Soil Mechanics

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SOIL MECHANICS (CE –40) 1 NOTE-1 (BY MOYUKH DE) INTRODUCTION Existence of matter is commonly found in basic three states i.e. solid ,liquids and gaskin. A soil mass in its natural state may consists of all three phases. The basic Ingredients is the solid grains which forms the soil skeleton, While the intermittent void spaces are filled by either air or water. Soil mass as three phase system can be expressed as under:- SOIL MASS AS A 3 PHASE SYSTEM.

Transcript of Soil Mechanics

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NOTE-1 (BY MOYUKH DE)

INTRODUCTION Existence of matter is commonly found in basic three states i.e. solid

,liquids and gaskin. A soil mass in its natural state may consists of all three

phases. The basic Ingredients is the solid grains which forms the soil

skeleton, While the intermittent void spaces are filled by either air or

water.

Soil mass as three phase system can be expressed as under:-

SOIL MASS AS A 3 PHASE SYSTEM.

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ORIGIN & FORMATION

Soil is formed by weathering of rocks followed by transportation of

weathered particles & systematic sedimentation of them by natural forces

such as wind, water etc.

TYPES OF SOIL & THEIR DISTRIBUTION

Types of soil & their distribution is shown by a map provided below:

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THE ABOVE MAP SHOWS VARIOUS TYPES OF SOIL IN INDIA

(SOURCE: THE GEOLOGICAL SURVEY OF INDIA)

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SOIL STRUCTURE

An arrangement & state of aggregation of soil particles in a soil mass is

known as soil structure.

TYPE OF SOIL STRUCTURE

# SINGLE GRAINED:

coarse grained soil (diameter>0.02mm) settle out of suspension in water

as individual grains independently of other grains. They settle out due to

gravity &may be deposited in a loose state having a high void ratio or in

dense state having low voids.

SINGLE GRAINED STRUCTURE

#Honey comb structure:

Grain of silts having size varying between 0.0002mm –0.02mm exhibits

honey comb structure. When the grains settle due to gravity in a

suspension, the surface forces at the contact areas (which are large

enough compared to the sub merged weight) prevent the grains from

rolling down immediately into positions of equilibriums among the grains

already deposited. The grains are held until miniature arches are formed,

bridging over relatively larger void spacing & forming a honey comb

structure.

Large amount of water is enclosed within the voids generally. The

structure has high void ratio & carry heavy loads without excessive

volume change.

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HONEY COMB STRUCTURE

# FLOCCULENT AND DISPRESED STRUCTURE

Flocculated structure of clay platelets is formed when there are edge to

edge contacts between the platelets.

Dispersed structure is formed when the platelets have face to face contact

in more or less parallel array.

NOTE: V.M. gold Schmidt (1926) explained flaky minerals in highly

sensitive clay can be arranged as unstable card house structure surplus

water can be enclosed in the space between them.

However Lambe (1953) explained the same structure as platelets

(colloidal chemical view point). Accordingly, the structure was said to be

controlled by attractive or repulsive forces (SURFACE FORCES).

In flocculent structure the net electric forces between adjacent soil

particles are attraction forces.

In dispersed the net electrical forces between adjacent soil particles at the

time of deposition are repulsive.

FLOCCULATED STRUCTURE DISPERSED STRUCTURE

i.e. flocculated structure can be transformed to a dispersed structure by

compaction remolding & consolidation.

Consolidation

Compaction

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Flocculent structure

Dispersed structure

Coarse grained skeleton structure: In coarse grained skeleton structure the

vlid space of the single grainted structure is filled with clay particles.

# COHESIVE MATRIX: Here particle to particle contact of coarse faction is

not possible. The coarse grains are embedded in large mass of cohesive

fine grains.

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SOIL STRUCTURE

# COMPOSITE SOIL STRUCTURE: Depends upon the relative proportion of

coarse grained & fine grained particles. Depending on which there may be

either coarse grained skeleton structure or cohesive matrix structure.

CLAY MINERAL

# SILICA TETRAHEDRAL UNIT: Encloses a silicon atom having four oxygen

or hydroxyls in configuration of tetrahedral.

The tetrahedron combines in a sheet structure so oxygen of the bases of

all tetrahedron is in a common place and each oxygen belong to two

tetrahedron

The silicon sheet is represented by

# Octahedral unit encloses on aluminum ion or magnesium ion having six

hydroxyl in a configuration of an octahedron. The octahedral units are put

together into a sheet structure which may be viewed as two layers of

COARSE GRAINED SOILS • Single grained

structure

FINE GRAINED SOILS • Honey comb • Flocculent • dispersed

SILICA TETRAHEDRAL UNIT

OCTAHEDRAL UNIT

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densely packed hydroxyls with cation between the sheets in octahedral

co-ordination

This is symbolized as:

This is known as gibbsite.

BASIC UNITS OF CLAY MINERALS SILICA & ALUMINUM SHEETS

IMPORTANT CLAY MINERALS & DESCRIPTIONS ARE AS FLOWS:

# KAOLINITE:-it is a common mineral of kaolin group .They are made up of

gibbsite sheet joined to silica sheet &can be expressed as:-

The size of this structural unit is about is about seven angstrom.

In kaolinite mineral these (structural units as shown above) are joined in

sheets by strong hydrogen bonds. A kaolinite crystal is formed by 100 or

more such stackings .

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KAOLINITE PARTICLES OCCUR IN CLAY AS PLATELETS.

Kaolinite is relatively stable and water is unable to penetrate between the

layers. Kaolinite consequently shows relatively little swell on wetting.

Example:- china clay

# MONTMORILLONITE:-it is made up of a gibbsite sheet (i.e. the

octahedral sheet) sand witched between two silica sheet it is symbolized

as :-

These basic units are stacked on one another & are attached by weak

bonds .The spacing between the elemental silica- gibbsite –silica sheets

depends upon the amount of available water to occupy the space .The

gibbsite layer may include atoms of aluminum, iron, magnesia etc. In

addition silica iron may interchange with these ions (mentioned above)

resulting in negative charge on the clay platelets.

Soils containing monmorillonite minerals exhibit high shrinkage &

swelling characteristics.

# ILLITE: - The structure is similar to montmorillonite but due to

substantial replacement of metallic ions of gibbsite(such as magnesium,

aluminum, etc.)By silicon ions, cations such as K+(potassium) are attacked

between the layers to balance the charge resulting from replacement and

to tie the sheets together.

Illite clay minerals donot swell as montmorillonite.

The structure can be represented as:-

+ +k+ ions

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STABILITY SERIES:_

Kaolinite> illite> montmorillonite

BOND STABILITY SERIES:-

Hydrogen bond> cation bond>water bond

STRUCTURE OF THE MAIN CLAY MINERALS: (A) KAOLINITE, (B) ILLITE AND

(C) MONTMORILLONITE, BASED ON COMBINED SHEETS (FROM CRAIG,

1990)

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CLAY MINERALS (FROM MITCHELL, 1993 AFTER TOVEY, 1971)

A) KAOLINITE; B) HALLOYSITE; C) MONTMORILLONITE D) ILLITE

(Note: images made by electron photomicrograph)

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GRANULAR PARTICLES (FROM MITCHELL, 1993, COURTESY OF K. L. LEE)

Note: images made by electron photomicrograph)