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
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?