G

11. Specific Gravity

s1

= 1

Shrinkage limitShrinkage ratio

G

12. Thixotrophy

It is a property of soil due to which it

regains a part of original strength while

reorienting its grains in presence of

adsorbed water without the change in

water content in a disturbed condition.


PRACTICE QUESTIONS

Q.1) A soil sample has a diameter of 38 mm

and a height of 76 mm. Its wet weight is 1.15

N. Upon drying, its weight reduced to 0.5 N,

Gs = 2.7. In the wet state the degree of

saturation and the water content of the soil

sample are.

a) 130% and 96%

b) 96% and 130%

c) 30% and 96%

d) 30% and 100%

Q.2) A soil has a bulk density of 22 kN/m3

and water content 10%. The dry density of

soil is

a) 18.6 kN/m3

b) 20.0 kN/m3

c) 22.0 kN/m3

d) 23.2 kN/m3

Q.3) Valid range for n, the percentage voids,

is

a) 0 < n < 100

b) 0 ≤ n ≤ 100

c) n > 0

d) n ≤ 0

Q.4) Select the correct statement

a) Unit weight of dry soil is greater than

unit weight of wet soil.

b) For dry soils, dry unit weight is less

than total unit weight.

c) Unit weight of soil increase due to

submergence in water.

d) Unit weight of soil decrease due to

submergence in water

Q.5) Varved clay is

a) A mixture of sand, silt, clay

b) A chemically bonded soil mixture

c) Alternate thin layers of silt and clay

d) Decomposed volcanic ash deposit

Q.6) A phenomenon involved in soil by

losing it strength due to remoulding and

increase in strength with passage of time is

called

a) Sensitivity

b) Thixotropy

c) Activity

d) Consistency index

Q.7) Inverse of Relative density of soil is

a)

max nat

max min

100%e e

e e

b)

max min

max nat

100%e e

e e

c)

nat max

max min

100%e e

e e

d)

max min

nat max

100%e e

e e

Q.8) Pick up the incorrect definition from

the following:

a) Ratio of the compressive strength of un-

confined undisturbed soil to that of

remoulded soil, is known as the sensitivity of

the soil sample

b) The rotation of soil particles into stable

state while remoulding, is known as the

thixotropy of soil


c) The water content at which a soil changes

from the liquid state to solid state, is known

liquid limit of the soil

d) The water content at which a soil flows, is

known plastic limit of the soil

Q.9) According to Atterberg, the soil is said

to be of medium plasticity index PI is

a) 0 < PI < 7

b) 7 ≤ PI ≤ 17

c) 17 ≤ PI ≤ 27

d) PI ≥ 27

Q.10) For a soil having liquid limit of 45%,

plastic limit of 25% and shrinkage limit of

15%, the plasticity index is

a) 20%

b) 40%

c) 50%

d) 60%

/

ORIGIN OF SOILS

FORMATION

weathering of rocks

Decomposition oforganic matter

Geological CycleWeathering -

Transportation -Deposition - Upheaval

Weathering Physical Eg. Gravel,Sands

Chemical Eg. Clays

Transportation

Sedimentary soils

Sedentary (Residual)soils Eg. Black

cotton soils, Lateritesoils River - Alluvial Soil,

Lake - Lacustrine soil,Sea - Marine soils, Wind - Aeolian soils,Gravitation - ColluvialGlacier - Glacierdeposited soil

Aeolian soil Eg. Sanddunes, Loess

Colluvial soil Eg.Talus

Glacier Eg. Drift, till

Gravitational forces arepredominant

Surface forces arepredominant

Special Soils

Varved Clay(Lacustrine deposit)alternate layers of

clay+silt

Moorum - Gravel +Red Clay

Loam - Sand+Silt +Clay

Bentonite -Decomposed volcanicash (contains high %

of montmorillonite)

