2010Lecture2a

2010Lecture2a - Geomechanics Week 2: Soils &...

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Unformatted text preview: Geomechanics Week 2: Soils & Clays 1.Weathering, genesis of soil clays 2.Clay mineralogy 3.Soil colloid properties and the diffuse double layer 4.Engineering applications What is soil? Soil is the outer layer of the earths crust capable of supporting plant growth (Pearson, 1967, Principles of Agronomy). (Perspective: Botany, Agronomy, Horticulture, ) Soil is a dynamic, heterogeneous, three-phase, porous media (Perspective: Soil Physics) Soil is the unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. (USDA Soil Science Glossary; Perspective: Pedology, Soil Science) Soil is the interface between the lithosphere (rocky part of the planet) and the biosphere, atmosphere, and hydrosphere (Perspective: Modern Biogeochemistry). Soil is unconsolidated, surficial material. (Perspective: Engineers, Soil Mechanics) Deep weathering profile vs. soil Oxisol and deep weathering, near Kuantan, Malaysia Soil of the soil scientist Deep weathering, Regolith, or Soil to the Geotechnical engineer mineral particles and organic matter between bedrock and vegetation Variation in physical properties of soil profiles Soil Colloids Organic and inorganic matter with very small particle size and a correspondingly large surface area per unit mass Four categories: Crystalline silicate clays (phyllosilicates) Noncrystalline silicate clays (gel-like structures) Iron and aluminum oxide clays (fine particles, not silicates) Organic matter (humus) Clay is . . . A particle size class (<0.002 mm) A mineral type with specific properties and characteristics (secondary mineral) (it only happens by coincidence of these properties that most clay mineral particles fall within the clay particle size class). Big smaller extremely small Relative Size and shape comparison of soil particles Sand and silt have essentially the same properties (slight changes in specific surface area, shape, etc) Major change in properties (specific surface area, activity of surfaces) Variation of clay minerals (mineral groups, but also their properties) Clay mineralogy is based on: numbers and combinations of structural units (tetrahedral and octahedral sheets) Number and type of cations in the octahedral sheet. Size and location of layer charge, if any (due to isomorphic substitution) Absence or presence of interlayer cations Two (major) general categories: 1:1, 2:1 Most important Soil Colloids Kaolinite Most common in soils, depends on environment....
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2010Lecture2a - Geomechanics Week 2: Soils &...

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