Soil Mechanics
-
Upload
mohit-suri -
Category
Documents
-
view
115 -
download
1
Transcript of Soil Mechanics
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.
SOIL MECHANICS (CE –40)
2
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:
SOIL MECHANICS (CE –40)
3
THE ABOVE MAP SHOWS VARIOUS TYPES OF SOIL IN INDIA
(SOURCE: THE GEOLOGICAL SURVEY OF INDIA)
SOIL MECHANICS (CE –40)
4
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.
SOIL MECHANICS (CE –40)
5
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
SOIL MECHANICS (CE –40)
6
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.
SOIL MECHANICS (CE –40)
7
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
SOIL MECHANICS (CE –40)
8
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 .
SOIL MECHANICS (CE –40)
9
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
SOIL MECHANICS (CE –40)
10
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)
SOIL MECHANICS (CE –40)
11
CLAY MINERALS (FROM MITCHELL, 1993 AFTER TOVEY, 1971)
A) KAOLINITE; B) HALLOYSITE; C) MONTMORILLONITE D) ILLITE
(Note: images made by electron photomicrograph)
SOIL MECHANICS (CE –40)
12
GRANULAR PARTICLES (FROM MITCHELL, 1993, COURTESY OF K. L. LEE)
Note: images made by electron photomicrograph)