Exhibits high degree ofshrinkage & swelling

Organic soils Eg.Peat,Humus

Peat, highly organicfibrous in nature

Humus, partlydecomposed

amorphous in nature

Muck is a mixture oforganic & inorganic

soils

/

PROPERTIES OF SOIL

PHASE SYSTEM

3 Phase system -Unsat (Partiallly sat) soil, 2 Phase system- Saturated or dry soil Vol indiacted on right

& wt indicated on leftside of phase diagram

Imp Properties of soil

Void ratio, e = Vv/Vs(value of e>0sometimes >1 also)e=n/(1-n)

Porosity, n = Vv/V(value 0<n<100)

n=e/(1+e)

Degree of sat, S =Vw/Vv (value0<=S<=100)

Air Content, ac =Va/Vv (value

0<=ac<=1) ac + S =1

% air voids, na =[Va/V]*100 (value

0<=na<=n) na = n.ac

Water Content, w =[Ww/Ws]*100 (valuew>0 sometimes >100

Unit Weights of Soilγs>γsat>γ>γd>γ'

Bulk unit weight,,γ=W/V

Dry unit weight, γd=[Ws (or) Wd]/V

Sat unit weight, γsat=(W @ ful sat)/V

Unit weight of solids,γs=(Ws orWd)/Vs (γs

is constant for a soil

Submerged unitweight, γ'=γsat-γw

(based on archimedi'sprinciple)

Specific GravitiesTrue Sp.Gravity orSp. Gravity, G = γs/

γw (G is constant for asoil) Apparent/mass/bulk

sp.gravity Gm = γ/γw(Gm<G & not constant)

Imp Relationships

S.e = w.Gγd=γ/(1+w), γd=Gγw/(1+e), γd=((1-na)Gγw)/(1+wG),

γ=γw (G+Se)/(1+e),γ=γd+S(γsat-γd) V1/V2 =γd1/γd2 =

(1+e1)/(1+e2 )

Water Content ExpMethod

Oven Drying -Accurate Mathod

Calcium Carbide -Quick Method

Temp 105-110 degCelsius for 24 hours

For high organic soil60 deg celsius is

prefered

If gypsum is present,not more than 80 deg

celsius but for longtime

Specific Gravity ExpMethod

Pycnometer method,Oven dried sample is

used

G =(W2-W1)/[ (W2-W1) - (W3-W4)]

W1 = pycnometer, W2=Pyc+ dry sample, W3= Pyc+drysample+water, W4 =pyc+water

/

INDEX PROPERTIES

Grain sizedistribution

Sieve analysis(coarse grained soil)

Sedimentationanalysis (fine grained

soil <75μ)

Based on stoke's law(valid only if size of

particle is b/w 0.2mmand 0.2 μ)

Pipette Method(accurate method)

Hydrometer MethodMeniscus Correction(always +ve)

Dispersing agentcorrection (always -ve)

Dispersion solution -33g of sodium

hexametaphosphate+7g of soadium

carbonate

Temp correction (+vewhen T>27 and -ve

when T<27)

v=(gD^2 (S-1))/18γ(approx v=902D^2)

RelativeDensity/Density Index(coarse grained soil)

ID= (emax-e) / (emax-emin)

<15% - very loose, 15to 35% - loose, 35 to65% - medium dense,

65 to 85%- dense,>85% - very dense

Grain sizedistribution curve

Well graded, uniformlygraded and gap

graded soil

Cu = D60/D10 (always>=1)

>6 for well graded soil

b/w 1 & 2, foruniformly graded soill

>4, for well gradedgravel

Cc =(D30^2)/(D10×D60 )

= 1, unifromly gradedsoil

b/w 1 & 3, for wellgraded soils

Atterberg'sConsistency limits(FIne grained soil)Liquid limit

Plastic limit

Shrinkage limit

Drop of cup is 1 cm

Shear strength @ LL =2.7 kPa

Flow Index (Slope offlow curve w vs n (log

plot))

