Effect of Rigid Boundaries on Measurement of Particle Concentration

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Courtesy of ASTM International

Soil specimens are often formed by placement in a mold with rigid boundaries, followed by striking off a plane surface with a rigid straightedge. These rigid boundaries and plane surfaces interrupt the packing of particles causing the measured void ratio to be greater than the value away from the boundary. A theory for boundary effects has been derived considering regular packings of uniform sized particles. For a cubical container, the difference between measured and true void ratios is proportional to the ratio of particle size to cube side length. Definition of an equivalent side length permits application of the theory to volumes with boundaries of arbitrary shape. Tests indicate that the theory works for angular particles and for graded materials providing particle size is represented by the size for which 10% of the particles (by mass) are finer. For container sizes normally used in practice, boundary effects are negligible for silt and clay soils and for cohesionless soils containing more than 10% finer than 0.074 mm.

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