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CIV3248 Groundwater,seepage and environmental engineering

CIV3248 Groundwater,seepage and environmental engineering. Clay Minerals K H McKenry. Clay and engineering significance. Plasticity – severe reaction with water. Hydraulic Conductivity. Chemically active: Sensitive to small amounts of Portland cement, Electrolyte content of soil water.

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CIV3248 Groundwater,seepage and environmental engineering

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  1. CIV3248Groundwater,seepage and environmental engineering • Clay Minerals • K H McKenry CIV3248 Clay Minerals

  2. Clay and engineering significance • Plasticity – severe reaction with water. • Hydraulic Conductivity. • Chemically active: • Sensitive to small amounts of Portland cement, • Electrolyte content of soil water. CIV3248 Clay Minerals

  3. Basic features • Size – smaller than the wavelength of light • Shape – sheet: minimum particle dimension << maximum dimension • COLLOID behaviour (large surface area) CIV3248 Clay Minerals

  4. Origin • Weathering of primary rock minerals • (except quartz / silica) • Weathering of clay minerals • Bauxite is the end product CIV3248 Clay Minerals

  5. Atomic structure – alumino-silicate sheet structures • Silicate structure (tetrahedral) • Si++++ bound to four O--, three in base layer, one in apex layer • Alumina structure (octahedral) • Al+++ bound to six OH-, three in top layer three in bottom layer CIV3248 Clay Minerals

  6. Silica lattice Layer representation Alumina lattice CIV3248 Clay Minerals

  7. CIV3248 Clay Minerals

  8. Mineral unitsApex O-- of silica sheet are at the same spacing as the OH- of the alumina sheet and attach together with the hydrogen bond. Silicate layer Silicate layer Alumina layer Alumina layer Silicate layer Three layer minerals -illite -montmorillonite Two layer minerals - kaolinite CIV3248 Clay Minerals

  9. Mineral structure • Extend laterally using the basic molecular sheet arrangement. • Sheets may be stacked on top of each other. • Inter-layer molecules – water, cations CIV3248 Clay Minerals

  10. CIV3248 Clay Minerals

  11. CIV3248 Clay Minerals

  12. CIV3248 Clay Minerals

  13. Clay mineral Kaolinite Illite Montmorillonite Typical thickness (nm) 50 - 2000 30 3 Typical diameter (nm) 300 - 4000 10000 100 - 1000 Specific Surface (m2 /g) 10 - 30 50 - 100 200 –800 Cation Exchange capacity meq/100g 3 25 100 Activity (PL / %clay) 0.3 – 0.5 0.5 – 1.3 1.5 - 7 Swell potential low medium high Clay mineral types CIV3248 Clay Minerals

  14. Mineral structural defects • Finite sheets – broken mineral bonds at the edges. • Isomorphous substitution – replacement of Si++++ or Al+++ with lower valency cation of a similar size. • CHARGED PARTICLES ! CIV3248 Clay Minerals

  15. Clay particle charges • Broken bonds at edges – positive • Isomorphous substitution - negative Clay particle charge distribution CIV3248 Clay Minerals

  16. Exchangeable cations • Positively charged ions (Na+ K+ , Ca ++ Mg++) • Associated with clay mineral to give electrical neutrality. • May be exchanged with other cations in the soil water. • Cation exchange capacity – ions required (milliequivalents per 100 grams of clay). CIV3248 Clay Minerals

  17. Structure of water • Water is a polar material • Binds to any charged particle, especially O- - • Water binds to itself • Water of hydration of many salts. H+ O- - - + H+ CIV3248 Clay Minerals

  18. CIV3248 Clay Minerals

  19. Adsorbed water layer (AWL) • Water layer immediately adjacent to the clay mineral surface. • Embedded with exchangeable cations. • Structure differs from free water • May sustain a tension of several MPa • (Free water vapourises at -100kPa) CIV3248 Clay Minerals

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  21. AWL thickness • Influences the macro structure of the clay soil • “Thickness” determined by nature of soil water electrolytes • Valency • Concentration CIV3248 Clay Minerals

  22. Valency of adsorbed cations • Half as many divalent ions (eg Ca++) needed for charge neutrality, compared with monovalent ions (eg Na+) • Half the AWL thickness required CIV3248 Clay Minerals

  23. CIV3248 Clay Minerals

  24. Electrolyte concentration in free water • Free water is in equilibrium with adsorbed water. • High free water concentration requires a high absorbed water concentration • For the same number of adsorbed ions, this needs a THINNER adsorbed water layer CIV3248 Clay Minerals

  25. CIV3248 Clay Minerals

  26. AWL thickness • THIN • Divalent exchangeable cations (eg Mg++) and/or high concentration of free water electrolytes • THICK • Monovalent exchangeable cations (eg K+) and/or low concentration of free water electrolytes CIV3248 Clay Minerals

  27. Dispersed structure Card house structure (Flocculated) Clay soil structure CIV3248 Clay Minerals

  28. Flocculated clay • Thin adsorbed water layer • Negative faces bond with positive edges • Card house structure • Rigid configuration • Apparent larger particles • Better engineering properties CIV3248 Clay Minerals

  29. Dispersed structure • Thick adsorbed water layer • Negative faces repel. • Clay plate stack parallel to each other • Large variation in water content (high PI) • Large shrink / swell capacity • Poor engineering properties CIV3248 Clay Minerals

  30. Characteristic Flocculated Dispersed Origin Marine deposition Fresh water deposition Stiffness high low Swell potential low high Strength high low Remoulding sensitivity high low Erosion potential low high Hydraulic conductivity high low Properties CIV3248 Clay Minerals

  31. Time • Dispersed structure may be changed to flocculated by adding Ca++. • For Portland cement or hydrated lime • (Soil stabilisation) • Alteration of the ground water electrolyte condition can change the flocculation / dispersion potential of a soil. CIV3248 Clay Minerals

  32. Closure • Clays behave as COLLOIDS. • Variety of clay minerals. • Sensitive to their chemical environment. • May be a sink for ground water pollutants. • Barrier to water flow. • Susceptible to erosion (dispersed) • Incorporated into engineering design CIV3248 Clay Minerals

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