Shrinkage ratio, SR =Gm @ its dry state

Important Indices

Plastic Index Ip = wL -wp

Shrinkage Index, Is =wp - ws

Flow index, If = (w1-w2)/log10 (n2/n1)

Toughness index, It =Ip/If (↑ value,better

strength@ wp)

Liquidity Index, IL =(w-wp)/Ip

Consistency index, Ic= (wL-wp)/Ip

Activity, A = Ip/C

A < 0.75, inactive

A = 0.75 to 1.25,normal

A > 1.25, Active

Volumetric Shrinkage =(w1-ws)*SR


Unit III

SOIL CLASSIFICATION & STRUCTURE

Particle Size Distribution

For measuring the distribution of particle

sizes in a soil sample, it is necessary to

conduct different particle size tests or

mechanical analysis.

By mechanical analysis is meant the

separation of a soil into its different size

fractions.

The mechanical analysis is performed in two

stages:

Sieve analysis

Sedimentation analysis

Sieve Analysis

In the BS (British Standards) and ASTM

(American Standard Test Method), the

sieve sizes are given in terms of the

numbers of openings per inch.

The number of openings per square inch is

equal to the square of the sieve.

The complete sieve analysis can be divided

into two parts as given below.

1. Wet sieving:

It is carried out for separating fine grains

from coarse grains by washing the soil

specimen on a 75µ sieve mess.

2. Dry sieve Analysis:

It is carried out when particles coarser than

75 µ. Samples (with fine removed) are

dried and shaken through a set of sieves of

descending sizes.

The weight retained on each sieve is

measured.

The cumulative percentage quantities finer

than sieve sizes (passing each given sieve

size) are then determined.

Calculation for percentage finer

The data obtained is presented as a distribution

curve with

Grain size along x-axis (Log scale)

Percentage passing along y-axis

(Arithmetic scale)

% finer passing through each sieve.

Table 3.1 Sieve Analysis

S.No. Sieve

size

% Retained = 𝑾𝟏

∑ 𝑾

Cumulative % Retained % finer = 100 – Cumulative % Retained

1. 2mm

2 1 mm

3 600µ

4 425 µ

5 300 µ

6 212 µ

7 150 µ

8 75 µ

9 Pan

Fig : 3.1


Fig : 3.2 A schematic curve showing grading characteristics of soil

GRAIN SIZE DISTRIBUTION CURVE

A grain size distribution plot is a special

purpose graph generally used to depict the

results of a sieve analysis, commonly known

as gradation test.

It is also known as grading curve.

Gradation curve is plotted between the

percentage finer y-axis (on natural scale).

vs the particle size (on log scale) x-axis.

To obtain the grading characteristics, three

points are located first on the grading curve.

D60 = Size at 60% finer by weight (retained =

40%)

D30 = Size at 30% finer by weight

D10 = Size at 10% finer by weight

The grading characteristics are obtained as

follows

Effective size = D10

Uniformity coefficient, Cu = 𝑫𝟔𝟎

𝑫𝟏𝟎

Curvature coefficient, Cc = (𝐃𝟑𝟎)𝟐

𝐃𝟔𝟎 × 𝐃𝟏𝟎

Observations from the Curve

For well graded soil (i.e. soil which has a

distribution of particles over a wide size

range) And,

Cu > 4 (for gravel)

Cu >6 (for sand)

1 < Cc< 3

For uniform soil (i.e soil which has a

narrow particle size range)

Cu=1

CLASSIFICATION OF SOILS

The Unified Soil Classification System

(USCS):

Originally developed by Casagrande.

According to USCS, the coarse grained

soils are classified on the basis of their

grain size distribution and the fine grained

soils (whose behavior is controlled by

plasticity) on the basis of their plasticity

characteristics.

All soils are classified into four major

groups:

(a) Coarse grained

(b) Fine grained

(c) Organic soils

(d) Peat

Coarse grained soils are those having 50%

or more retained on the 0.075 mm sieve.

