Handout1 - University o f Missouri-Columbia Dept o f Civil...

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University of Missouri-Columbia Dept of Civil & Environmental Engineering Ori2in and Nature of Soils Soil Fonnation: The ultimate origin of soil is bedrock, which is broken down into successively finer and finer particles by physical (mechanical) and chemical disruption. This process is called weathering. Mechanical Weathering: (Disintegration) Breakdown of masses by mechanical means. 1. Abrasive Action of wind, water, ice 2. Gravity -~ landslides, rockfalls, cave-ins 3. Action of organisms -~ plant roots, wonns, microorganisms 4. Tectonic action - volcanoes (heat/pressure) 5. Freezing/Thawing (ice expansion cycles) 6. Rapid temperature changes (deserts significant) 7. Action of man - mining, oring, blasting, excavations, etc. "Soils formed by mechanical weathering basically retain the properties of the original rock since they have the same mineral composition. " Chemical Weathering: Breakdown of masses by chemical processes (decomposition). 1. Dissolution - dissolving action of water 2. Hydration - combining with water e.g., Anhydrite CaS04 (G s = 2.9) e.g., Gypsum CaS04 2H20 (G s = 2.3) 3. Hydrolysis - fonnation of hydroxides (OH-) with H+ separated from water molecule 4. Oxidation - losing electrons 5. Reduction - gaining electrons 6. Carbonatization - formation of weak carbonic acid to produce carbonates and clay minerals e.g., calcite (CaC03) reacts with bicarbonate to produce calicum cations (Ca+ 2 ) 7. Base exchange - cation exchange on colloid surfaces, dispersion and flocculation.
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University of Missouri-Columbia Dept of Civil & Environmental Engineering Origin and Nature of Soils (Con't) p2 "Soils formed by chemical weathering usually have chemical and
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