Introduction to Soil Mechanics

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Introduction to Soil Mechanics

• D. A. Cameron
• 2003

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Reference

• Barnes, G E
• Soil Mechanics, Principles and Practice,
  MacMillan Press
• (2nd Edition)
• Civil Engineering students will need
• in 3rd year

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The engineering behaviour of soil

• Need to understand
• How soils are formed
• The basic units which form soil material
• Engineering concepts of sand, silt and clay
• The Unified Soil Classification System
• Stress in soil, total and effective
• Water flow in saturated soils
• Erosion, scour or piping
• Physical improvement of soil (compaction)
• 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
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Soil from Rocks Residual

• SAND - quartz, silica
• SILT - finer quartz silica
• CLAY - clay minerals
• (from weathered feldspar and mica )
• very fine particles

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Particle Interactions

• Coarse soils v. Fine soils
• sand and gravel silt and clay
• STRENGTH DERIVED FROM
• Friction, interlock v.
• physico-chemical interaction

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Fine - Grained Soils

• Cohesion
• Apparent cohesion ? tensile strength
• arising from
• electrostatic forces
• (are stronger, the finer the particle)

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Molecular Structure of the Clay Minerals

• Lecture 1a
• Intro to Civil and Mining Engineering

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• http//pubpages.unh.edu/harter/crystal.htm
• Phyllosilicates
• ..are the clay building blocks
• ? Tetrahedrons and 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 left
• with 11/2 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 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
Hydrogen bonding
Micelle
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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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Clay mineral 1x10-7 m
3. Aggregate 1 to 4 x10-5 m
2. Clay mineral stack 0.1x10-6 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
• - are able to be moulded
• SO, moisture affects clay soil properties

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Properties of the clay minerals

• Can absorb or lose water between the silicate
  sheets
• -ve 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

• Differ in the form of bonding
• Illite - a 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 water capacity

• 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 in soil pores
• Electrostatic forces give rise to COHESION in
  soils with clay minerals

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Uses of Kaolinite

• Ceramics (China clay)
• A filler for paint, rubber and plastics
• Glossy paper production in the paper industry

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Uses of Montmorillonite Smectite group

• facial powder (talc)
• filler for paints and rubbers,
• an electrical, heat and acid resistant
  porcelain,
• plasticizer in moulding sands
• drilling muds
• repairing leaking farm dams

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Summary

• The basic building blocks of clays are small
• Si, O, H and Al are the chief ingredients
• Tetrahedral and octahedral sheets possible
• Different combinations of these form the basic
  micelles of clay minerals
• Clay mineral properties vary because of 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?