Further, the coarse grained soils are

designated as gravel (G) if 50% or more of

the coarse fraction is retained on the 4.75

mm sieve otherwise, they are designated as

sands (S).

Both Gravel and sand are subdivided into

four sub groups

(a) Well graded (W)

(b) Poorly graded (P)

(c) Silty (M)

(d) Clayey(C)

Well graded soil (W) has wide distribution

of grain sizes present, whereas, a poorly

graded (P) soil is either uniform or gap

graded.

Whether a soil is well graded or poorly

graded can be determined by plotting the

grain size distribution curve and

computing the coefficient of uniformity

(Cu) and the coefficient of curvature (Cc).

Table 3.2 Prefix and suffix of USCS:

Soil type Prefix Subgroup Suffix

Gravel G Well graded W

Sand S Poorly graded P

Silt M Silty M

Clay C Clayey C

Organic O WL < 50% L

Peat Pt WL ˃ 50% H

Fine grained soils having more than 50%

material passing the 0.075 mm sieve are

further subdivided into silt (M) and clay


Fig : 3.3 Plasticity chart for laboratory classification

Fig : 3.4

(C), based on their liquid limit and

plasticity index.

Note:

If percentage of fines

< 5% passing 75𝜇 – GW,GP,SW,SP

>12% passing 75𝜇- GM,GC,SM,SC

5 - 12% passing 75𝜇 – GW-GM,SP-SC

(dual symbol)

The A-line on Casagrande’s plasticity

chart generally separates the more clay like

materials from those that are silty and also

the organic from the inorganic.

Silt, clay and organic fractions are further

subdivided into soils possessing low (L) and

high (H) plasticity when the LL is less than

50% and more than 50% respectively.

Some soils possessing the characteristics

of more than one group are termed as

Boundary.

Soils & hence, are assigned dual group

symbols e.g., GW-GC, which means well

graded gravel with some clay fines.

“AASHTO” SOIL CLASSIFICATION

SYSTEM

According to AASHTO system, the soils

are classified into eight groups: A-1

through A-7 with an additional group A-8

for peat or muck.

The system includes several subgroups.

Soils within each group are evaluated

according to the group index calculated

from empirical formula:

GI = 0.2 a + 0.005 ac + 0.01 bd

Where,

a = that part of the percent passing the 75 µ

sieve greater than 35 and not exceeding 75.

b = that part of the percent passing the 75µ

sieve greater than 15 and not exceeding 55.

c = that part of liquid limit greater than 40 and

not greater than 60.

d = that part of plasticity index greater than 10

and not exceeding 30.

Greater the G.I. value, the less desirable a

soil is for highway construction with in

that subgroup.

Table 3.3

GI SOIL TYPE

0 – 2 Good

2 – 5 Fair

5 – 10 Poor

10 - 20 Very Poor

Indian Standard Soil Classification

System (ISSCS):

It is based on USCS with the modification

that the fine grained soils have been

subdivided into three subgroup of low,

medium and high compressibility as

against only two in the USCS.

Equation of A-line = 0.73(WL – 20)

The plasticity chart of ISSCS is:


Fig : 3.5 Grading curves

Table 3.3 Prefixes and suffixes of ISSCS:

Soil type Prefix Subgroup Suffix

Gravel G Well graded W

Sand S Poorly graded P

Silt M Silty M

Clay C clayey C

Organic O WL< 35% L

Peat Pt 35 <WL< 50 I

WL> 50 H

Typical grading curves are given below

Curve A a poorly graded medium sand

Curve B a well graded gravel sand

Curve C a gap graded cobbles sand

Curve D a sandy silt

Curve E a silty clay.

