Introduction to Soil Engineering

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Introduction to Soil Engineering

• D. A. Cameron
• 2007

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StaffCIVIL ENGINEERING

• Dr. Don Cameron
• donald.cameron_at_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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Water Transport and Soil Development
Mountains
Coastline
River valleys
Lakes, estuaries, deltas
Ocean
B, C
G
G
S
M silts
C, O (organic)
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Soil from Rocks Residual

• SAND - quartz, silica
• SILT - finer quartz silica (842)
• 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
angular particles from quarry
1 mm 1000?m
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Fine - Grained Soils

• Cohesion
• Apparent cohesion
  ? apparent tensile strength,
• arising from
• electrostatic 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
• 11 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
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Kaolinite
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21 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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Clay mineral 1x10-7 m
3. Aggregate 1 to 4x10-5 m
2. Clay mineral stack 0.1x10-6 m

1. 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
• Expand !
• 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
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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 11 clay mineral is formed
• How does the Mica group of clay minerals differ
  from the 11 clay minerals?