Geotechnical Engineering I

• Soil


  Relatively loose mass of mineral and organic materials and sediments found above the bedrock, which can be relatively easily broken down into its constituent mineral or organic particles

  
• Soil consists of layers of minerals constituents of variable thickness, which differ from the parent materials in the morphological, physical, chemical and mineralogical characteristics

  
• Soil


  A natural product of weathering of rocks and decomposition of organic matter
• Soil


  An accumulation of individual particles that are bonded together by mechanical or attractive means, the strength of the bonds being a small fraction of the mineral particles
• Soil


  Particles may range from colloidal size to small boulders
• Soil mechanics


  A branch of engineering mechanics that describes the behaviour of soils
• Geotechnical engineering


  The branch of civil engineering concerned with the engineering behaviour of earth materials
• Geotechnical engineering project


  1. Review project needs to define required material properties

  2. Site investigation of soil, rock, fault distribution and bedrock properties

  3. Determine and design type of foundations, earthworks and pavement subgrades required
• Weathering


  The process of breaking down rocks
• Types of weathering


  • Physical weathering
  • Chemical weathering
• Physical weathering


  • Breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure, without any change in chemical condition
  • Soil formed will be cohesionless (sand and gravel)
• Chemical weathering


  • Changes the composition of rocks by decomposing the parent minerals, transforming them into new compounds such as clay silica particles, carbonates and iron oxides
  • Soil produced will be cohesive (silt and clay)
• Chemical weathering processes


  • Oxidation
  • Carbonation
  • Hydration
  • Leaching
• Residual soils


  Soils which have remained over the parent rock from which they have been formed, relatively shallow in depth

  
• Alluvial soils


  Soils which have been transported and subsequently deposited by flowing water
• Alluvial soil deposits


  • Alluvial fan
  • Flood-plains
  • Delta
• Residual soils


  Soils that have remained over the parent rock from which they have been formed, relatively shallow in depth, characterized by a gradual transition from soil through partially weathered rocks, fractured and fissured rock, to bedrock

  
• Alluvial soils


  Soils that have been transported and subsequently deposited by flowing water, usually stratified due to fluctuations in velocity of flowing water, average particle size decreases with increasing distance from the source of stream, delta soils consist of silt and clay, marine deposits are formed when fine-grained soils are carried beyond deltas into the sea, lacustrine soils are deposited at the bed of lakes
• Mineral


  An inorganic chemical compound formed in nature, may occur in an amorphous state or in a crystalline state
• Crystal


  A homogenous body bounded by smooth plane surfaces, molecules of minerals are composed of atoms of chemical elements arranged in a definite orderly manner to form a three dimensional net-work, called a "lattice"
• Ion


  A charged particle formed when a molecule loses or gains one or more electrons, positively charged ions are called "cations" and negatively charged ions are called "anions"
• Primary bonds


  Bonds that combine atoms into molecules, including ionic bonds and covalent bonds
• Secondary bonds


  Bonds that link atoms in one molecule to atoms in another, including hydrogen bonds and Van der Waals bonds, much weaker than primary bonds
• Ionic bond


  The simplest and strongest of the bonds which hold atoms together, formed between oppositely charged ions by the exchange of electrons, causes a separation between centres of positive and negative charge in a molecule forming a "dipole"
• Covalent bond


  Formed when one or more bonding electrons are shared by two atoms so that they serve to complete the outer shell for each atom
• Hydrogen bond


  The attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen, or fluorine, that comes from another molecule
• Van der Waals bonds


  The sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or ionic bond, allow sliding and rupture to occur
• Silica tetrahedral unit


  Consists of a silicon ion surrounded by four oxygen ions arranged in the form of a tetrahedron, the bases of the tetrahedrals are all in the same plane and the tips all point in the same direction
• Aluminium (or Magnesium) octahedral unit


  Has an aluminium ion or a magnesium ion endorsed by six hydroxyl radicals or oxygens arranged in the form of an octahedron
• Kaolinite


  Each structural unit is a combination of two layers with apeces of a silica layer joined to one of a gibbsite layer, successive layers are held together by strong hydrogen bonds
• Montmorillonite


  The basic structural unit is made up of a gibbsite sheet sandwiched between two silica sheets, the bonding between layers is weak Van der Waals forces allowing water and exchangeable cations to enter and separate the layers, exhibits high swelling and shrinkage characteristics
• Illite


  The basic structural unit is the same as montmorillonite but with some substitution of aluminium for silicon in the silica sheet, the charge deficiency is balanced by potassium ions which bond the layers, the bond is weaker than hydrogen bond in kaolinite but stronger than water bond in montmorillonite, does not swell as much as montmorillonite
• Single grained structure


  Found in coarse-grained soil deposits where particles settle independently of each other due to gravitational forces, with particle-to-particle contact
• Honeycomb structure


  Associated with silt deposits, particles are attracted to each other and already deposited particles leading to formation of arches and a honeycomb structure with high void ratio

  
• Flocculated structure


  Found in clay deposits, particles have edge-to-edge and edge-to-face contact due to net attractive electrical forces between them, results in high void ratio
• Dispersed structure


  Found in clay deposits, particles are oriented randomly with face-to-face contact due to net repulsive electrical forces between them, results in low void ratio
• Void ratio


  High void ratio in honeycomb structure, leading to large reduction in volume when disturbed due to breakdown of structure
• Flocculated structure


  Edge-to-edge and edge-to-face contact between particles, due to attractive net electrical forces between adjacent particles at time of deposition
• Dispersed or oriented structure


  Face-to-face contact between particles, due to repulsive net electrical forces between adjacent particles at time of deposition
• Flocculated structure


  • High void ratio, remoulding or compaction leads to slippage of particles and formation of dispersed structure with decreased void ratio