Fig : 3.5

Table 3.4 Soil classification using group symbols

Group Symbol Classification

GW Coarse Soils

Well graded gravel

GP Poorly-graded gravel

GM Silty gravel

GC Clayey gravel

SW Well graded sand

SP Poorly graded sand

SM Silty sand

SC Clayey sand

ML Fine soils

Silt of low plasticity

MI Silt of medium plasticity

MH Silt of high plasticity

CL Clay of low plasticity

CI Clay of medium plasticity

CH Clay of high plasticity

OL Organic soil of low plasticity

OI Organic of medium

plasticity

OH Organic of high plasticity

Pt Peat

IMPORTANT POINTS

Only particles of coarse grained soil

can be seen by naked eye.

Best soil for homogeneous rolled

embankment dam–—GC

Best soil for core of rolled earthen

dam—GC

Best soil for shell of rolled earth

dam—G W

Best soil for foundation when seepage

not important—GM

Best soil for foundation when seepage

important.—GW

Best soil for road surfacing—GC

Soil Structures

Soil structure is determined by how

individual soil granules clump or bind

together and aggregate, and therefore, the

arrangement of soil pores between them.

Soil structure has a major influence on

water and air movement, biological

activity, root growth and seeding

emergence.

For coarse-grained soils the soils structures

can be classified as

Single Grained Structure

Single grained structure is characteristic of

coarse grained soils, with a particle size

greater than 0.02 mm.

Cohesion less soils such as gravel and sand

in which gravitational forces are more

predominant than surface forces, take an

equilibrium position.

The deposition may occur in a loose state,

with large voids or in a sense state with

less of voids.

Fig : 3.6 Single grained structure


Honeycomb Structure

This structure occurs in soils having small

granular particles which have cohesion

because of their fineness.

Soils in honeycomb structure are loose.

They can support loads only under static

condition.

The structure has a large voids spaces and

may carry high loads without a significant

volume change.

The structure can be broken down by

external disturbances.

Clay consisting of very fine particles or

colloids may have two types of structure

such as flocculated and dispersed.

Both of the structures are described below

Flocculated Structures

The clay particles haves large surface area

and therefore electrical forces are

predominant in such soils.

The clay particles haves negative charge

on the surface and a positive charge on the

edges.

Inter particle contact develops between the

positively charged edges and negatively

charged faces.

Such a structure is formed when the net

electrical forces between adjacent soils

particles at the time of depositions are

attraction forces.

Dispersed Structures

These structures develop in clays that have

been reworked or remolded.

The particles develop almost parallel

orientation.

Dispersed or oriented structure is formed

when the platelets have face-to-face

contact in more or less parallel array.

Such a structure is formed if the net

electrical forces between adjacent soil

particles at the time of deposition are

repulsion.

TWO LAYER SHEET MINERALS

The basic kaolinite unit is a two layer unit

that is formed by stacking a gibbsite sheet on a

silica sheet.

Kaolinite Minerals

Kaolinite is the most common mineral of

the kaolin group.

The basic units of kaolinite minerals (i.e.,

gibbsite and silica sheet) are stacked one

on top of the other to form a lattice of the

mineral.

The units are held together by hydrogen

bonds. The strong bond does not permit

water to enter the lattice, thus kaolinite

minerals are stable and do not expand

under saturation.

Fig : 3.7 Honeycomb Structure

Fig : 3.8 Flocculated Structure

Fig : 3.9 Dispersed Structure

Fig : 3.10


Fig : 3.13 Polar representation of water molecule (dipole action)

THREE LAYER SHEET MINERALS

Typically, the three layer sheet minerals

are categorized as follows

Montmorillonite Minerals

The basic structural unit consists of an

alumina sheet sandwiched between two

silica sheets.

The two successive structural units are

joined together by a link between oxygen

ions of the two silica sheets.

The negatively charged surfaces of the

silica sheet attract water in the space

between two structural units.

This result in an expansion of the mineral,

thus montmorillonite minerals exhibit high

shrinkage and high swelling

characteristics.

IlIite Minerals

The basic structural unit is similar to that

of Montmorillonite; there is always a

substantial amount of isomorphous

substitution of silica by aluminum in silica.

The link between different structural units

is non-exchangeable potassium ion (K+).

