DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 1

Introduction to Soil Engineering

D. A. Cameron

2007

DIVISION OF INFORMATION TECHNOLOGY, ENGINEERING AND THE ENVIRONMENT 2

StaffCIVIL ENGINEERING

Dr. Don Cameron

donald.cameron@unisa.edu.au

P2-35

ph 8302 3128

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Reference

Barnes, G E

“Soil Mechanics, Principles and Practice,” MacMillan Press

Civil Engineering students will need

this text in 3rd year

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The engineering behaviour of soil

1. How soils are formed 2. The basic units which form soil material 3. Engineering concepts of sand, silt and clay 4. The Unified Soil Classification System 5. Stress in soil, total and effective 6. Water flow in saturated soils 7. Erosion, scour or piping 8. Physical improvement of soil (“compaction”)9. Terminology

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Origins of Soils

• Residual

• Alluvial

• Aeolian = wind blown

• Glacial

• Marine

• Lacustrine

• Organic

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Mountains

Lakes, estuaries, deltas OceanOcean

River valleys

Coastline

G

B, ‘C’G

S

C, O (organic)

M = silts

Water Transport and Soil DevelopmentWater Transport and Soil Development

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Soil from Rocks = Residual

SAND - quartz, silica

SILT - finer quartz & silica (8:4:2)

CLAY - clay minerals

(from weathered feldspar & mica )− very fine “clay” particles

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Particle Interactions

Coarse soils v. Fine soils

[sand and gravel] v. [silt and clay]

STRENGTH DERIVED FROM

Friction, interlock v.

physico-chemical interaction

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Clean Sand - under the microscope

1 mm = 1000m

angular particles from quarry

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Fine - Grained Soils

Cohesion

“Apparent” cohesion

“apparent” tensile strength,

arising from

electrostatic forceselectrostatic forces

(are stronger, the finer the particle)

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Molecular Structure of the Clay Minerals

Lecture 1

Civil Engineering Practice

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http://pubpages.unh.edu/~harter/crystal.htm#

Phyllosilicates

are the clay “building blocks”

Tetrahedrons & Octahedrons

• Clays form from weathering and secondary sedimentary processes

• Clays are usually mixed − other clays

− microscopic crystals of carbonates, feldspars, micas and quartz

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1. The Tetrahedron Unit

Silica, Si4+

forms a tetrahedron

with 4 x O2-

Has a nett -ve charge of 4-

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1. Silica Tetrahedron Unit

8-, 4+

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Tetrahedral sheets

• Formed by sharing of O2- between

units

• Corner O2- shared, creating the

sheet

• Nett –ve charge at top of

tetrahedral sheets!

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Sharing

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2. The Aluminium Octahedral Unit

Al3+ with six O2-

Each oxygen ion is left

with 1.5 –ve charge

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Aluminium Octahedra

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Octahedral sheets

• Octahedral sheets formed by each

oxygen being bonded to two Al ions

• Each O ion left with one –ve charge

• IF charge satisfied by hydrogen ions,

the Gibbsite mineral is formed

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Sharing

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The Kaolinite CLAY Mineral

• Top oxygen ions in Silica sheet

bonded to Aluminium sheet

– “1:1 clay mineral”

• Each top oxygen ion shared by 2 Al

and 1 O ion

• This unit = “a clay micelle”

(approx. 0.7 nm thick and 10 x10 nm)

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Kaolinite micelle

Gibbsite layer

Silicate layer

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Kaolinite clay mineral

…consists of stacks of micelles

Usually hydrogen bonds

micelles together:

a strong bond

stable clay mineral

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Kaolinite

Hydrogen bonding

Micelle

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Kaolinite

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2:1 Clay Minerals “The Mica Group”

3 sheets, 2 silica tetrahedra,

1 aluminium octahedron = a micelle

Many different clay minerals occur

with this basic unit

e.g. “Illite” (Adelaide clays) and

“Montmorillonite” (basaltic clays)

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Smectite (includes montmorillonite)

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2. Clay mineral stack

0.1x10-6 m

1. Clay mineral 1x10-

7 m

3. Aggregate

1 to 4x10-5 m

4. Clod

0.1 mm = 1x10-4 m?

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Properties of the clay minerals

When mixed with a little water,

clays become “plastic”

i.e. are able to be moulded

SO, moisture affects clay soil

engineering properties

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Properties of the clay minerals

• Can absorb or lose water between the

silicate sheets

− negative charge attracts H2O

• When water is absorbed, clays may

ExpandExpand !!

− water in spaces between stacked layers

− Montmorillonite most expandable

− Kaolinite the least

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Illite v Montmorillonite Different forms of bonding between these minerals

Illite - main component of shales and other argillaceous rocks

- stacks keyed together by K+

- nett negative charge

Montmorillonite

- stacks keyed together by Na++ or Ca++

and H2O

- greater nett negative charge

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Clay Minerals – capacity for water

i) Kaolinite (China clay) Water absorption, approximately 90%

ii) Montmorillonite (Bentonite, Smectite)

Water absorption, approximately 300 -

700%

iii) Illite Intermediate water absorption

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“Specific surface” = grain area/grain mass

TYPE OF

SOIL

SPECIFIC SURFACE

(m2/g)

Kaolin 80

Glauconite 400

Black earth 440 - 990

Bentonite 1300 - 1390

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The influence of charges

“The greater the surface area, the

greater the charge”

− the greater the affinity for water

− some water strongly adsorbed in a very

thin layer

− other water “free” in the soil “pores”

Electrostatic forces give rise to

COHESION in soils with clay minerals

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Uses of Kaolinite

Ceramics (China clay)

Filler for paint, rubber & plastics

Glossy paper production

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Uses of Montmorillonite The “Smectite” group

facial powder (talc)

filler for paints & rubbers

an electrical, heat & acid resistant porcelain

plasticizer in moulding sands

drilling muds

repairing leaking farm dams

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In Summary

1. The basic building blocks of clays are small

2. Si, O, H and Al are the chief ingredients

3. Tetrahedral & octahedral sheets possible

4. Different combinations of sheets form the basic

micelles of clay minerals

5. Clay mineral properties vary due to the nature of

bonding of the sheets between micelles

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Revision

• What is a clay micelle?

• Describe how a 1:1 clay mineral is formed

• How does the Mica group of clay minerals differ from the 1:1 clay minerals?