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G1-Lecture10-The-MicroStructure-of-Soils

G1-Lecture10-The-MicroStructure-of-Soils - CEGCEG-4011...

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CEG CEG-4011 Geotechnical Engineering I 4011 Geotechnical Engineering I Lecture #10 The Micro-Structure of Soils - Particle or Grain size analysis - Particle shapes - Relative density of sands - The Atterberg limits of silts and clays Luis Prieto-Portar 2009
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SOIL COMPOSITION. The texture of a soil is described by its appearance , and depends on the size and shape of its solid phase. The soil is either a coarse or fine grained: - COARSE GRAINED (are visible to the naked eye) : G (gravels) + S (sands) - FINE GRAINED (invisible to the naked eye) : M (silts) + C (clays) Coarse grained soils are also called , and the fine grained soils are commonly granular referred to as cohesive . The name "gravel" comes from the French word “ greve ”, which is typical of southern French beaches and are composed of small rounded pebbles. It’s symbol is G . The symbol S for sand comes from the French word “ sable ”. The symbol for silt M comes from the Swedish word “ mäjala ”. Finally, the symbol C for clay is self- evident.
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The Grain Size Analysis of Soils: - The sieve analysis (used for gravels and sands) - The hydrometer analysis (used for silts and clays) Particle Sizes and their Shapes. Each individual solid particle that forms a soil can have different size and shape . These two properties will determine how the soil behaves under engineering requirements.
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A) Particle Sizes. In this course, we will classify the soil in two ways: (1) the USCS (Unified Soil Classification System) and (2) the AASHTO (American Association of State and Highway Transportation Officials) system. In both, particles larger than 3 inches (75 mm) as called rock fragments . Smaller particles are soils . These soils are divided into gravels, sands, silts and clays according to their size. The first two, gravel G and sands S are classified using sieves . These sieves are a grid of wires that permit soils of a smaller size to pass through. The finer two types of soils, silts M and clays C are classified using a hydrometer .
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The Sieve Analysis.
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The Hydrometer Analysis. The easiest way to measure the particle size D at any time t is through a hydrometer (shown at left suspended in a solution of soil and water). The largest particles settle out first (shown in the bottom of the graduated cylinder). As the water loses these particles, it becomes less dense. The floating hydrometer will therefore sink in the less dense water a distance L that is proportional to the reduced density. Measuring L at different time t periods will provide the diameter D of the particles that have settled out of the solution.
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