Properties of mineral illite are somewhat

intermediate between that of kaolinite and

montmorillonite.

CLAY WATER RELATIONSHIP

Normally clayey soils are associated

with water and its properties are

significantly influenced with the

presence of water whereas granular

soils are not sensitive to the amount of

water present

Water is a dipolar molecule. The

centre of gravity of +ve & –ve charges

do not coincide. Hence it is treated as a

bar magnet.

Clay particles always carry a net

negative charge except at edges.

Reason for charge accumulation

1. Isomorphous substitution by lower

valency cation.

2. Breakage of particles

3. Dissociation of hydroxyl (OH-) radical

into hydrogen ions.

Fig : 3.11

Fig : 3.12


General Mechanism of Clay water

Interaction

Due to the attraction of +ve and –ve

charges close to the clay surface,

results in formation of adsorbed layer

of water and double diffused layer.

Following are the ways by which water is held

to the clay surface

(a) Attraction between negatively charged

faces of clay and +ve ends of dipoles

(electrostatic attraction).

(b) Attraction between cations in the water

and negatively charged ends of

dipoles.

(c) Hydrogen atom of the water molecule

is attracted to the oxygen or hydroxyl

ion on the surface of clay crystals.

Adsorbed Water

Water held on the surface of particle

by powerful forces of electrical

attraction and virtually in a solid state,

this layer is very thin (10A°)

Adsorbed water cannot be removed by

oven drying at 105 - 110°C and may

therefore be considered to be part of

the solid grain. Heating at 105 – 110°C

will change clay soil structure.

Fig : 3.18

Cations distribute themselves around

the negatively charged surface of clay

particles with the greatest density near

the surface.

Beyond this there is an outer layer

which is attracted to lesser degree and

is more mobile. This layer which

extends up to the limit is called as

diffused double layer and its density

decreases with the increasing distance

from the surface.

Ultimately the concentration

becomes equal to that in water in the

void space.

Rigid layer and diffused layer

combinedly is called Electrical double

layer.

The oriented water is considered to

affect the behaviour of clay particles

Fig : 3.16

Fig : 3.17


when subjected to stress because it

comes between the particle surface.

For a given particle, the thickness of

the cation layer depends mainly on the

valency and concentration of cations.

An increase in valency (due to cation

on exchange) or an increase in

concentration will lead to decrease in

the layer thickness.

CATION EXCHANGE CAPACITY

The ability of clay particles to absorb

ions on its surface or edges is called

base or cation exchange capacity

which is a function of the mineral

structure of the clay and the size of the

particles.

CEC = f [Mineral structure, size of

particles]

Cation is termed as exchangeable

because one cation can easily be

exchanged with the other.

Behaviour of clay soil is dependent on

the exchangeable cation in the clay.

Different clay has different change

deficiencies and has varying ability to

adsorb exchangeable cations.

The Base Exchange capacity is

expressed in terms of the weight of a

cation, which may be held on the

surface of 100 g of dry soil material. It

is measured in milli equivalent (meq)

per 100 gm of dry soil where 1 meq is

1mg of hydrogen or the portion of any

ion which will combine with or

displace 1 mg of hydrogen.

Montmorillonite has 10 times Base

Exchange capacity as compared to

Kaolinite. CEC of Illite is in between

Kaolinite and Montmorillonite.

Depositional environment, weathering

action, leaching etc. will govern the

kind of ions present in clay.

Marine clay has predominantly Na &

Mg cations.

Valence of the cations is the basic

factor in the process of replacement of

exchange.

Replacement ability of various cations

are as follows:

Al3+

> Ca2+

> Mg2+

> K+ > H

+ > Na

+ > Li

+

This principle of cation exchange can

be used with advantage in many

practical situations, as for example, in

the stabilization of sodium clay soil by

using lime. Here, calcium ions replace

the sodium ions by virtue of their

Superior replacing power and reduces

the swellings of sodium

Montmorillonite, because the adsorbed

layer of water would be thinner when

the valence of the cation is larger.

Clay Particles Interaction

Soil structure of clay depends on the

force of attraction and repulsion

between the soil particles. But there is

an adsorbed layer of water between

two particles. Hence particles interact

through adsorbed layer.

Thus factors such as nature of ions

present, their concentration, size etc.

affects the soil structure.

Fig : 3.19

Fig : 3.20


The repulsive force between clay

particles are due to similar charges in

clay particles. This force comes into

play when particles come closer to

such an extent that adsorbed water

layer starts interacting.

Attractive forces between clay

particles are due to Vander wall forces

various types of linkage such as

hydrogen bond etc.

Force of repulsion and attraction both

act between clay mineral particles.

Repulsion occurs between like charges

of the double layer of cation and the

force of repulsion depending on the

characteristics of layers.

An increase in cation valency or

concentration will result in decrease in

repulsive force and vice versa.

Attraction between clay particles is

due to Vander wall forces which are

independent of double layer.

If two particles come closer and

interact such that Total energy

decreases then net force is attractive.

This leads to Flocculated structure.

If however, net force is repulsive,

Dispersed structure will be formed

Flocculated structure (are mostly edge

to face but it can be Edge to edge

also).

Dispersed structure have face to face

configuration.

Tendency to flocculate depends on

1. Concentration of electrolytes

2. Valency of ions

3. Temperature

Dispersed structure with face to face

configuration has lower void ratio as

compared to flocculated structure

which has large void ratio.

Marine clay has flocculated structure,

lacustrine clay has dispersed structure.

Sodium clay can absorb more water

than Ca clay because absorbed layer

thickness in calcium clay is smaller

(diffused layer being thinner, there by

tendency to absorb water being less).

Thus liquid limit of sodium clay is

larger as compared to calcium – clay.

As sodium clay can adsorb more water

its swelling property is more. If Na is

replaced by calcium, tendency to

swelling is less. This is used in soil

stabilization.

Note: Decrease in layer thickness, decrease

repulsion.

Table 3.5 Comparison between Clay Minerals

Fig : 3.21 Flocculated Structure

Fig : 3.22

Fig : 3.23 Dispersed Structure


Clay Mineral Grain size Base exchange

capacity

Ip Dry strength Active tip

Montmorillonite Min largest Max Max largest

Illite Intermediate Intermediate Intermediate Intermediate Intermediate

Kaolinite Max Least Min Min Least


PRACTICE QUESTIONS

Q.1) The following data refer to a sample of

soil

Percent passing 4.75 mm IS siever – 71

Percent passing 75 μ IS siever – 4

Uniformity coefficient – 7.5

Coefficient of curvature – 2.7

The soil is classified as

a) SW

b) GW

c) SM

d) GM

Q.2) Dispersed type of soil structure is an

arrangement comprising particles having

a) face to face or parallel orientation

b) edge to edge orientation

c) edge to face orientation

d) all of the above

Q.3) Of the following soils, characterized by

their USCS group symbol, which one is

most suitable as fill for a road embankment?

a) SM

b) GC

c) GW

d) CL

Q.4) For general engineering purposes, soils

are classified by

a) particle size classification system

b) textural classification system

c) High Way Research Board (HRB),

d) unified soil classification system.

Q.5) An arrangement composed of ‘flocs’ of

soil particle is known as

a) Dispersed structure

b) Cohesive matrix structure

c) Honey comb structure

d) Flocculent structure

Q.6) For a soil sample, degree of shrinkage

is less than 5%. The quality of the soil

sample is

a) poor

b) very poor

c) medium good

d) good

Q.7) Unified soil classification system

(USCS) was developed by

a) Kozney

b) Atterberg

c) Casagrande

d) Rankine

Q.8) The group index of a soil depends on

a) Liquid limit

b) Plastic limit

c) All of the mentioned

d) None of the mentioned

Q.9) According to IS classification, the

range of silt size particle is

a) 4.75 mm to 2.00 mm

b) 2.00 mm to 0.425 mm

c) 0.425 mm to 0.075 mm

d) 0.075 mm to 0.002 mm


Q.10) The number of basic soil components

in IS classification system (IS: 1498) is

a) 4

b) 5

c) 7

d) 18

/

SOIL CLASSIFICATION

Based on indexproperties

Particle sizeclassification

< 2μ - Clay, 2 to 75μ -Silt, 75μ to 4.75mm -Sand, 4.75 to 80mm -Gravel, 80 to 300 mm

Cobble, > 300mm -Boulder

AASHTO/HRB SoilClassification

Based on particle sizedistribution &

plasticitycharacteristics

Soil is classified into 7groups (A1, A2..A7)

Black cotton fallsunder A7 (smaller

group number, bettersoil for pavement)

Group Index =0.2a+0.005ac+0.01 bd

(0 to 20)

Depends on % soilpassing 75μ sieve,

Liquid limit & Plasticlimit

GI = 0 best soil & 20poor soil for pavement

Suitable forpavement design

USCS Classification

Suitable for"Air fieldclassification" by

casagrande

Based on particle sizedistribution, plasticity

& compressibilitycharacteristics

IS Soil Classification

Derived from USCS

Primary SoilClassification (18)

Coarse Grained soil(% retained on 75μ is

> 50%)

Gravel (more than50% of coarse fractionretained on 4.75mm)

Sand (more than 50%of coarse fractionpasses 4.75mm)

Fine Grained soil (%passing on 75μ is >

50%)Inorganic Silts (M)

Inorganic Clay (C)

Organic silts & clays(O)

Highly Organic soil(Peat, Pt)

Plasticity Chart (Ip vswL)

Equation of A line =0.73 (wL - 20)

Inorganic Clay aboveA line & Inorganic Silts

below A line

wL<35 - low comp,35<wL<50 - int comp,

>50 - High comp

Organic silts & claysbelow A line

Well graded gravel(GW) - fines<5%,

Cu>4 & Cc b/w 1&3

Poorly graded gravel(GP) - fines < 5% &

not meeting allgradation req of GW

Silty Gravel (GM) -fines>12%, below A

line or Ip<4

Clayey Gravel (GC) -fines>12%, above A

line with Ip>7

Dual Symbols

Fines b/w 5% and 12%, GP-GM, SW-SC,

GW-GC, SP-SM etc

Ip lies b/w 4 & 7, GM-GC, SM-SC etc

/

SOIL STRUCTURE &CLAY MINERALOGY

Behaviour of soildepends on soil

structure

Types of soilstructure

Single grainedstructure (eg. gravel

& sand)

Honey combedstructure (eg. Fine

sand or silts)

Flocculated structure(eg. clays)Dispersed structure

(eg. remoulded clays)

Edge to faceorientation

has high shearstrength, high

permeability & lowcompressibility

Formed where there isnet attractive forceexist b/w particles

Formed where there isnet repulsive b/w

particles

Face to Faceorientation

has low shearstrength, low

permeability & highcompressibility

Remoulding

Loss of strength incohesive soil

Thixotropy

Regain of lost strengthwith passage of time,

with no change inwater content

Clay MineralogyKaolinite

Illite

Montmorillonite

HalloysiteGibbsite & silicasheets one over

another with H-bond

Causes no swelling &no shrinkage

Stable Mineral eg.china clay

Contains nonexchangeablepotassium ions

Medium swelling &shrinkageExhibits large

swelling & shrinkage

highest % ofmontmorillonite is seen in bentonite followedby black cotton soil

Behaviour similar tokaolinite

Specific Surface Area(m^2/g or cm^2/g)

Finer the soil, higherSSA

Montmorillonitehigher SSA = 800

m^2/g

Diffuse Double layer

Present only in clays

adsorbed waterimparts plasticity to the

clay

SOIL MECHANICS

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